Protection against arthropod parasites Technical domain [0001] The present invention concerns a product for protecting humans and host animals against arthropod parasites. The invention also concerns a method of selecting suitable active agents for said products. Related art [0002] Arthropod parasites comprising arthropod ectoparasites and arthropod endoparasites impact on human and animal health. Mosquitoes, ticks, fleas, lice and mites attack humans or burrow in human skin inflicting injury. In addition, arthropod ectoparasites and endoparasites affect livestock production, reducing animal well- being and performance in terms of weight gain and productivity. Arthropod ectoparasites also serve as vectors of major pathogens that have a substantial worldwide socioeconomic impact causing morbidity and mortality in humans and farmed animals. [0003] Agents employed to control arthropod parasites are primarily chemical, accounting for nearly a quarter of the total animal health market. Yet, with only a few exceptions, active ingredients mostly originate from chemicals developed for crop protection. Macrocyclic lactones, such as ivermectin, are used extensively against arthropod endoparasites and ectoparasites. Other ectoparasiticides include pyrethroids, carbamates, neonicotinoids, spinosyns, insect growth regulators and isoxazolines. [0004] With the exception of isoxazolines, one of the few active ingredients introduced this century, resistance to the other chemical classes of parasiticides is widespread and growing rapidly. Control of malaria, transmitted by mosquitoes, is currently highly dependent on arteminisin-based drug combination therapies but resistance to arteminisin has developed rapidly in Asia and Africa over the past decade. Added to this problematic are changing environmental conditions which, allied with transport of people and animals on a global scale, enable arthropod parasites to colonize further geographic zones. The spread of these parasites in combination with their ever-increasing resistances to conventional control agents are of growing concern for human and animal health. [0005] There is therefore an urgent need for the identification and development of advanced compositions and active agents with alternative modes of action against arthropod parasites. In order to protect the health of host species in a changing environment, new strategies for controlling arthropod parasite growth and survival are required. Short disclosure of the invention [0006] It is an aim of the present invention to provide an alternative to existing methods and compositions for reducing damage inflicted by arthropod parasites on their host organisms. [0007] It is a further optional aim of the invention to provide an alternative method to control the population of arthropod parasites. In particular, growth of the population should be curtailed and the parasites capacity to spread should be reduced. [0008] Moreover, the alternative should preferably be suited to prevent blood- feeding arthropod parasites from transmitting parasites and/or infectious agents onto and between their hosts. [0009] According to the invention, these aims are attained by the object of the attached claims, and especially by the independent claims. [0010] In particular, the aims are achieved by using a product comprising one or more purinergic receptor antagonists to protect animals and/or humans from arthropod parasites by preventing the parasite from feeding on host tissue, including blood. [0011] Arthropod parasites all belong to a monophyletic group of molting animals. Arthropod parasites may, for example, be biting insects, ticks, mites and crustaceans. [0012] Hematophagous parasites, also called blood-feeders, comprise amongst others the group of blood-feeding arthropods, such as biting insects, as well as ticks. [0013] Known purinergic receptors (PRs) are plasma-membrane molecules activated by extracellular purines, including nucleotides such as adenosine triphosphate (ATP) and nucleosides such as adenosine. While ATP is well known to drive energy-consuming processes within cells, purines also modulate biological processes through their extracellular action on these PRs. [0014] In humans, for example, extracellular ATP serves as a co-transmitter with classic neurotransmitters within the central nervous system and the peripheral nervous system, as well as a signalling molecule for most human tissues and cell types. Due to their significance in pathologies, human PRs have been subject to extensive studies. [0015] Vertebrate PRs are broadly divided into P1 PR and P2 PR classes. [0016] The first class comprises metabotropic P1 PRs. P1 PRs are adenosine- sensitive PRs formed by a single polypeptide with 7 trans-membrane α-helices set perpendicular to the plasma membrane which respond, for example, to extracellular adenosine. P1 PRs can oligomerise to form homo-oligomers or hetero-oligomers such as the P1A ₁ -A _2A dimer. P1 PRs can also oligomerise with other receptor types. P1 PR subtypes show different affinities for adenosine, with the A ₁ subtype showing the highest affinity. Adenosine and adenosine 5’-monophosphate (AMP) are full agonists for human P1A ₁ PR. [0017] The second class comprises P2 PRs, which respond to ATP and other nucleotides such as α,β-methylene ATP. This class is further subdivided into ionotropic ATP-gated P2X PRs and metabotropic nucleotide-sensitive P2Y PRs that respond to ATP, adenine 5’-diphophate (ADP) and uridine 5’-triphosphate (UTP). Ligand-gated P2X PRs are composed of P2X ₁ -P2X ₇ subunits that form homomeric or heteromeric trimers. ATP is a full agonist for P2X ₂ , P2X ₃ and P2X _2/3 P2 PR subtypes. [0018] While P2X PRs have primarily been studied in connection with their implication in neurological pathology, members of this class have also been found in organisms as diverse as an amoeba, a single-celled green algae and invertebrate species. However, the homologous receptors identified in these species show a low sequence homology of less than 45% with human P2X PRs. [0019] Despite the pharmaceutical interest in these receptors, the role of PRs in arthropod parasites, including insect vectors of disease, remains obscure today. One reason for this is lack of evidence for P2 PRs in the sequenced genome in well- investigated insect vectors, such as the African malaria mosquito Anopheles gambiae. In fact, P2X PRs are considered absent from insects. Whole genome sequences of blood-feeding insect vectors of disease are apparently devoid of genes coding for P2X PR homologues (Hou, Z. and Cao, J.2016 Comparative study of the P2X gene family in animals and plants. Purinergic Signalling 12, 269-281). [0020] Despite this lack of any molecular evidence for known PRs in insect vectors of disease, the inventors could prove the presence of a purine-sensitive cell on the mouthparts of the malaria mosquito, which responds to ATP and adenosine by increasing its action potential frequency. In addition, the inventors have established that ATP acts as a feeding stimulant for the malaria mosquito as measured by increased feeding by this mosquito species on a feeding medium to which ATP is added in solution. These findings are contrary to what is reported in the art. [0021] Moreover, the inventors show for the first time that known P1, P2X ₂ , P2X ₃ , P2X _2/3 antagonists of vertebrate PRs are effective inhibitors of the electrophysiological response of the purine-sensitive cell of arthropod parasites, such as mosquitos. Adding an antagonist of a P1 PR in solution with adenosine causes inhibition in action potential frequency recorded from the purine-sensitive cell compared to when adenosine is presented without the antagonist. Likewise, adding an antagonist of P2X ₂ , P2X ₃ , P2X _2/3 PRs in solution with ATP causes inhibition in action potential frequency recorded from the purine-sensitive cell compared to when ATP is presented without the antagonist. [0022] The inventors provide evidence of the presence of a purine-sensitive cell, which can be inhibited by PR antagonists, including known antagonists of human PRs, in the mouthpart of the malaria mosquito Anopheles gambiae. [0023] Remarkably, the inventors demonstrate that by interfering with the activation of PRs, PR antagonists block the blood-feeding behaviour of hematophagous arthropod parasites, such as biting insects and ticks. [0024] It could be shown that the same P1, P2X ₂ , P2X ₃ , P2X _2/3 PR antagonists of vertebrate PRs inhibit feeding of two mosquito species, two biting fly species, and three tick species on a feeding medium in which either adenosine or ATP, or adenosine plus ATP is present as a feeding stimulant. PR antagonists, such as P1, P2X ₂ , P2X ₃ , P2X _2/3 PRs antagonists, are therefore promising alternatives to existing methods to control blood-feeding arthropod parasites on humans and host animals through inhibition of their PRs. [0025] As will be described in greater detail below, the efficacy of specific PR antagonists to inhibit parasites from feeding on host blood may vary according to the arthropod species and the type of PR a specific species expresses. [0026] The invention concerns the use of a product comprising a purinergic receptor antagonist, wherein the antagonist inhibits the arthropod parasite from feeding on host tissue. [0027] Products comprising PR antagonists may be directed at arthropod ectoparasites, for example a blood-feeding ectoparasite. [0028] Products comprising PR antagonists may also be directed at arthropod endoparasite. Such products may be used therapeutically. They may also be used as a preventative measure to protect against infection by an arthropod endoparasite. The products may be used to prevent the establishment of an arthropod endoparasite in the host body. [0029] The product may comprise only one PR antagonist compound. The product may also comprise different PR antagonist compounds. The product my comprise different compounds which are antagonists to one or more P2X PR subtypes. The product may comprise different compounds, which are antagonists to one or more of P1 PR subtypes. The product may also comprise a mixture of antagonists directed towards P1 PRs and antagonists directed against P2X PRs. Additionally, the product may comprise metal modulators, which are suitable for inhibiting PRs. [0030] PR antagonist compounds, including PR antagonists that act selectively on either P1 or on P2 PR classes, or that act selectively on a particular PR subtype within a PR class, are known in the art. Several examples of PR antagonist compounds are mentioned in subsequent sections. It is understood that this invention is not limited to the examples of known PR antagonist compounds mentioned herein. The skilled person understands that other PR antagonists are equally suitable for the purpose of this invention, most importantly the claimed use of the product. [0031] The products may be directed against different kinds of arthropod parasites. The product may for example be used against arthropod ectoparasites, preferably against biting insects. The product may also be used to protect against arachnids, such as ticks, for example. [0032] Moreover, the product may be used against endoparasites that may be arthropod endoparasites. If the product is used against endoparasites, it be may be provided in form of a therapeutically active formulation. [0033] The products should be used to protect against arthropod parasites having at least one PR and/or one purine-sensitive cell. The PR may be P1 type PR or a P2 type PR. The PR may also be a homolog to a known vertebrate PR. It may also be a new type of PR. Depending on the kind of PR the arthropod parasite expresses and displays on its cells, it may respond to nucleotides, preferably ATP or to nucleosides, preferably adenosine, as feeding stimulant. It is also possible, that the arthropod parasite responds to both feeding stimulants, to ATP and/or other nucleotides, and to adenosine and/or other nucleosides and to a combination nucleotide and nucleoside feeding stimulants. [0034] A suitable formulation comprising PR antagonists may be administered systemically to a host. It is also possible, that the product will be formulated for a targeted delivery of the PR antagonist compounds, respectively the PR metal modulators. [0035] Compounds that inhibit P1, P2X ₂ , P2X ₃ , P2X _2/3 PRs of a blood-feeding arthropod parasite may be formulated using known methods and deployed to inhibit feeding by an arthropod parasite on or in a host animal. However, this invention is not limited to any specific formulation of the PR antagonist comprising product. [0036] Depending on its intended application, the product comprising PR antagonist compounds and PR metal modulators may be provided in different forms. The product may for example be provided as a liquid composition. The liquid composition is preferably water-based, optionally containing a molecule or molecules that enhances solubility of the PR antagonist in said liquid composition. It may be a water dispersible emulsion. [0037] A liquid product could be suitable for spraying onto surfaces, for example clothes or mosquito nets, or onto host animals and humans, or onto a habitat occupied by an arthropod parasite. [0038] The product may also be provided in solid form, for example as a powder. [0039] The concentration of the PR antagonists in the product should be chosen according to the inhibitory activity, as preferably determined by an inhibitory concentration IC, for example the IC ₅₀ value, or an inhibitory dose ID, for example the ID ₅₀ value, of each PR antagonist compound. In addition, the mode of application, for example topically or systemically, should also be considered when determining a suitable concentration of PR antagonists in the product. A liquid product for example, may comprise up to 50 wt% or up to 5 wt% of total PR antagonist compounds. Depending on their mode of use, less concentrated products may be preferable. The content of PR antagonist compounds in a less concentrated product may, for example, be no less than 10 ^-9 wt% or more than 10 ^-3 wt %. [0040] It was furthermore shown for blood-feeding arthropod parasites that metal modulators inhibit the response of PRs to its agonists. Metal modulators are suitable for inhibiting blood-feeding behaviour in hematophagous parasites. To inhibit blood- feeding behaviour metal modulators may for example be provided in combination with PR antagonist compounds. [0041] The invention also concerns methods for screening and selecting PR antagonist candidate compounds for their efficacy in inhibiting blood-feeding behaviour of arthropod parasites. [0042] The methods may be based on screens of test animals. Such test animals may be arthropod parasites, such as blood-feeding ectoparasites. The test animals may also be transgenic animals. Alternatively, the method may be based on a heterologous cell systems. [0043] The present invention provides a means to inhibit feeding by an arthropod parasite by interfering with its ability to sense nucleosides and nucleotides. This is achieved by inhibiting the arthropod parasite’s P1, P2X ₂ , P2X ₃ or P2X _2/3 PR or any combination of said PRs using PR antagonists. [0044] The present invention therefore provides a novel means to stop or so reduce feeding by an arthropod parasite and to prohibit it from inflicting injury to the host animal through biting, through establishing a feeding site, through injury, through loss of blood, through loss of vital body fluids, through loss of essential nutrients and/or through loss of tissue or cells by the host animal. [0045] Reduced feeding on the host animal also reduces the capacity of an arthropod parasite to transmit pathogens to its host. [0046] Furthermore, reduced feeding by the arthropod parasite diminishes its capacity to grow and multiply which, in turn, leads to a lower population density of the arthropod parasite in subsequent generations. [0047] This invention provides a protection against arthropod parasites based on an entirely new mechanism targeting a molecular structure, i.e. a purinergic receptor, which has never been used for this purpose before. By providing this new protection mechanism, the invention overcomes the current acute problem of resistances against known insecticides, acaricides and parasiticides employed to control these parasites. Terms and definitions used herein: [0048] Action potential: The terms action potential and spike are used synonymously herein. [0049] Activates: As used herein the terms “activates”, “activation” and “activated”, mean to enhance the function. In respect of receptors and in particular of a P1, P2X ₂ , P2X ₃ and/or P2X _2/3 PR activate means to enhance the functioning of said receptor. [0050] Agonist: The term “agonist” as used herein means a compound that enhances the action of a molecule or a molecular structure, for example a receptor, or a function of said molecule or molecular structure. Specifically, a “PR agonist” is a compound that activates a molecule or molecular structure which is a PR and/or which exhibits a PR function. Use of the term agonist herein comprises a plurality of agonists. [0051] Antagonist: The term “antagonist” as used herein means a compound that inhibits, reduces, or prevents the action of another molecule or molecular structure, such as a receptor, or the function of said molecule or molecular structure. Specifically, a “PR antagonist” is a compound that inhibits, reduces, or prevents the action of another molecule, or prevents the action of a molecule or molecular structure which is a PR and/or which exhibits a PR function. [0052] Arthropod parasite: The term arthropod parasite as used herein means a parasitic arthropod living on or within a host animal that obtains nutrients from the host animal through routes comprising an arthropod parasite’s mouthparts, or by absorption or adsorption of host animal nutrients through the integument or plasma membrane of an arthropod parasite for survival, growth, and reproduction. Nutrients obtained from the host animal may comprise blood or blood cells, nutrients from within a cell, and nutrients may be obtained when an arthropod parasite feeds on a cell or cells, on tissue, on keratinous particles, on epidermal particles, on fur, on feathers, on nutrients from hair, on feather follicles, on hypodermal tissue, on tissue fluids, on sebaceous and exocrine gland secretions, on lymph, on inflammation products, on lysed tissue, on content of the gastrointestinal system, on mucous membranes, on mucus, and when feeding on blood is restricted to a particular life stage of an arthropod parasite. Accordingly, an arthropod parasite comprises such means of obtaining nutrients from the host animal. Arthropod parasites comprise arthropod ectoparasites and arthropod endoparasites. Arthropod parasites also comprise micro-predators. Arthropod parasites are important vectors of disease. Use of the term arthropod parasite herein comprises a plurality of arthropod parasites. [0053] Blood-feeder: The term “blood-feeder” as used herein means blood- feeding arthropod parasite. [0054] Blood-feeding insect: The term “blood-feeding insect” as used herein is synonymous with biting insect. [0055] Biting insect: The term “biting insect” as used herein comprises mosquitoes, sandflies, tsetse flies, black flies, tabanid horse and deer flies, stable flies Stomoxys calcitrans, horn flies Haematobia irritans and other members of the genus Haematobia, snipe flies, midges, fleas, lice, true bugs, bed bugs and triatomine bugs. These insects can serve as vectors of diseases and infections. During blood-feeding, biting insects may transmit parasites and infectious agents into humans and host animals as for example into domesticated farm animals. Parasites transmitted by biting insects comprise, for example, malaria, trypanosomiasis, surra, leishmaniasis, onchocerciasis, filariasis and loiasis. Infectious agents transmitted by biting insects comprise, amongst others, dengue virus, yellow fever virus, Zika virus, West Nile virus, chikungunya, plague, Bacillus anthracis, Bluetongue virus and lumpy skin disease virus. Use of the term biting insect herein comprises a plurality of biting insects. [0056] Arthropod ectoparasite: The term “arthropod ectoparasite” as used herein means a parasitic arthropod that survives on blood, on or in the skin or epidermis, or that feeds on outgrowths of the skin or epidermis, or that feeds on subcutaneous or intracutaneous tissue or other tissue or on any other fluids of a host animal and comprises ectoparasitic arthropods that include, amongst others, biting insects, face flies, sheep head flies, louse flies or keds of the family Hippoboscidae, bat flies of the superfamily Hippoboscoidea, sucking lice, chewing lice, biting lice, poultry lice, fleas, ticks, feather mites, follicle mites, quill mites, depluming mites, fur mites, chigger mites, chicken mite or roost mite Dermanyssus gallinae, northern fowl mite Ornithonyssus sylviarum, tropical fowl mite Ornithonyssus bursa, cattle ear mite, sheep itch mite, otoacariosis mite, scaly-leg mite Knemidocoptes mutans, the tracheal mite and varroa mite of honey bees, isopod parasites of fish and aquatic invertebrates, copepod parasites of mollusks, amphipod or scud parasites of mollusks and other aquatic invertebrates, copepods parasites on fish such as species of the family Ergasilidae or gill lice, lernaeid anchor worms of the copepod genus Lernaea on freshwater fish such as carp and goldfish, copepod anchor worms on marine fish, cuckoo copepods or nicothoid parasites of lobsters and spider crabs, sea lice of Lepeophtheirus spp. and Caligus spp. on salmonids, and sea louse host-animal infective free-living copepodid life stages. Ectoparasites can transmit parasites and infectious agents and can also debilitate host animals through blood-feeding, anaemia, irritability, dermatitis, loss of weight, skin necrosis, secondary infection, inoculation of toxins, paralysis, and death. The term bleed-feeding ectoparasite as used herein comprises also animals classified as blood-feeding micro-predators. Use of the term arthropod ectoparasite herein comprises a plurality of arthropod ectoparasites. [0057] Arthropod endoparasite: The term “arthropod endoparasite” as used herein means a parasitic arthropod that lives and grows inside a host animal and comprises endoparasitic arthropods of vertebrate animals that are intracutaneous-, tissue- and cavity-dwelling organisms including, amonst others, mites of the Sarcoptidae family causing sarcoptic mange or scabies in the skin of humans and animals, mites of the nasal cavity, tracheae, lungs and of the gastric mucosa of birds and mammals, sarcoptic mites of the buccal mucosa of mammals, botfly larvae of the family Oestridae parasitizing the pharyngonasal cavity and throat of mammals, cattle grub flies or gadflies of the genus Hypoderma responsible for subdermal cysts or warbles on host animals, botfly larvae of the family Gasterophilidae in the gastric mucosa of horses and donkeys, fly larvae feeding on vertebrate host tissue, for example, myiasis or flesh flies of the families Calliphoridae, Muscidae and Sacrophagidae such as the New World Screwworm Cochliomyia hominivorax, Old World Screworm Chrysomya bezziana and Wohlfahrtia magnifica, and larvae of the botfly families Cuterebridae and Hypodermatidae. Endoparasitic arthropods of invertebrates include parasitic barnacles such as the rhizocephalan parasites or Sacculina spp. barnacle parasites of the reproductive system of crabs. Use of the term arthropod endoparasite herein comprises a plurality of arthropod endoparasites. [0058] Feed: The term “feed” as used herein means obtaining nutrients by a mosquito, biting fly, tick, mite, or by any other arthropod parasite relying on host- animal fluid or host-animal tissue for survival. Accordingly, “feeding” is the process or state of bringing host-animal fluid, cells, tissue, or nutrients inside an arthropod parasite. [0059] Feeding inhibitor: The term “feeding inhibitor” as used herein means a compound which modifies an arthropod parasite’s behaviour, such that the parasite does not feed on host resources comprising blood, nutrients within a cell, a cell or cells, tissue, tissue fluids, lymph, content of the gastrointestinal system, mucus or host mucous membranes. The terms “inhibit feeding”, “inhibits feeding”, “inhibition of feeding”, “inhibiting feeding”, “inhibitor of feeding”, “reduce feeding”, “prevent feeding”, “block feeding” and “control feeding” as used herein comprise what a feeding inhibitor for an arthropod parasite achieves. A feeding inhibitor for an arthropod parasite can be composed of one or more compounds. [0060] Engorgement: The terms “engorgement” and “engorged” as used herein mean to swell with blood, cells, tissue, tissue fluids, lymph, or with any other body fluid or nutrient taken from a host animal during the process of feeding by an arthropod parasite. The terms “engorgement” and “engorged” as used herein also mean to swell with fluid taken from a feeding medium by an arthropod parasite during the process of feeding. Accordingly, the terms engorgement, engorged, feeding and fed are used synonymously herein. [0061] Express: The terms “express”, “expression”, “expresses”, “expressing” and “expressed” as used herein mean the biological process of a cell by which a protein such as a receptor protein is synthesized by transcription and translation from information stored in a gene into a polypeptide which can be further processed within the cell such as by protein folding. The polypeptides constituting receptors of relevance to this disclosure are in addition transported into the plasma membrane where they organize to confer on the cell its purine-sensitive phenotype. [0062] Feeding-stimulant: The term “feeding-stimulant” as used herein means a compound or combination of compounds which induce feeding behaviour of an arthropod parasite. The feeding stimulants used in this invention are purinergic compounds. [0063] Hematophagous: The terms “hematophagous” and “blood-feeding” are used synonymously herein. [0064] Host animal: The terms “host” or “host animal” as used herein comprise animals which can carry or harbour an arthropod parasite or may be prey for a micro- predator. Hosts or host animal include, without limitation, mammals, humans and animals such as cattle, buffalo, bison, pigs, donkeys, horses, sheep, goats, camels, deer, reindeer, dogs, cats, hamsters, rabbits, birds, cage birds, canary, parrot, finch, hens, chickens, ducks, geese, turkeys, pigeons, ostrich, reptiles, turtles, fish, farmed fish, aquarium fish, salmon, caged salmon, cod, trout, carp, sole, bass, sturgeon, turbot, tilapia, arthropods, crustaceans, shrimps, crabs, lobsters, crayfish, prawns, insects, honey bees, molluscs, slugs, snails, squid, mussels, scallops, clams and oysters. Use of the term host animal herein comprises a plurality of host animals. [0065] Inverse agonist: The term “inverse agonist” as used herein means a compound that can reduce constitutive receptor activity regulating cellular signalling in absence of an agonist. [0066] Micro-predator: The term “micro-predator” as used herein is an hematophagous arthropod which attacks more than one host, reducing each host's fitness by at least a small amount, and is only in contact with any one host intermittently. This behaviour makes micro-predators suitable as vectors, as they can pass smaller parasites from one host to another. The term micro-predator as used herein is comprised in the term ectoparasite defined above. [0067] Modulator: The terms “modulator”, “modulate”, “modulation”, “modulated” and “modulating” as used herein mean a change in the activity of a target molecule or a target multi-molecular structure forming a receptor. Modulation may cause an increase or decrease in activity of said target molecule or a target multi- molecular structure, for example through changes in binding capacity, in ion flux changes in receptor molecules, ion channel opening, ion channel conductance or in other functional characteristics of receptor molecules. Accordingly, a “modulator” is a compound that interacts with another molecule or multi-molecular structure, or which alters the function of a molecule or multi-molecular structure. Use of the term modulator herein comprises a plurality of modulators. [0068] Non-parasitic organism: The term “non-parasitic organism” as used herein comprises organisms that do not feed on a host animal to survive, for example humans, rat and the fruit fly Drosophila melanogaster. [0069] Organism: The terms “animal” and “organism” are used synonymously herein. [0070] Parasite: The term “parasite” is an organism that lives in or on an organism of another species, a host, and benefits by deriving nutrients at the other’s expense. [0071] Pharmacologically acceptable salts: Reference herein to pharmacologically acceptable salts includes solvates, hydrates and polymorph crystal forms of same. [0072] Purinergic receptor: The term “purinergic receptor” or “PR” as used herein means a plasma-membrane molecule or molecular structure activated by, amongst others, adenosine 5’-triphosphate (ATP), related nucleotides adenine 5’-diphophate (ADP) and uridine 5’-triphosphate (UTP), and/or by adenosine 5’-monophosphate (AMP) or adenosine. A PR permits an arthropod parasite to detect and respond to nucleotides and/or nucleosides, for example, on a host animal’s skin, in a host animal’s blood, in a host animal’s body fluids, in a host animal’s tissues, in a cell of a host animal, in a host animal’s tissue fluids, in a host animal’s lymph, in a host animal’s gastrointestinal content and in a host animal’s mucous membranes. Purinergic receptors (PRs) are divided into two broad classes, P1 PRs activated by adenosine and P2 PRs activated by ATP, ADP and UTP. Use of the term PR herein includes a plurality of purinergic receptors (PRs). [0073] Purine-sensitive cell: The term “purine-sensitive cell” as used herein means a sensory neuron expressing at least one PR that allows an arthropod parasite to detect a nucleoside such as adenosine and/or a nucleotide such as ATP in the cell’s surroundings, for example, on a host animal’s skin, in a host animal’s blood, in a host animal’s body fluids, in a host animal’s tissues, in a cell of a host animal, in a host animal’s tissue fluids, in a host animal’s lymph, in a host animal’s gastrointestinal content and in a host animal’s mucous membranes. A purine-sensitive cell forms part of the purine-sensing system of an arthropod parasite. [0074] Tick: The term “tick” as used herein means an organism belonging to the Arachnida and, like biting insects, is an arthropod that feeds on blood. Ticks are found throughout the world and are divided into Ixodidae, hard ticks, and Argasidae, soft ticks. Parasites transmitted by ticks comprise theileriosis, babesiosis, anaplasmosis and heartwater disease. Infectious agents transmitted by ticks comprise Lyme disease, bovine borreliosis, relapsing fever, tularaemia, Q-fever, Crimean-Congo haemorrhagic fever, tick-borne encephalitis and louping-ill. Use of the term tick herein comprises a plurality of tick species. [0075] Vertebrate: The term “vertebrate” as used herein means an animal that develops and possesses a vertebral column or backbone. Vertebrates comprise, for example, humans and domesticated animals. Use of the term vertebrate herein comprises a plurality of vertebrate species. Short description of the drawings [0076] Exemplar embodiments of the invention are disclosed in the description and illustrated by the drawings in which: Figure 1A and B: Mean spike frequencies / 2 s (± s.e.m.) generated in response to 200 pM doses of nucleotides, nucleosides, uracil and adenine presented to Anopheles gambiae (A. gambiae) labellum sensilla (Figure 1A) and representative 2 s recordings (Figure 2B). Responses, which are significantly different to the water negative control are indicated by “h” (P<0.05) and spike frequencies significantly lower than those recorded for ATP are indicated by “a” (P<0.05). Treatment repetitions (n) are reported in Materials and Methods. Figure 2A to 2D: Number of spikes (mean ± s.e.m.) per 2s recorded from A. gambiae labellum sensilla in response to increasing doses of ATP and AMP-CPP (Figure 2A), to adenosine and AMP (Figure 2B), to 50 pM of adenosine, 50 pM of ATP, and to an equimolar mixture of 50 pM ATP and adenosine with water as negative control (0 spikes) (Figure 2C), and to increasing equimolar doses of ATP admixed with adenosine (continuous line) compared to responses to increasing doses of ATP and adenosine (respectively, dashed and dotted lines, data as in A and B) (Figure 2D). Treatment repetitions (n) are reported in Materials and Methods. Figure 3A: Two second recordings from a female A. gambiae labellum sensillum presented with ATP, adenosine and an equimolar 200pM mixture of ATP plus adenosine. Discrimination for action potentials of similar amplitudes and durations (on right) indicate that treatments elicited responses characterized by one spike class. Figure 3B: Typical 2 second recordings from the sensillum of Figure 3A presented with 200pM ATP, 1mM sucrose and 1mM sucrose admixed to 200pM ATP. Spike amplitudes elicited by sucrose are 1.7 times higher than those elicited by ATP or adenosine, permitting discrimination of two spike classes each distinguished by similar amplitudes and durations (on right) elicited by sucrose (open arrowhead at bottom) and by ATP (closed arrowhead at bottom). A spike doublet (d) resulting from near simultaneous firing by the sucrose-sensitive and ATP-sensitive cells is indicated. Treatment repetitions (n) are reported in Materials and Methods. Figures 4A to 4D: The effect of CPT (Figure 4A) and suramin (Figure 4B) on mean (± s.e.m.) action potential frequencies recorded from A. gambiae labellum sensilla in response to adenosine and ATP each at 200 pM. The test sequence in Figures 4A and 4B was agonist, agonist admixed with antagonist, followed by agonist, presented to the sensillum at approximately 2 min intervals; doses of CPT and suramin are indicated. Antagonism lasted for a least 2 min (final response to an agonist compared to the initial response). Typical 2s spike trains from such recording sequences where 10 nM CPT (Figure 4C) and 1 nM suramin (Figure 4D) were admixed with adenosine. * P <0.05, **P <0.001 indicate significant reductions in spike frequency. Treatment repetitions (n) are reported Materials and in Methods. Figure 5A: Number of spikes (mean ± s.e.m.) per 2s recorded from A. gambiae labellum sensilla in response to increasing doses of ATP (solid line; data as in Figure 2A), and to increasing doses of TMP (black dots) and A-317491 (triangles) admixed with 200 pM ATP (positive control, 0 on abscissa). Doses up to 100 nM TMP are presented for purposes of clarity, for full dose range of TMP tested see Figure 6E. Figures 5B and 5C: Two second recordings obtained with 10 nM doses of TMP and A- 317491 admixed with 200 pM ATP following the sequence ATP (top trace), ATP + antagonist and ATP, presented at approximately 2 min intervals to the sensillum. Antagonism lasted at least 2 min (spike count for the final response to ATP compared to the initial response). Treatment repetitions (n) are reported in Materials and Methods. Figures 6A to 6F: Number of spikes (mean ± s.e.m.) per 2s recorded from A. gambiae labellum sensilla presented with 200 pM ATP (positive control, 0 on abscissa) and to increasing doses of the P2 PR antagonists suramin (Figure 6A) and PPADS (Figure 6B), the P2X ₂ PR antagonist RB-2 (Figure 6C), the P2X ₃ / P2X _2/3 PR antagonist A-317491 (Figure 6D), and the P2X ₃ PR antagonists TMP (Figure 6E) and spinorphin (Figure 6F), each dose admixed with 200 pM ATP. Treatment repetitions (n) are reported in Materials and Methods. Figure 7A to 7I: Number of spikes (mean ± s.e.m.) per 2s recorded from A. gambiae labellum sensilla presented in sequence with 200 pM ATP, the indicated nM dose of a PR antagonist admixed with 200 pM ATP, and to 200 pM ATP: suramin (Figure 7A), PPADS (Figure 7B), MRS 2159 (Figure 7C), RB-2 (Figure 7D), A-317491 (Figure 7E), TMP (Figure 7F), spinorphin (Figure 7G), paroxetine (Figure 7H) and A-438079 (Figure 7I). Treatments were presented to sensilla at approximately 2min intervals. The dose of antagonist used approximates its estimated ID ₉₅ value, whereas the dose of a substance failing to inhibit is the maximum dose tested (see Table 4). * P <0.05, ** P <0.01 compared to the initial response to ATP, NS not significant. Treatment repetitions (n) are reported in Materials and Methods. Figure 8A Mean percentage (± s.e.m.) A. gambiae feeding on increasing doses of ATP in the feeding solution. Figure 8B: Percentage A. gambiae feeding on the feeding solution containing 20 µM ATP (positive control, 0 on abscissa) with increasing doses of TMP (black dots) and suramin (triangles) added. Treatment repetitions (n) are reported in Materials and Methods. Figure 9A to 9E: Mean proportion (± s.e.m.) of A. gambiae feeding on a solution containing 20 µM ATP (positive control, 0 on abscissae) and on the same solution containing 20 µM ATP with increasing doses added of the P2 PR antagonists suramin (Figure 9A) and PPADS (Figure 9B), the P2X ₂ PR antagonist RB-2 (Figure 9C), the P2X ₃ , P2X _2/3 PR antagonist A-317491 (Figure 9D), and the P2X ₃ PR antagonist TMP (Figure 9E). Treatment repetitions (n) are reported in Materials and Methods. Figure 10: Percentage A. gambiae feeding on the feeding solution (buffered NaCl/dextrose), on 20 µM ATP + 200 µM adenosine as feeding stimulant in the feeding solution, on the P1A ₁ PR antagonist CPT at 20 µM + the P2X ₃ PR antagonist TMP at 7.5 µM + the feeding stimulant in the feeding solution, and on TMP alone at 7.5 µM + the feeding stimulant in the feeding solution. Bars accompanied by same lower-case letter indicate a significant difference. Treatment repetitions (n) and exact P values are reported on Table 6. Detailed Description and embodiments of the present invention Basic embodiments [0077] The present invention comprises a means to inhibit feeding by an arthropod parasite using purinergic-receptor antagonists identified for at least one P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of said organism using the means described in the invention. The present invention also comprises methods for selecting compounds that antagonize P1, P2X ₂ , P2X ₃ , P2X _2/3 PRs of an arthropod parasite for the purpose of using such antagonists to inhibit feeding by an arthropod parasite on or in a host animal. [0078] A suitable PR antagonist is for example Suramin. Suramin is a symmetric molecule of formula 1: wherein R1 is -methyl in Suramin, and wherein the methyl group may be replaced in suramin derivatives which may be used as PR antagonists according to this invention, by moieties selected from a group consisting of - H, an ethyl group, an isopropyl group, a phenyl group, -F, -Cl, a methoxy group, or -CH ₂ OCH ₃ . [0079] Derivatives of suramin, which may be used as PR antagonists according to this invention, include compounds of formula 1 wherein the carbamide moiety forming R2 is replaced by methylene, carbonyl, or carbonate. [0080] Derivatives of suramin, which may be used as PR antagonists according to this invention also include compounds of formula 1 wherein benzene ring with amide linkers forming R3 is replaced by benzyl, benzofuran, and indole. [0081] Derivatives of suramin, which may be used as PR antagonists according to this invention further include compounds of formula 1 wherein the amidotoluene forming R4 is replaced by thiazolidinone, thiazinanone, or thiobenzamide. [0082] Derivatives of suramin, which may be used as PR antagonists according to this invention further include compounds of formula 1 wherein the naphthalene rings forming R5 are replaced by benzene rings, cyclohexane rings, or 1,3,4-triazine rings. [0083] Derivatives of suramin, which may be used as PR antagonists according to this invention further include compounds of formula 1 wherein sulfonate group as in R6 is replaced by a methyl group, a methoxy group, or an amino group. [0084] Suramin-derived P2X2, P2X3 and P2X2/3 PR antagonist also comprise the following compounds: PR antagonists may also be small molecule inhibitors. P2PX2 PR antagonists may be selected from compounds comprising an anthraqinone moiety of the formula: such as compound RB-2 having the formula: P2PX2 PR antagonists may also be selected from compounds comprising a pyridoxal phosphate moiety of the formula: such as PPADS having the following formula: [0085] P2PX ₃ and P2X _2/3 PR antagonists may also be selected from the following compounds: A317491: AF-906: BLU-5937: S-600918: [0086] The present invention also concerns assay methods for selecting compounds capable of modulating at least one P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of an arthropod parasite, in particular a blood-feeding arthropod parasite. Such assay methods can be physiological and/or biochemical assays and/or behavioural assays. Examples of behavioural assays are described herein in Material and Methods and in Examples, but suitable assays are not limited to the examples provided in Material and Methods and in Examples herein. Assay methods can be used to select compounds for their ability to modulate a P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of an arthropod parasite from available libraries containing hundreds of thousands of molecules or substances. [0087] In one embodiment of this invention a product comprising at least one PR antagonist is directed against arthropod parasites of a vertebrate host, preferably warm-blooded vertebrate host. [0088] In another embodiment, the product is directed against parasites of an invertebrate host, such as an insect, for example a bee. [0089] In an embodiment the product is directed against arthropod parasites. The product may be directed against arthropod endoparasites such as mange mites, botfly larvae, myisasis fly larvae, and/or against arthropod ectoparasites such as blood- feeding micro-predators, mites and ticks. [0090] In an embodiment the product is directed against arthropod biofouling, such as against barnacles. [0091] In an embodiment at least one P1 PR of an arthropod parasite, preferably a blood-feeding arthropod parasite, is modulated by a P1 PR antagonist or by a combination of P1 PR antagonists; [0092] In an embodiment at least one P1 A ₁ PR of an arthropod parasite, preferably a blood-feeding arthropod parasite, is modulated by a P1 A ₁ PR antagonist or by a combination of P1 A ₁ PR antagonists; [0093] In an embodiment at least one P2X ₂ PR of an arthropod parasite, preferably a blood-feeding arthropod parasite, is modulated by a P2X ₂ PR antagonist or by a combination of P2X ₂ PR antagonists. [0094] In an embodiment at least one P2X ₃ PR of an arthropod parasite, preferably a blood-feeding arthropod parasite, is modulated by a P2X ₃ PR antagonist or by a combination of P2X ₃ PR antagonists; [0095] In an embodiment at least one P2X _2/3 PR of an arthropod parasite, preferably a blood-feeding arthropod parasite, is modulated by a P2X _2/3 PR antagonist or by a combination of P2X _2/3 PR antagonists; [0096] In an embodiment at least one P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of an arthropod parasite, preferably a blood-feeding arthropod parasite, is modulated by a combination of P1, P2X ₂ , P2X ₃ 3, P2X _2/3 PR antagonists; [0097] In an embodiment the invention comprises modulation of any combination of P1, P2X ₂ , P2X ₃ and P2X _2/3 PRs of an arthropod parasite, preferably a blood-feeding arthropod parasite, by a combination of P1, P2X ₂ , P2X ₃ and P2X _2/3 PR antagonists; Methods for selecting PR antagonists [0098] The present invention comprises a method for selecting an antagonist for a P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of an arthropod parasite by making, for example, electrophysiological recordings from a cell expressing at last one of said PRs. This is achieved when a test compound is presented in combination with an agonist such as adenosine or ATP and brought into contact in solution with the PR. A PR antagonist causes a decrease, for example, in action potential frequency recorded from the cell compared to the action potential frequency recorded from the same cell when an agonist such as adenosine or ATP is presented alone as positive control. [0099] In an embodiment, the invention comprises an electrophysiological method for selecting an antagonist for a purine-sensitive cell of an arthropod parasite expressing at least one P1, P2X ₂ , P2X ₃ or P2X _2/3 PR. In a preferred embodiment the test compound is presented in solution to contact the cell at increasing doses over the 1 pM to 100 nM range in combination with a fixed dose of an agonist such as adenosine or ATP. Alternatively, 1 pM to 100 µM dose range for the test compound can be chosen for testing. Inhibition induced by an antagonist occurs within seconds as measured by the reduction in the number of action potentials compared to the number of action potentials recorded for adenosine or ATP presented alone as positive control. This is shown herein in Examples on Figure 4 A and C for the P1 antagonist 8- cyclopentyl-1,3-dimethylxanthine (CPT), and on Figure 5 for the P2X ₃ PR antagonist 2,3,5,6-tetramethylpyrazine (TMP) and for the P2X ₃ / P2X _2/3 PR antagonist A-317491. This permits calculation of a dose-response function and ID ₅₀ value for a PR antagonist which can be in the 300 pM to 15 nM range as shown herein in Examples on Figures 5 and 6, and on Table 4 for the P2 PR antagonists suramin and PPADS, for the P2X ₂ PR antagonist RB-2, for the P2X ₃ PR antagonists TMP and spinorphin, and for the P2X ₃ / P2X _2/3 PR antagonist A-317491. Nevertheless, an ID ₅₀ value of an antagonist for a P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of an arthropod parasite is not limited to a particular molar range. [00100] The present invention comprises a method for selecting an inverse agonist for a P1, P2X ₂ , P2X P2X ₃ or P2X _2/3 PR of an arthropod parasite by making, for example, electrophysiological recordings from a cell expressing at last one of said PRs. This is achieved when a test compound is brought into contact in solution with the PR. An inverse agonist causes, for example, a decrease in action potential frequency recorded from the cell compared to the action potential frequency recorded from the same cell when the inverse agonist is absent from the solution. Use of transgenic animals [00101] For the selection of suitable PR antagonists it may be advantageous to use transgenic animals. [00102] In an embodiment the present invention provides for a method for selecting an antagonist for a P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of an arthropod parasite by making, for example, recording from a cell, organ or tissue of a transgenic arthropod parasite containing an altered allele for a reporter gene that encodes and expresses for example a fluorescent protein in addition to at least one functional P1, P2X ₂ , P2X ₃ or P2X _2/3 PR. Presence of the encoded fluorescent protein permits recording of Ca++ transients in the cell, organ or tissue. Selection of a PR antagonist can be achieved when a test compound is presented in combination with an agonist such as adenosine or ATP and brought into contact in solution with the cell, organ or tissue of the transgenic arthropod parasite. A PR antagonist causes, for example, a decrease in cellular activity as measured by Ca ⁺⁺ transients recorded from said cell, organ or tissue compared to Ca ⁺⁺ transients recorded from the same cell, organ or tissue when adenosine or ATP is presented alone as positive control. In this case, modulation by the PR antagonist comprises inhibition of functional PR activity in the transgenic arthropod parasite’s cell, organ or tissue. [00103] In an embodiment the present invention provides for a method for selecting an antagonist for a P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of an arthropod parasite by making, for example, electrophysiological recording from a cell of a non-parasitic transgenic animal containing an altered allele for a gene that naturally encodes and expresses at least one functional P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of an arthropod parasite. This is achieved when a test compound is presented in combination with an agonist such as adenosine or ATP and brought into contact in solution with the PR. A PR antagonist causes, for example, a decrease in action potential frequency recorded from said cell compared to the action potential frequency recorded from the same cell when adenosine or ATP is presented alone as positive control. In the present case, modulation comprises inhibition of the functional PR of the non-parasitic transgenic animal. [00104] In an embodiment the present invention provides for a method for selecting an antagonist for a P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of an arthropod parasite by making, for example, electrophysiological recordings from a cell of a non-parasitic transgenic animal containing an altered allele for a gene that naturally encodes and expresses at least one functional P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of a non-parasitic organism. It is understood that the functional P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of the non- parasitic organism responds to an agonist of at least one P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of the arthropod parasite. Selection of an antagonist is achieved when a test compound is presented in combination with an agonist such as adenosine or ATP and brought into contact in solution with the PR. A PR antagonist causes, for example, a decrease in action potential frequency recorded from said cell compared to the action potential frequency recorded from the same cell when adenosine or ATP is presented alone as positive control. In the present case, modulation comprises inhibition of the functional PR of the transgenic animal. [00105] In one embodiment the selection method is performed using a transgenic animal containing an altered allele for the gene that naturally encodes and expresses a functional P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of a non-parasitic organism such as humans or a rat; [00106] In an embodiment the selection method is performed using a transgenic animal containing an altered allele for the gene that naturally encodes and expresses a functional P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of an arthropod parasite such as a biting insect or a tick; [00107] In an embodiment the selection method is performed using a transgenic animal possessing an altered allele for a functional P1, P2X ₂ , P2X ₃ or P2X _2/3 PR with a phenotype that shows enhanced sensitivity to adenosine and/or ATP as a feeding stimulant compared to the wild-type animal. It is understood that such as transgenic animal is a non-parasitic one; [00108] In an embodiment the selection method is performed using a transgenic animal possessing an altered allele for a non-functional P1, P2X ₂ , P2X ₃ or P2X _2/3 PR that renders it less sensitive to adenosine or ATP as a feeding stimulant. Such a transgenic animal is a knockout (KO) transgenic animal. Further, such a KO transgenic animal can be either an arthropod parasite or a non-parasitic animal. Use of heterologous system to select PR antagonists [00109] It may furthermore be useful to base a selection method for a PR antagonist on a heterologous system. [00110] In an embodiment the selection method is performed using a heterologous cell system, using host cells of vertebrate or invertebrate origin, containing an allele for the gene that naturally encodes and expresses at least one functional P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of an arthropod parasite for the purpose of selecting antagonists for said PRs. The host cell in such a system expresses a nucleic acid sequence or sequences that is not part of its own genome. Host cells can have a variety of origins, for example being human in origin such as human HEK cells, or in another example being amphibian in origin such as the Xenopus oocyte. In such a system a functional PR is constituted when polypeptide subunits synthesized by the host cell combine in the cell membrane to form a functional PR. The PR is activated by a nucleoside such as adenosine or a nucleotide such as ATP in solution through ion flux, and/or ion channel activation and/or induction of secondary messengers. Such activation by an agonist can be used to select antagonists for P1, P2X ₂ , P2X ₃ , P2X _2/3 PRs of an arthropod parasite through changes brought about by the antagonist in solution such as changes in ion flux when the antagonist contacts the functional PR. Such changes in ion flux can be quantified by a variety of techniques well known in the art as indicative of cellular activity, for example by measurement of changes of cell membrane potential, measurement of cell membrane currents, using radiolabelled ion flux assays, using fluorescence assays for voltage sensitive dyes, and by patch clamp electrophysiological techniques. In the case of P1 PRs such measurements that are indicative of cellular activity can also include quantification of secondary messengers such as adenylate cyclase and inositol 1,4,5-triphosphate (IP3). Automation of such measurements permits high-throughput screening of compound libraries using said heterologous systems to select antagonists of said PRs. Alternatively, the ability of cells expressing said functional PRs of an arthropod parasite to grow in a feeding medium in presence of a test compound can be used to select antagonists for P1, P2X ₂ , P2X ₃ , P2X _2/3 PRs of an arthropod parasite. [00111] In an embodiment the selection method is performed using a heterologous cell system for the purpose of selecting antagonists for P1, P2X ₂ , P2X ₃ , P2X _2/3 PRs of an arthropod parasite where host cells express a functional P1, P2X ₂ , P2X ₃ or P2X _2/3 PR or any combination of said PRs belonging to any organism within the Protozoa or belonging to any invertebrate or vertebrate organism. It is understood that the functional P1, P2X ₂ , P2X ₃ or P2X _2/3 PR or said PR combination expressed in the heterologous cell system responds to an agonist of at least one P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of the arthropod parasite. Human astrocytoma cells expressing human and rat P2X ₃ and P2X _2/3 PRs permitted identification of A-317491 as a non-nucleotide antagonist of Ca ⁺⁺ flux by ATP-gated P2X ₃ and P2X _2/3 PRs of humans and the rat as documented by Jarvis et al., 2002 in “A-317491, a novel potent and selective non-nucleotide antagonist of P2X ₃ and P2X _2/3 PRs, reduces chronic inflammatory and neuropathic pain in the rat” in PNAS 99(26), doi:10.1073/pnas.252537299. [00112] As described herein on Figures 5, 6 and 7 and Table 4 of Examples, A- 317491 inhibits the malaria mosquito PR at an ID ₅₀ of 320 pM in the electrophysiological assay and, as described herein on Figure 9 and Table 7 of Examples, A-317491 inhibits feeding by the malaria mosquito at an ID ₅₀ of 240 pM. Furthermore, as described herein on Tables 8, 9 and 10 of Examples, A-317491 inhibits feeding by the Asian mosquito vector of malaria, by biting flies and by ticks. Use of ex vivo and in vivo systems to select PR antagonists [00113] It is also possible, to perform the method of selecting PR antagonists using an ex vivo or an in vivo system. [00114] In an embodiment the method of selecting antagonists for P1, P2X ₂ , P2X ₃ , P2X _2/3 PRs of an arthropod parasite is performed employing cell lines, organoids and tissue originating from arthropod parasites or originating from any organism within the Protozoa or from any invertebrate or vertebrate organism that expresses native P1, P2X ₂ , P2X ₃ and P2X _2/3 PRs or expresses any combination of said PRs. Alternatively, transformed cell lines, organoids and tissue from said organisms expressing P1, P2X ₂ , P2X ₃ and P2X _2/3 PRs may be used to select antagonists for P1, P2X ₂ , P2X ₃ , P2X _2/3 PRs of an arthropod parasite. The ability of cells expressing said native PRs or of said transformed cells expressing PRs to grow in a feeding medium in presence of a test compound can be used to select antagonists for P1, P2X ₂ , P2X ₃ , P2X _2/3 PRs of an arthropod parasite. Notably, sensory cells of dorsal root ganglia and nodose ganglia of vertebrate animals express P1 PRs that respond to adenosine and P2X ₂ , P2X ₃ and P2X _2/3 PRs that respond to ATP and accordingly provide an appropriate medium in which to select antagonists for P1, P2X ₂ , P2X ₃ , P2X _2/3 PRs of an arthropod parasite. [00115] In an embodiment the present invention provides for a method for selecting an antagonist for a P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of an arthropod parasite by making, for example, recording ex vivo from a cell, organ or tissue of a transgenic arthropod parasite containing an altered allele for a gene that encodes and expresses for example a fluorescent protein in addition to at least one functional P1, P2X ₂ , P2X ₃ or P2X _2/3 PR. Examples of an arthropod-parasite organ could be the labellum of a mosquito or the hypostome of a tick. Presence of the encoded fluorescent protein in the arthropod parasite permits recording of Ca ⁺⁺ transients in the cell, organ or tissue indicative of activation of said PRs in presence of an agonist such as adenosine or ATP. Selection of a PR antagonist can be achieved when a test compound is presented in combination with an agonist such as adenosine or ATP and brought into contact in solution with the cell, organ or tissue of the transgenic arthropod parasite. A PR antagonist causes, for example, a decrease in cellular activity as measured by Ca ⁺⁺ transients recorded from said cell, organ or tissue compared to the Ca ⁺⁺ transient activity recorded from the same cell, organ or tissue when adenosine or ATP is presented alone as positive control. In this case, modulation by the PR antagonist comprises inhibition of functional PR activity in the transgenic arthropod parasite’s cell, organ or tissue. [00116] In an embodiment the selection method is performed by selecting antagonists for P1, P2X ₂ , P2X ₃ , P2X _2/3 PRs of an arthropod parasite by using for example electrophysiological recordings of action potentials, recording cell membrane potentials, measurement of cell membrane currents, use of ion flux assays, use of fluorescence assays for voltage sensitive dyes and fluorescent proteins, patch clamp electrophysiological recording techniques or cell imaging techniques that measure cellular activity of any cell, tissue or organ of any organism within the Protozoa or of any invertebrate, including arthropod parasites, or of any vertebrate organism expressing a P1, P2X ₂ , P2X ₃ or P2X _2/3 PR or expressing any combination of said PRs that responds to an agonist of at least one P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of the arthropod parasite. Methods of selecting feeding inhibitors [00117] In an embodiment the present invention provides for a method of selecting antagonists of P1, P2X ₂ , P2X ₃ , P2X _2/3 PRs that inhibit feeding by an arthropod parasite through inhibition of at least one of its P1, P2X ₂ , P2X ₃ , P2X _2/3 PRs. [00118] In an embodiment the present invention provides for selecting a compound that inhibits feeding by an arthropod parasite possessing at least one P1, P2X ₂ , P2X ₃ or P2X _2/3 PR. This is achieved by measuring feeding by the arthropod parasite on a feeding medium to which a test compound plus a feeding stimulant such as adenosine or ATP is added in solution. A compound that inhibits feeding causes a reduction in feeding by the arthropod parasite compared to its feeding, for example, on the positive control where only the feeding stimulant is added to the feeding medium. In this case, modulation comprises inhibition by the antagonist of the PR that interferes with the ability of the arthropod parasite to sense the feeding stimulant in the feeding medium. [00119] In an embodiment, a compound that inhibits feeding can be selected using an arthropod parasite expressing at least one P1, P2X ₂ , P2X ₃ or P2X _2/3 PR. In a preferred embodiment the test compound is presented in solution to the arthropod parasite at increasing doses over the 10 nM to 10 µM range as shown herein on Figures 8 B of Examples for TMP and suramin when added to a solution containing a fixed dose of ATP. Alternatively, a 1 pM to 1 mM dose range of a test compound can be chosen for testing. Inhibition of feeding induced by a test compound is measured within a period of a few minutes to over an hour, depending on the arthropod parasite in question, and is expressed, for example, relative to the number of the arthropod parasites feeding on the positive control where only the feeding stimulant is added to the feeding medium. This permits calculation of a dose-response function and ID ₅₀ value for a feeding inhibitor which can be in the 200 pM to 800 nM range as shown herein on Figures 8 and 9 and on Table 7 of Examples for the P2 PR antagonists suramin and PPADS, for the P2X ₂ PR antagonist RB-2, for the P2X ₃ PR antagonists TMP and spinorphin, and for the P2X ₃ / P2X _2/3 PR antagonist A-317491. Nevertheless, the ID ₅₀ value for a compound that inhibits feeding by an arthropod parasite is not limited to a particular molar range. [00120] An embodiment of the present invention comprises selecting a compound that inhibits feeding by an arthropod parasite possessing at least one P1, P2X ₂ , P2X ₃ or P2X _2/3 PR where a test compound is presented on or anywhere around a substrate such as a membrane or surface of a reservoir held at a temperature at or near 37°C on which the arthropod parasite has the ability to feed on a feeding medium. This is achieved by measuring feeding by the arthropod parasite on the feeding medium to which a feeding stimulant such as adenosine or ATP is added in solution and where the test compound is present on the substrate over the feeding medium. Inhibition of feeding induced by a test compound is measured within a period of a few minutes to over an hour, depending on the arthropod parasite in question. A compound that inhibits feeding causes a reduction in feeding by the arthropod parasite compared to its feeding on the positive control where, for example, an untreated membrane is placed over the feeding medium incorporating the feeding stimulant. This is shown herein on Tables 10 and 11 of Examples where the P2X ₃ antagonist TMP applied onto the feeding membrane inhibits feeding by ticks and biting flies. In this case, modulation comprises inhibition by the antagonist of the PR that interferes with the arthropod parasite’s ability of the feed through the membrane. Feeding rates of the arthropod in the presence and in the absence of at least one test compound are compared. The comparison allows for selection of compounds which are suitable feeding inhibitors. [00121] An embodiment of the present invention comprises selecting a compound that inhibits feeding by an arthropod parasite possessing at least one P1, P2X ₂ , P2X ₃ or P2X _2/3 PR where a test compound is presented on or anywhere around a substrate or warm body held at a temperature at or near 37°C. Once an arthropod parasite, such as a mosquito, has contacted said substrate or warm body the arthropod parasite starts probing the surface with its mouthparts leading to biting attempts. Such probing and biting activity on said substrate or warm body is typical of mosquito behaviour once it has landed on a host animal. This biting behaviour induced on a warm body has been exploited to test repellents for arthropod parasites as described in Kröber et al., 2010, “An in vitro assay for testing mosquito repellents employing a warm body and carbon dioxide as a behavioral activator”, Journal of the American Mosquito Control Association 26, 381-386; doi.org/10.2987/10-6044.1. Selection of a compound that inhibits feeding is made by measuring the biting rate by the arthropod parasite on the warm body onto which the test compound is applied. Inhibition of the biting rate induced by the test compound is measured within a period of a few minutes to over an hour, depending on the arthropod parasite in question. A compound that inhibits feeding causes a reduction in the biting rate by the arthropod parasite compared to its biting rate on the positive control where, for example, an untreated warm body is presented. In this case, modulation comprises inhibition by the antagonist of the PR that interferes with the arthropod parasite’s ability of bite. Biting rates of the arthropod in the presence and in the absence of at least one test compound are compared. The comparison allows for selection of compounds which are suitable feeding inhibitors. [00122] The present invention comprises a method for selecting an antagonist for a P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of an arthropod parasite by recording, for example, a feeding electrogram from said parasite expressing at last one of said PRs. The feeding electrogram technique is used to measure the feeding rate by the arthropod parasite on a feeding medium to which a test compound plus a feeding stimulant such as adenosine or ATP is added in solution. The feeding electrogram permits recording of changes in potential, measured in mV, across the mouthparts and action potential frequencies generated by cells of the arthropod parasite’s mouthparts. A regular feeding rate is sustained, for example, by sucking bouts induced by activation of pharyngeal muscles, and salivation induced by closing of the pre-pharyngeal valve accompanied by lowering of the salivarium floor. Use of the feeding electrogram to measure the feeding rate of an arthropod parasite is described in Lösel et al., 1992, “Feeding electrogram studies on the African cattle brown ear tick Rhipicephalus appendiculatus: evidence for an antifeeding effect of tick resistant serum”, Physiological Entomology 17, 342-350; doi.org/10.1111/j.1365- 3032.1992.tb01032.x. The arthropod parasite can feed on said medium with the test compound plus the feeding stimulant added either across a membrane, or directly on said medium with the test compound and the feeding stimulant added that is held, for example, in a micro reservoir such as a glass capillary. Inhibition of feeding induced by the test compound is measured within a period of a few minutes to over an hour, depending on the arthropod parasite in question. Both the membrane-feeding and glass-capillary feeding methods are described herein in Materials and Methods. Feeding rates measured from the arthropod parasite in the presence and in the absence of at least one test compound are compared to detect, for example, a decrease in the frequency of sucking events caused by the test compound. The comparison allows for selection of compounds which are suitable feeding inhibitors. Sucking and salivation events that indicate the feeding rate of an arthropod parasite can also be measured by macro-video photography as described in Lösel et al., 1992, ibid. [00123] An embodiment of the present invention comprises selecting a compound that inhibits feeding by an arthropod parasite through use of a non-parasitic transgenic animal containing cells with an altered allele for a gene that naturally encode and express at least one functional P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of an arthropod parasite. This is achieved when feeding by the transgenic animal is lower on a feeding medium to which a feeding stimulant such as adenosine or ATP plus a test compound is added in solution than feeding by the same transgenic animal on the feeding medium that serves as positive control to which only the feeding stimulant is added. In this case, modulation comprises inhibition by the feeding inhibitor of the functional PR that interferes with the ability of the transgenic animal to sense the feeding stimulant in the feeding medium. [00124] In an embodiment the present invention provides for selecting a compound that inhibits feeding by an arthropod parasite using of a non-parasitic transgenic animal possessing an altered allele for a gene that naturally encodes and expresses at least one functional P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of a non-parasitic organism. It is understood that the functional P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of the non- parasitic organism responds to an agonist of at least one P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of the arthropod parasite. Selection of an antagonist is achieved when feeding by the transgenic animal is lower on a feeding medium to which a feeding stimulant such as adenosine or ATP plus a test compound is added in solution than feeding by the same transgenic animal on the feeding medium that serves as positive control to which only the feeding stimulant is added. In this case, modulation comprises inhibition by the feeding inhibitor of the functional PR that interferes with the ability of the transgenic animal to sense the feeding stimulant in the feeding medium. Embodiments for feeding inhibitors [00125] In an embodiment the present invention comprises an antagonist of at least one P1, P2X ₂ , P2X ₃ or P2X _2/3 PR of an arthropod parasite, for example a blood- feeding arthropod parasite, such as a biting insect, as a feeding inhibitor for said organism. [00126] In an embodiment the present invention comprises an antagonist of any combination of P1, P2X ₂ , P2X ₃ and P2X _2/3 PRs of an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, as a feeding inhibitor for said organism. [00127] In an embodiment the present invention comprises an antagonist of at least one P1 PR of an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, as a feeding inhibitor for said organism. [00128] In an embodiment the present invention comprises an antagonist of at least one P1 A ₁ PR of an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, as a feeding inhibitor for said organism. [00129] In an embodiment the present invention comprises an antagonist of at least one P2X ₂ PR of an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, as a feeding inhibitor for said organism. [00130] In an embodiment the present invention comprises an antagonist of at least one P2X ₃ PR of an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, as a feeding inhibitor for said organism. [00131] In an embodiment the present invention comprises an antagonist of at least one P2X _2/3 PR of an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, as a feeding inhibitor for said organism. [00132] In an embodiment the present invention comprises an antagonist of any combination of P1, P2X ₂ , P2X ₃ and P2X _2/3 PRs of an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, as a feeding inhibitor for said organism. [00133] For the purposes of this invention it is understood that inhibition of P1, P2X ₂ , P2X ₃ , P2X _2/3 PRs by an antagonist can be achieved in different ways, for example, by binding to the ion channel pore thus blocking opening of the ion channel by the agonist, by blocking the ion channel pore in the open state, by acting as a competitive agonist for binding by the agonist with the PR, or by binding to a non-agonist allosteric site on the PR to prevent its activation. [00134] In an embodiment this invention comprises suramin (antrypol, a polysulfonated naphthylamine), PPADS (pyridoxal-phosphate-6-azophenyl-2,4- disulfonate), RB-2 (an anthraquinone-sulfonic acid), spinorphin (a heptapeptide), 2,3,5,6-tetramethylpyrazine (TMP) or A-317491 (a tricarboxylate) as an antagonist for P2X ₂ , P2X ₃ , P2X _2/3 PRs of an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, that respond to ATP or AMP-CCP as an agonist, as disclosed herein on Figures 4 B and D, 5, 6 and 7 and on Table 4 of Examples. The invention also comprises any combination of said PR antagonists and pharmacologically admissible salts of said PR antagonists for an arthropod parasite. An antagonist for P2X ₂ , P2X ₃ , P2X _2/3 PRs of an arthropod parasite may also comprise compounds selected from P2X ₂ , P2X ₃ , P2X _2/3 PR antagonists listed herein. [00135] In an embodiment this invention comprises suramin, PPADS, RB-2, spinorphin, TMP or A-317491 as a feeding inhibitor for an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, where said organism responds to ATP or AMP-CPP as a feeding stimulant, as disclosed herein on Figures 8 B, 9 and 10, and on Tables 7, 8, 9, and 10 of Examples. The invention also comprises any combination of said feeding inhibitors and pharmacologically admissible salts of said feeding inhibitors for an arthropod parasite. A feeding inhibitor for an arthropod parasite may also comprise compounds selected from P2X ₂ , P2X ₃ , P2X _2/3 PR antagonists listed herein. [00136] In an embodiment this invention comprises 1,3-dimethyxanthine (theophylline) or 8-cyclopentyl-1,3-dimethylxanthine (8-cyclopentyltheophylline or CPT) as an antagonist of a P1 PR of an arthropod parasite that respond to adenosine or AMP as an agonist, as herein disclosed on Figure 4 A and C and in the corresponding text in Examples. The invention also comprises a combination of said antagonists and pharmacologically admissible salts of said antagonists. An antagonist for P1 PRs of an arthropod parasite may also comprise compounds selected from P1 PR antagonists listed herein. [00137] In an embodiment this invention comprises 8-cyclopentyl-1,3- dimethylxanthine as a feeding inhibitor for an arthropod parasite, for example a blood- feeding arthropod parasite, such as a biting insect, that responds to adenosine as a feeding stimulant, as herein disclosed on Table 6 of Examples. The invention also comprises a pharmacologically admissible salt of 8-cyclopentyl-1,3-dimethylxanthine as a feeding inhibitor for an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect. A feeding inhibitor for an arthropod parasite may also comprise compounds selected from P1 PR antagonists listed herein. [00138] In an embodiment this invention comprises the P1A ₁ PR antagonist 8- cyclopentyl-1,3-dimethylxanthine combined with the P2X ₃ PR antagonist 2,3,5,6- tetramethylpyrazine as a feeding inhibitor for an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, that responds to adenosine with ATP as a feeding stimulant, as herein disclosed on Figure 10 and Table 6 of Examples. A feeding inhibitor for an arthropod parasite may also comprise combinations of compounds selected from P1 PR antagonists listed herein. [00139] In an embodiment this invention comprises combinations of PR antagonists. A combination of said PR antagonists may comprise suramin with 8- cyclopentyl-1,3-dimethylxanthine. A combination of said PR antagonists may comprise suramin with 1,3-dimethylxanthine. A combination of said PR antagonists may comprise suramin with 2,3,5,6-tetramethylpyrazine. A combination of said PR antagonists may comprise suramin with a tricarboxylate such as A-317491. A combination of said PR antagonists may comprise suramin with spinorphin. A combination of said PR antagonists may comprise PPADS with 8-cyclopentyl-1,3- dimethylxanthine. A combination of said PR antagonists may comprise PPADS with 1,3- dimehtylxanthine. A combination of said PR antagonists may comprise RB-2 with 8- cyclopentyl-1,3-dimethylxanthine. A combination of said PR antagonists may comprise RB-2 with 1,3-dimehtylxanthine. A combination of said PR antagonists may comprise 2,3,5,6-tetramethylpyrazine with 1,3-dimehtylxanthine. A combination of said PR antagonists may comprise A317491 with 8-cyclopentyl-1,3-dimethylxanthine. A combination of said PR antagonists may comprise A317491 with 1,3-dimehtylxanthine. A combination of said PR antagonists may comprise spinorphin with 8-cyclopentyl-1,3- dimehtylxanthine. A combination of said PR antagonists may comprise spinorphin with 1,3-dimehtylxanthine. A combination of said PR antagonists may comprise RB-2 with 2,3,5,6-tetramethylpyrazine. A combination of said PR antagonists may comprise s RB- 2 with A317491. A combination of said PR antagonists may comprise RB-2 with spinorphin. A combination of said PR antagonists may comprise PPADS with 2,3,5,6- tetramethylpyrazine. A combination of said PR antagonists may comprise PPADS with A317491. A combination of said PR antagonists may comprise PPADS with spinorphin. A combination of said PR antagonists may comprise s 2,3,5,6-tetramethylpyrazine with spinorphin. A combination of said PR antagonists may comprise suramin with 2,3,5,6- tetramethylpyrazine and A-317491. A combination of said PR antagonists may comprise suramin with 8-cyclopentyl-1,3-dimethylxanthine and 2,3,5,6- tetramethylpyrazine. A combination of said PR antagonists may comprise suramin with 8-cyclopentyl-1,3-dimethylxanthine and A317491. A combination of said PR antagonists may comprise suramin with 1,3-dimethylxanthine and A317491. A combination of said PR antagonists may comprise suramin with 1,3-dimethylxanthine and 2,3,5,6-tetramethylpyrazine. A combination of said PR antagonists may comprise RB-2 with 2,3,5,6-tetramethylpyrazine and A-317491. A combination of said PR antagonists may comprise RB-2 with 8-cyclopentyl-1,3-dimethylxanthine and 2,3,5,6- tetramethylpyrazine. A combination of said PR antagonists may comprise RB-2 with 1,3-dimethylxanthine and 2,3,5,6-tetramethylpyrazine. A combination of said PR antagonists may comprise RB2 with 8-cyclopentyl-1,3-dimethylxanthine and A317491. A combination of said PR antagonists may comprise 2,3,5,6-tetramethylpyrazine with A-317491. [00140] In an embodiment of the present invention a feeding inhibitor for an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, comprises an antagonist of P1, P2X ₂ , P2X ₃ or P2X _2/3 PRs or an antagonist for any combination of said PRs belonging to any invertebrate or vertebrate organism. [00141] In an embodiment of the present invention a feeding inhibitor for an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, comprises antagonists of P1, P2X ₂ , P2X ₃ or P2X _2/3 PRs of said arthropod parasites as disclosed herein, combined with a compound or compounds selected from P1 PR antagonists listed herein, such as DPCPX, and/or combined with a compound or compounds selected from P2X ₂ , P2X ₃ or P2X _2/3 PR antagonists listed herein, such as AF- 353. [00142] In an embodiment of this invention a feeding inhibitor for an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, comprises one or more compounds selected from a group consisting of: pyridoxalphosphate-6-azophenyl-2’,5’-disulfonic acid (iso-PPADS); 2-[[2-hydroxy-5- (hydroxymethyl)-2-oxo-4H-[1,3,2]dioxaphosphinino[4,5-c]pyrid in-8- yl]diazenyl]benzene-1,4-disulfonic acid (MRS 2220); MRS 2160; [3-[[4-formyl-5- hydroxy-6-methyl-3-(phosphonooxymethyl)pyridin-2-yl]diazenyl ]-5- [[hydroxy(oxido)phosphoryl]methyl]phenyl]methyl-hydroxyphosp hinate (MRS 2257); P ¹ P ⁵ -di-(inosine-5’)pentaphosphate (Ip5I); aurintricarboxylic acid; 2’,3’-O-benzyldiene- ATP; 3ʹ-deoxy-3’-(3,5- dimethoxybenzamido)-ATP (DT-0111); 4-[(2-amino-3,5- dibromophenyl)methylamino]cyclohexan-1-ol (ambroxol); 5-O-[2- [benzyl(methyl)amino]ethyl] 3-O-methyl 2,6-dimethyl-4-(3-nitrophenyl)-1,4- dihydropyridine-3,5-dicarboxylate (nicardipine). [00143] In an embodiment of this invention a feeding inhibitor for an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, comprises one or more compounds selected from a group consisting of: polysulfonated aromatic compound; sulfonamide tethered (hetero)aryl ethylidene compound; diaminopyrimidine compound; biaryl benzamide compound; teatrahydropyrido-pymiridine compound; pyrrolopyrimidin-7-one; pyrrolinone; minodronic acid; benzamide compound containing piperazine, tetrazole, thiazole, imidazole, triazole or pyrazole. The invention also comprises a pharmacologically admissible salt of said compounds. [00144] In an embodiment of this invention a feeding inhibitor for an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, comprises one or more compounds selected from a group consisting of: imidazopyridine; piperidino-aminopyrimidyl; benzamide; pyrrolidino-aminopyrimidyl; benzoimidazole; phenoxy-diaminopyrimidine; benzyl-diaminopyrimidine; carboxamide; benzoxazole; pyrrol; oxo-triazine; pyrimidinone. The invention also comprises a pharmacologically admissible salt of said compounds. [00145] In an embodiment of this invention a feeding inhibitor for an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, comprises one or more compounds selected from a group consisting of: diaminopyrimidine; arylamide; pyrazole; oxazole; pyridine; pyrimidine; pyrazolo- pyrimidine; imidazo-pyridine. The invention also comprises a pharmacologically admissible salt of said compounds. [00146] In an embodiment of this invention a feeding inhibitor for an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, comprises one or more compounds selected from a group consisting of: 2‘(3‘)-O- (2,4,6-trinitrophenyl)-adenosine 5‘-triphosphate (TNP-ATP); 2ʹ,3ʹ-O-(2,4,6- trinitrophenyl)-ADP (TNP-ADP); 2ʹ,3ʹ-O-(2,4,6-trinitrophenyl)-AMP (TNP-AMP); 2ʹ,3ʹ-O- (2,4,6-trinitrophenyl)-GTP (TNP-GTP); 5-[[4,5-dimethoxy-2-(1- methylethyl)phenyl]methyl]-2,4-pyrimidinediamine (RO-3); 2ʹ,3ʹ-O- cyclohexylideneATP; 2ʹ,3ʹ-O-cyclohexylidene-α,β-methylene ATP; 5-(5-iodo-2- isopropyl-4-methoxy-phenoxy)-pyrimidine-2,4-diamine (AF-353 or RO-4); tetrasodium 4-amino-6-[[4-[4-[(8-amino-1-hydroxy-5,7-disulfonatonaphthal en-2-yl)diazenyl]-3- methylphenyl]-2-methylphenyl]diazenyl]-5-hydroxynaphthalene- 1,3-disulfonate (Evans Blue); sodium 1-amino-9,10-dihydro-9,10-dioxo-4-(phenylamino)-2- anthracenesulfonate (Acid Blue 25); 5-(2,4-diamminopyrimidin-5-yloxy)-4-isopropyl-2- methoxybenzenesulfonamide (AF-219, MK-7264 or gefapixant); 9-(5-iodo-2-isopropyl- 4-methoxybenzyl)-N ⁶ -methyl-9H-purine-2,6-diamine; 2-(R)-(4-fluoro-2-methyl- phenyl)-4-(5)-((8aS)-6- oxo-hexahydro-pyrrolo[1,2-a]-pyrazin-2-yl)-piperidine-1- carboxylic acid [l-(R)-(3,5-bis-trifluoromethyl-phenyl)- ethyl]-methylamide (orvepitant); 5-(4-methoxy-5-methylsulfonyl-2-propan-2-ylphenoxy)pyrimidin e-2,4-diamine (AF- 130); 5-(5-isobutyltetrazol-1-yl)- 4ʹ-methylbiphenyl-3-carboxylic acid ((S)-2-hydroxy-1- methylethyl)amide; N-[1(R)-(5-fluoropyridin2-yl)ethyl]-3-(5-methylpyridin-2-yl) -5- [5(S)-(2-pyridyl)-4,5-dihydroisoxazol-3-yl]benzamide (MK-3901); 3-(5-methylthiazol-2- yl)-5-(((R)-tetrahydrofuran-3-yl)oxy)-N-((R)- 1-(2-(trifluoromethyl)pyrimidin-5- yl)ethyl)benzamide (eliapixant or BAY-1817080); methyl (S)-3-((2-(2,6- difluoro-4- (methylcarbamoyl)phenyl)-7-methylimidazo [1,2-a] pyridin-3-yl)methyl)piperidine-1- carboxylate (BLU-5937); (2S)-3-[(4E)-3-[(4-chlorophenyl)methyl]-2,6-dioxo-4-({4- [(pyridin-2-yl)oxy]phenyl}imino)-1,3,5-triazinan-1-yl]-2-met hylpropanoic acid (sivopixant or S-600918); 5-cyclohexyl-3-hydroxy-1-(4-(isoxazol-4-yl)phenyl)- 4-(4- methoxybenzoyl)-1,5-dihydro-2H-pyrrol-2-one; biphenyl and phenyl-pyridine amides; oxazolone and pyrrolidinone-substituted arylamides; amino quinazoline; fused heterocyclics; triazole-substituted arylamide; oxalamide; indole arylamide; indazole arylamide; benzimidazole arylamide. The invention also comprises a pharmacologically admissible salt of said compounds. [00147] In an embodiment of this invention a feeding inhibitor for an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect, comprises one or more compounds selected from a group consisting of: 8-[[3-[[3- [(4,6,8-trisulfonaphthalen-1- yl)carbamoyl]phenyl]carbamoylamino]benzoyl]amino]naphthalene -1,3,5-trisulfonic acid (NF023); tetrasodium 4-[[3-[[3,5-bis[(4- sulfonatophenyl)carbamoyl]phenyl]carbamoylamino]-5-[(4- sulfonatophenyl)carbamoyl]benzoyl]amino]benzenesulfonate (NF110); hexasodium 8- [[4-[[4-[[4-[[4-[(4,6,8-trisulfonatonaphthalen-1- yl)carbamoyl]phenyl]carbamoyl]phenyl]carbamoylamino]benzoyl] amino]benzoyl]amin o]naphthalene-1,3,5-trisulfonate (NF279); 5-methoxy-3-[[3-[[3-[[3-[[5-[(8-methoxy- 3,6-disulfonaphthalen-2-yl)carbamoyl]-2- methylphenyl]carbamoyl]phenyl]carbamoylamino]benzoyl]amino]- 4- methylbenzoyl]amino]naphthalene-2,7-disulfonic acid (NF770); 2-[[4-Methyl-5-[[3-[[3- [[2-methyl-5-[(1,3,7-trisulfonaphthalen-2- yl)carbamoyl]phenyl]carbamoyl]phenyl]carbamoylamino]cyclohex a-2,5-diene-1- carbonyl]amino]cyclohexa-2,4-diene-1-carbonyl]amino]naphthal ene-1,3,7-trisulfonic acid (NF778); 1-amino-4-[3-[(4,6-dichloro-1,3,5-triazin-2-yl)amino]anilino ]-9,10- dioxoanthracene-2-sulfonic acid (PSB-10211); 1-amino-4-[3-(4,6- dichloro[1,3,5]triazine-2-ylamino)-4-sulfophenylamino]-9,10- dioxo-9,10- dihydroanthracene-2-sulfonate (PSB-1011); 2-[[4-amino-5-(5-iodo-4-methoxy-2- propan-2-ylphenoxy)pyrimidin-2-yl]amino]propane-1,3-diol (AF-906 or RO-51); N- [(2R)-1-(4-acetylpiperazin-1-yl)propan-2-yl]-1-methyl-3-phen ylthieno[2,3-c]pyrazole-5- carboxamide (RO-85); 5-[5-Methyl-2-(1-methylethyl)phenoxy]-2,4-pyrimidinediamine (TC-P 262); 8-cyclopentyl-1,3-dipropylxanthine (DPCPX): 1,3-dipropyl-8-[2-(5,6- epoxy)norbornyl] xanthine (ENX); (S-1,3-dipropyl-8[2-(5,6- epoxynorbornyl)]xanthine (CVT124); (5R)-5-[[9-methyl-8-[methyl(propan-2-yl)amino]purin-6- yl]amino]bicyclo[2.2.1]heptan-2-ol (WRC-0571); [2,8-bis(trifluoromethyl)quinolin-4- yl]-piperidin-2-ylmethanol (mefloquine); trans-4-[(2-Phenyl-7H-pyrrolo[2,3- d]pyrimidin-4-yl)amino]cyclohexanol (SLV 320); 8-(Hexahydro-2,5-methanopentalen- 3a(1H)-yl)-3,7-dihydro-1,3-dipropyl-1H-purine-2,6-dione (KW 3902); N-(2- Aminoethyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl- 1H-purin-8-yl)phenoxy]- acetamide dihydrochloride (XAC); 3-[[5-(2,3-dichlorophenyl)tetrazol-1- yl]methyl]pyridine (A-438079). The invention also comprises a pharmacologically admissible salt of said compounds. [00148] In an embodiment this invention comprises a peptide, a polypeptide or a protein, and a pharmacologically admissible salts thereof, as a feeding inhibitor for an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect. Purotoxin-1, a 35 amino acid peptide of the wolf spider, antagonizes rat and human P2X ₃ PRs at nanomolar doses. Spinorphin is an endogenous antinociceptive heptapeptide and a known antagonist of human P2X ₃ PRs. [00149] As herein disclosed in Figures 6 and 7 and in Table 4 of Examples, spinorphin serves as an antagonist of a purine-sensitive PR of the malaria mosquito at sub-nanomolar doses, and spinorphin inhibits feeding by the malaria mosquito at 100 nM as disclosed herein on Table 7 of Examples. A feeding inhibitor for an arthropod parasite may also comprise compounds structurally related to spinorphin, such as heptapeptides AVVYPWT, LAVYPWT, LVAYPWT, LVVAPWT and the cyclic peptide LWYPWT that serve as selective P2X ₃ PR antagonists. [00150] In an embodiment this invention comprises a natural product and pharmacologically admissible salts thereof as a feeding inhibitor for an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect. The isoflavone puerarin and polyphenols emodin and resveratrol are reported as P2X ₃ PR antagonists. Theophylline, a constituent of tea, coffee and chocolate, is a non-selective P1 PR antagonist and as disclosed herein in Examples, theophylline antagonizes the response of the malaria mosquito P1 PR to adenosine.2,3,5,6-Tetramethylpyrazine (TMP), synonym to ligustrazine, from Ligusticum wallichii, employed as an analgesic in Chinese medicine, is a P2X ₃ PR antagonist. As disclosed herein on Figures 5, 6 and 7 and on Table 4 of Examples, TMP antagonizes a PR of the malaria mosquito at an ID ₅₀ of 12 nM. Furthermore, and as disclosed herein on Figures 8 and 9 and on Table 7 of Examples, TMP inhibits feeding by the malaria mosquito at an ID ₅₀ of 15 nM. In addition, TMP inhibits feeding by the Asian mosquito vector of malaria and by the horn fly as disclosed herein on Tables 8 and 10 of Examples. As disclosed herein on Tables 9 and 10 of Examples, TMP inhibits feeding when applied onto a membrane through which a biting fly and tick species feeds on a feeding medium with ATP added as feeding stimulant. [00151] In an embodiment this invention comprises an antibody as a feeding inhibitor for an arthropod parasite, for example a blood-feeding arthropod parasite, such as a biting insect,. A means of using vascular endothelial growth factor (VEGF) antibody to inhibit the expression of the P2X ₂ and P2X ₃ PRs has been described, and application of monoclonal antibodies produced against human P2X ₃ PRs induce significant inhibition of human P2X ₃ and P2X _2/3 PRs. Embodiments for metal modulators of P2X ₂ , P2X ₃ , P2X _2/3 PRs [00152] In an embodiment the product used for the protection against arthropod parasites comprises metal ions as modulators of P2X ₂ , P2X ₃ , P2X _2/3 PRs of an arthropod parasite further comprising PR antagonist compounds. Metals such as copper, magnesium, iron, nickel and zinc modulate P2X ₂ and P2X ₃ PR function. As disclosed herein in Table 5 of Examples, Zn ⁺⁺ significantly increases the response to ATP by the malaria mosquito purine-sensitive cell at 100 pM but not at double that dose, and 200 pM Mg ⁺⁺ inhibits the response of the malaria mosquito purine-sensitive cell to ATP. [00153] In an embodiment the present invention comprises a product for inhibiting feeding by an arthropod parasite using at least one metal ion modulator of its P2X ₂ , P2X ₃ , P2X _2/3 PRs [00154] In an embodiment the present invention comprises a product for inhibiting feeding by an arthropod parasite using at least one metal ion modulator of its P2X ₂ , P2X ₃ , P2X _2/3 PRs further comprising PR antagonist compounds. Embodiments for inhibiting feeding by an arthropod parasite , preferably a blood- feeding arthropod parasite, using an antagonist for at least one of its PRs as a feeding inhibitor [00155] In an embodiment the present invention comprises a product for inhibiting feeding by an arthropod parasite, preferably a blood-feeding arthropod parasite, comprising an antagonist of at least one of its PRs. [00156] In an embodiment the invention comprises a product for inhibiting feeding by an arthropod parasite, preferably a blood-feeding arthropod parasite, comprising an antagonist for at least one of its P1, P2X ₂ , P2X ₃ or P2X _2/3 PRs or for any combination of said PRs where said antagonist is selected by means of assay methods described in Materials and Methods and in Examples herein, or selected using other suitable assay methods described herein. Compounds structurally related to the selected antagonist, isomers thereof, esters thereof, prodrugs thereof and pharmaceutically admissible salts thereof are herein included. [00157] In an embodiment, the invention comprises a product for inhibiting feeding by an arthropod parasite, preferably a blood-feeding arthropod parasite, comprising an antagonist for at least one of its P1 PRs. [00158] In an embodiment, the invention comprises a product for inhibiting feeding by an arthropod parasite, preferably a blood-feeding arthropod parasite, comprising an antagonist for at least one its P1A ₁ PRs. [00159] In an embodiment, the invention comprises a product for inhibiting feeding by an arthropod parasite, preferably a blood-feeding arthropod parasite, comprising an antagonist for at least one of its P2X ₂ PRs. [00160] In an embodiment, the invention comprises a product for inhibiting feeding by an arthropod parasite, preferably a blood-feeding arthropod parasite, comprising an antagonist for at least one of its P2X ₃ PRs. [00161] In an embodiment, the invention comprises a product for inhibiting feeding by an arthropod parasite, preferably a blood-feeding arthropod parasite, comprising an antagonist for at least one its P2X _2/3 PRs. [00162] In an embodiment, the invention comprises a product for inhibiting feeding by an arthropod parasite, preferably a blood-feeding arthropod parasite, comprising an antagonist for any combination of its P1, P2X ₂ , P2X ₃ and P2X _2/3 PRs. [00163] In an embodiment the invention comprises a product for inhibiting feeding by an arthropod parasite, preferably a blood-feeding arthropod parasite, through inhibition of at least one of its P1, P2X ₂ , P2X ₃ or P2X _2/3 PRs by at least one P1, P2X ₂ , P2X ₃ , P2X _2/3 PR antagonist. [00164] In an embodiment the invention comprises a product for inhibiting feeding by an arthropod parasite, preferably a blood-feeding arthropod parasite, comprising an antagonist for at least one of its P2X ₂ , P2X ₃ , P2X _2/3 PRs accompanied by an antagonist for another PR of the arthropod parasite. For example, feeding inhibition can be achieved employing an antagonist for at least one P2X ₂ , P2X ₃ , P2X _2/3 PR combined with an antagonist for at least one P1 PR of the arthropod parasite as disclosed herein on Figure 10 and Table 6 of Examples where the P1 PR antagonist 8- cyclopentyl-1,3-dimethylxanthine combined with the P2X ₃ PR antagonist 2,3,5,6- tetramethylpyrazine serves to inhibit feeding by the malaria mosquito. [00165] In an embodiment the invention comprises a product for inhibiting feeding by an arthropod parasite, preferably a blood-feeding arthropod parasite, using at least one metal modulator of P2X ₂ , P2X ₃ or P2X _2/3 PRs further comprising at least one P1, P2X ₂ , P2X ₃ , P2X _2/3 PR antagonist. [00166] In an embodiment the invention comprises a product for inhibiting feeding by an arthropod parasite, preferably a blood-feeding arthropod parasite, comprising an antagonist for at least one of its P1, P2X ₂ , P2X ₃ , P2X _2/3 PRs and further comprising at least one metal modulator of P2X ₂ , P2X ₃ or PX _2/3 PRs. [00167] In an embodiment the invention comprises a product for inhibiting feeding by an arthropod parasite, preferably an arthropod parasite, comprising an antagonist for at least one of its P1, P2X ₂ , P2X ₃ or P2X _2/3 PRs where said antagonist can be modified through derivatization, conjugation, or esterification to facilitate, among others, stability, miscibility, resistance to light exposure and for target delivery to the arthropod parasite. [00168] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite, preferably a blood-feeding arthropod parasite, using a feeding inhibitor comprising an antagonist for at least one of its P1, P2X ₂ , P2X ₃ or P2X _2/3 PRs where said inhibitor is applied onto a host animal for the prevention and/or treatment of an infestation by the arthropod parasite, preferably a blood-feeding arthropod parasite. This control method aims to inhibit the arthropod parasite, preferably a blood-feeding arthropod parasite, from feeding on the host animal. For example, in the case of a flying biting insect such as a mosquito or a biting fly, this can lead to the organism flying away in a starved state from a host animal. [00169] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite using a product comprising a PR antagonist where said product affects an endosymbiont of the arthropod parasite, thereby adversely affecting development, reproduction and survival of the arthropod parasite. [00170] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite using a PR antagonist where said antagonist is applied onto a host animal for the prevention and/or treatment of an infestation by the arthropod parasite. Said control method aims to inhibit host recognition, biting, entering the host animal’s body, establishing a feeding site, feeding on host animal blood or on body fluids or on body tissues by the arthropod parasite, and/or inhibit the arthropod parasite from absorbing nutrients from the host animal through its integument or plasma membrane so as to prevent the arthropod parasite’s survival, growth and reproduction. Examination of an adequate number of host animals for presence of the arthropod parasite made at regular intervals following application of the antagonist, such as is made by following a test protocol, allows assessment of the PR antagonist as a feeding inhibitor to control the arthropod parasite infestation, and permits establishment of an effective dose of the feeding inhibitor by a person skilled in the art. [00171] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite, preferably a blood-feeding arthropod parasite, using a PR antagonist where said antagonist is applied at an effective dose for the prevention and/or treatment of an infestation by the arthropod parasite. An effective dose is the amount of a feeding inhibitor that achieves the desired effect of inhibiting feeding by the arthropod parasite, preferably a blood-feeding arthropod parasite. [00172] Establishing an effective dose of a PR antagonist that achieves the desired effect of inhibiting feeding by an arthropod parasite on a host animal considers several variables comprising, in case of arthropod parasites for example, the life stage and species of the arthropod parasite in question, the population density of the arthropod parasite, the type of compound, formulation of the compound, the method of application of said formulation and the required duration of the effect. Accordingly, the effective dose of a feeding inhibitor in a formulation or in any use form of the feeding inhibitor may vary over a wide range from 10 ^-9 wt% to 50 wt%, preferably from 10 ^-7 wt% to 5 wt% of the feeding inhibitor as a proportion of the formulation or of any use form of the feeding inhibitor. Details of suitable formulation methods for active ingredients used in the control of parasites are known in the art. A person skilled in the art of controlling an arthropod parasite infestation can therefore determine the amount of a compound of this invention that is required as a feeding inhibitor for the prevention and/or treatment of the infestation. [00173] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite for the purpose of preventing and/or treating an infestation by said parasite on a host animal using a product comprising a PR antagonist where said product is applied topically onto a host animal’s pelage or skin, and/or onto a neck collar, a saddle, a cover for a host animal such as a horse cover or fly rug, and/or onto a fly screen or any other protective screen or netting for a host animal, and/or onto a fly mask, cap or blinkers for a host animal, and/or onto a harness, bridle, or any means of restraining a host animal. [00174] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite for the purpose of preventing and/or treating an infestation by said parasite on a host animal using a product comprising a PR antagonist where said product is applied in the form of a spray onto a host animal’s pelage or skin, and/or onto a neck collar, a saddle, a cover for a host animal, such as a horse cover or fly rug, and/or onto a fly screen or any other protective screen or netting for a host animal, and/or onto a fly mask, cap or blinkers for a host animal, and/or onto a harness, bridle, or any means of restraining a host animal. [00175] In an embodiment the invention comprises inhibiting feeding by an arthropod ectoparasite for the purpose of preventing and/or treating an infestation by said parasite on a host animal using a product comprising a PR antagonist where said product is applied topically in the form of a solution or in the form of a spray onto a predilected biting or feeding site or onto a preferred egg deposition site of the arthropod parasite on a host animal. [00176] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite for the purpose of preventing and/or treating an infestation by said parasite on a host animal using a product containing PR antagonist where said product that may optionally contain a surfactant is applied, for example, in the form of a solution, emulsion, shampoo, cream, gel, lotion, ointment, by microencapsulation, as a nanoparticle formulation such as a nanoemulsion, as a sustained-release formulation, or as an ear tag preparation, or by applying any other suitable pour-on or pharmaceutical preparation, or by applying a solution or by applying a device onto the host animal’s pelage or skin, and/or onto a neck collar, a saddle, a cover for a host animal, such as a horse cover or fly rug, and/or onto a fly screen or any other protective screen or netting for a host animal, and/or onto a fly mask, cap or blinkers for a host animal, and/or onto a harness, bridle, or any means of restraining a host animal. [00177] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite for the purpose of preventing and/or treating an infestation by said parasite using a product comprising a PR antagonist where said product is applied at an effective dose onto clothing of a person by use of a spray or by immersing a person’s clothing in an appropriate solution containing said feeding inhibitor. [00178] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite for the purpose of preventing and/or treating an infestation by said parasite using a product comprising a PR antagonist where said product is applied at an effective dose onto a bed net by use of a spray or by immersing the bed net in an appropriate solution containing said PR antagonist. [00179] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite for the purpose of preventing and/or treating an infestation by said parasite using a product comprising a PR antagonist where said product is applied topically in the form of a solution onto the arthropod parasite. [00180] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite for the purpose of preventing and/or treating an infestation by said parasite using a product comprising a PR antagonist where said product is applied in the form of the spray onto the arthropod parasite. [00181] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite for the purpose of preventing and/or treating an infestation by said parasite using a product comprising a PR antagonist where said product is applied topically in the form of a solution or in the form of a spray onto any life stage of the arthropod parasite. [00182] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite for the purpose of preventing and/or treating an infestation by said parasite using a product comprising a PR antagonist where said product is incorporated at an appropriate dose into an attractive bait, for example a sugar bait, fed on by the arthropod ectoparasite. [00183] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite for the purpose of preventing and/or treating an infestation by said parasite using a product comprising a PR antagonist where said product that may optionally contain a surfactant is applied in the form of a spray or solution, or applied using a slow-release device, or applied by microencapsulation, or applied in a sustained-release long-lasting formulation onto and/or into, for example, any facility for a host animal comprising a refuge, and/or a tent, and/or a dwelling, and /or a barn, and /or a stable, and /or a perch, and/or a cage, and/or an animal rearing or housing facility or any part thereof, and/or a breeding site of an arthropod parasite, and/or a swarm occupied by an arthropod parasite, and/or a terrestrial zone, and/or an aquatic zone, and/or a biotope, and/or a natural or man-made habitat occupied by the arthropod parasite or by any life stage of said parasite. Details of suitable preparation methods for P1, P2X ₂ , P2X ₃ or P2X _2/3 antagonist application are found in Knowles D.A. 1998, “Chemistry and Technology of Agrochemical Formulations”, Springer Dordrecht. [00184] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite for the purpose of preventing and/or treating an infestation by said parasite using a product comprising a PR antagonist where said product optionally comprises an insecticide, insecticide synergist, acaricide, parasiticide, repellent or a chemosterilant, each at its effective dose, in the form of a solution or a spray, in the form of a shampoo, or from a slow-release device, or by microencapsulation, or in a sustained-release long-lasting formulation for topical application onto a host animal’s pelage or skin, onto any life stage of the arthropod parasite, and/or for application onto a neck collar, a saddle, a cover for a host animal such as a horse cover or fly rug, and/or onto a fly screen or any other protective screen or netting for a host animal, and/or onto a fly mask, cap or blinkers for a host animal, and/or onto a harness or bridle, or any means of restraining a host animal. [00185] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite for the purpose of preventing and/or treating an infestation by said parasite using a product comprising a PR antagonist where said product optionally comprises an insecticide, insecticide synergist, acaricide, parasiticide, repellent or a chemosterilant, each at its effective dose, in the form of a solution or a spray, or from a slow-release device, or by microencapsulation, or in a sustained-release long-lasting formulation for application onto or into any facility for a host animal comprising a refuge, a tent, a dwelling, a barn, a stable, a perch, a cage, an animal rearing or housing facility or any part thereof, a breeding site of the arthropod parasite, a swarm occupied by the arthropod parasite, a terrestrial zone, an aquatic zone, a biotope, or a natural or man-made habitat occupied by the arthropod parasite or occupied by any life stage of the arthropod parasite. An effective dose of an insecticide, insecticide synergist, acaricide, parasiticide or a chemosterilant is one which permits controlling the arthropod parasite infestation. [00186] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite for the purpose of preventing and/or treating an infestation by said parasite using a product comprising a PR antagonist where said product optionally comprises an insecticide, insecticide synergist, acaricide, repellent or parasiticide or a chemosterilant, each at its effective dose, for application onto a person’s clothing and/or onto a bed net in the form of a spray or by immersion of the clothing or bed net in an appropriate solution containing the combination of said product and the insecticide, insecticide synergist, acaricide, parasiticide or chemosterilant. Embodiments for inhibiting feeding by an arthropod parasite with a therapeutically effective does of a PR antagonist [00187] In an embodiment the invention comprises inhibiting feeding by an arthropod endoparasite using a product comprising a PR antagonist where said product is administered as a pharmaceutical formulation onto or internally to a host animal for the prevention and/or treatment of an arthropod endoparasite infestation . This control method aims to inhibit the arthropod endoparasite from recognizing the host, entering the host animal’s body, establishing a feeding site, inhibit the arthropod endoparasite from feeding on host animal blood or on body fluids or on body tissues, and/or inhibit the arthropod endoparasite from absorbing or adsorbing nutrients from the host animal through its integument or plasma membrane so as to prevent the arthropod endoparasite’s survival, growth and reproduction. Examination of an adequate number of host animals for presence of the arthropod endoparasite made at regular intervals following administration of the PR antagonist, such as is made by following a test protocol, allows assessment of a compound of this invention as a feeding inhibitor and permits a person skilled in the art to determine a therapeutically effective dose of the PR antagonist comprised in said product for the prevention and/or treatment of the arthropod endoparasite infestation. [00188] In an embodiment the invention comprises inhibiting feeding by an arthropod endoparasite or an arthropod ectoparasite for the prevention and/or treatment of an infestation by said parasite using a product comprising a PR antagonist where said product is administered at a therapeutically effective dose onto or internally to a host animal. The product, that may optionally contain a surfactant, can be administered as a pharmaceutically acceptable formulation suitable for parenteral, oral, buccal, enteral, rectal, vaginal, nasal, intramuscular, intravenous, intraarterial, intrathecal, subcutaneous, and transdermal delivery, or by any suitable means for systemic delivery of said pharmaceutical formulation to the host animal. Establishing a therapeutically effective dose for the PR antagonist that achieves the desired effect of inhibiting feeding by the arthropod endoparasite or an arthropod ectoparasite considers several variables, comprising the type of compound, the method of administration of the formulated compound onto and/or internally to the host animal, the pharmacokinetics of the compound, the pharmacodynamics of the compound, the species of endoparasite being targeted, the severity of a endoparasite’s burthen on the host animal, the age and health of the host animal and the duration of protection against the endoparasite that is desired. [00189] The therapeutically effective dose of a PR antagonist that achieves the desired effect of inhibiting feeding by an arthropod endoparasite or an arthropod ectoparasite to be used in a pharmaceutically acceptable formulation, or in a sustained-release formulation or in any use form of the feeding inhibitor may vary over a wide range from 0.000000001 wt% to 50 wt%, preferably between 0.0000001 wt% to 5 wt% by weight of the PR antagonist as a proportion of the pharmaceutical formulation or of any use form of the feeding inhibitor. A person skilled in the art of controlling endoparasitic and ectoparasitic arthropods can determine the amount of a compound or compounds of this invention that is required to inhibit feeding by the endoparasite for the prevention and/or treatment of the infestation by said parasite. [00190] In an embodiment the invention comprises inhibiting feeding by an arthropod parasite using a product comprising a PR antagonist where said product is purposely synthesized to inhibit P1, P2X ₂ , P2X ₃ or P2X _2/3 PRs of both humans and animals, formulation of said compound as a pharmaceutical preparation and administration of said compound at a therapeutically effective dose for the prevention and/or treatment of an infestation by an arthropod parasite. Details of pharmaceutically suitable preparation methods for P1, P2X ₂ , P2X ₃ or P2X _2/3 PR antagonist administration onto or internally to a host animal are provided in Felton L. 2013, in “Remington: Essentials of Pharmaceutics”, Pharmaceutical Press London. [00191] In an embodiment the invention comprises inhibiting feeding by an arthropod endoparasite or an arthropod ectoparasite for the prevention and/or treatment of an infestation by said parasite using a product comprising a PR antagonist where said product optionally comprises an insecticide, insecticide synergist, acaricide, parasiticide, endectocide or a chemosterilant, each at its therapeutically effective dose, for administration onto or internally to a host animal as a pharmaceutical formulation, or as a sustained-release formulation or in any use form suitable for parenteral, oral, buccal, enteral, rectal, vaginal, nasal, intramuscular, intravenous, intraarterial, intrathecal, subcutaneous, and transdermal delivery, or by any suitable means for systemic delivery said pharmaceutical formulation to the host animal. A therapeutically effective dose of an insecticide, insecticide synergist, acaricide, parasiticide or a chemosterilant is one which permits controlling the arthropod endoparasite. Materials and Methods Mosquitoes, ticks and biting flies: [00192] Anopheles gambiae Giles (Diptera, Culicidae) 16cSS-strain colony derived from wild-caught adults in Lagos, Nigeria in 1974. The Anopheles stephensi mysorensis Liston (Diptera, Culicidae) colony originated from the Swiss Tropical Institute, Basel. Colonies were maintained under standardized conditions. A 10% sucrose solution and water were supplied ad libitum on cotton for adults. Females were deprived of sucrose the night before electrophysiological recordings. [00193] N3 or N4 nymphs of the eyeless tampan, Ornithodorus moubata Murray (Ixodida, Argasidae) , were from an in-house culture of individuals collected in Ulanga District, Tanzania and originally maintained at the Swiss Tropical Institute, Basel. Ixodes ricinus Linnaeus (Ixodida, Ixodidae) and Amblyomma hebraeum Koch (Ixodida, Ixodidae) were from colonies at Novartis, Centre de Recherche Santé Animale SA, St Aubin, Switzerland. Glossina pallidipes Austen (Diptera, Glossinidae) pupae were obtained from the International Atomic Energy Agency (IAEA) Siebersdorf Laboratories, Austria. Adult tsetse were sexed at emergence and maintained separately in cotton-netting cages (1-mm mesh; 25 x 15 x 15 cm, tg® tubular bandage, Lohmann & Rauscher, St Gallen, Switzerland) in an environmental cabinet at 24°C, 80% relative humidity (RH) under 10 h photophase:14 h scotophase with 1 hour light ramps in the first and last hour of the photophase. Pupae of the horn fly, Haematobia irritans Linnaeus (Diptera, Muscidae), obtained from a rearing at BAYER Animal Health GmbH, Monheim, Germany were maintained in cages as tsetse using 0.5-mm mesh netting and provided with water-soaked cotton on the cage roof from emergence. Chemicals: [00194] Compounds used for the characterisation of purine-sensitive sensory cells in A. gambiae are listed in Tables 1 and 2. Compounds were dissolved in nanopure water and serially diluted. Solutions were kept at -20°C for less than a week and defrosted before experiments. D-(+)-sucrose (purity >99.5%) was from Sigma-Aldrich, Switzerland. Electrophysiology: [00195] The tip recording method was used to record action potentials (spikes) from sensory cells in sensilla on A. gambiae labellum lobes. Contrary to other labellum sensilla, the six most distally placed trichoid type 1 (T1) 8v sensilla do not house water- sensitive neurones in A. gambiae, allowing water to be used as a negative control in recordings. [00196] Sensilla were exposed to test compounds and PR antagonists for ca.2 - 4 s and spikes were counted in the first 2 s of recordings. Up to 4 test compounds were tested on a sensillum to evaluate potential agonists, each presented at 200pM. To study responses as a function of dose for agonists ATP, adenosine, AMP, AMP-CPP and ATP + adenosine, up to 7 recordings were made at increasing doses of agonist from the same sensillum. Unless otherwise stated, recordings were made throughout from 4 to 16 sensilla from individual 4 - 6-day-old females per dose of agonist, antagonist and metal ion modulator. For example, a response curve established over 6 doses of an antagonist using 8 sensilla per dose involved recording from a sensillum of 48 separate mosquitoes. A given dose of a PR antagonist or metal ion modulator was only presented once to a sensillum. Two-second recordings were imported as ASCII files into DataView software for analysis of spike amplitude. Differences in spike amplitude generated by stimulation of one 8v sensillum/female with 1 mM sucrose (n =294 spikes from 6 females) 200 pM adenosine (n =207 spikes from 7 females) and 200pM ATP (n =192 spikes from 7 females) were compared by measuring the spike amplitude in mV between the highest point of depolarisation and the lowest point of hyperpolarisation. Feeding assays: [00197] Experiments with A. gambiae and A. stephensi were made in a climate chamber at 28°C and 80% RH during the last 6 hours of scotophase. As a sugar meal can interfere with the blood meal by reducing the number of females that feed, the sucrose solution supplied in rearing cages to mosquitoes employed for feeding experiments was reduced to 5%. This did not affect mosquito survival but induced higher appetence. The 5% sucrose solution was presented to mosquitoes from emergence on four 1 cm diam. cotton dental rolls (Hartmann AG, Neuhausen, Switzerland) inserted in 10 ml upturned glass vials placed in holes at each corner of the rearing cage ceiling. Cotton rolls were turned daily in vials and renewed every two days. Mosquitoes were not fed blood and were deprived of sucrose 17 to 20 hours before experiments. Thirty min before a feeding experiment, between 10 – 25, 6-day- old females that responded to activation by human breath were removed with a mouth aspirator from the rearing cage wall and placed in polystyrol cylinders with the floor cut out (10 cm dm x 15 cm high; Semadeni, Ostermundigen, Switzerland) that served as feeding cages. Each cage was covered with mosquito netting at one end and a soft polyurethane lid at the other. [00198] Mosquitoes were fed for 30 min using a feeding system (Hemotek ^® , Hemotek Ltd, Blackburn, UK) consisting of 3.5 ml stainless steel reservoirs 37 mm in diameter covered with a 60 – 90 µm thick silicone-impregnated membrane (Kröber and Guerin, 2007 in "Feeding assays for hard ticks", Trends in parasitology 23, 445-449; doi 10.1016/j.pt.2007.07.010). Feeding reservoirs reached a temperature of 37°C within 10 min and were placed on the netting of feeding cages. Mosquitoes were activated using a 200 ms pulse of pure CO2 (volume 6.25 ml) from a pressurised cylinder (Carbagas, Switzerland) using a solenoid valve that caused the CO2 level in the feeding cage to rise to at least 1,500 ppm. Between 3 and 4 sets of 10 feeding cages held in a custom-built Ertacetal® POM frame were tested per day, depending of the number of female mosquitoes available from the colony. [00199] All compounds were tested in a mosquito feeding solution (FS) consisting of 25 mM dextrose, 25 mM NaCl buffered with 8.93 mM NaHCO ₃ adjusted to pH 7.4. Feeding responses of A. gambiae and A. stephensi to ATP as a function of dose were tested with between 7 to 10 feeding cages per dose. Each feeding experiment with an antagonist systematically included a treatment containing the FS as negative control along with a treatment containing the FS with 20 µM ATP added as positive control for A. gambiae and with 160 µM ATP added as positive control for A. stephensi. These positive and negative controls permitted to monitor any effect of experimental day on feeding responses as a function of dose for antagonists that were recorded over several days. Each dose of PR antagonist was tested in the FS with 20 µM ATP for A. gambiae and with 160 µM ATP for A. stephensi. Four to 10 cages each with 10 to 29 mosquitoes were tested per antagonist dose. The experimental procedure for testing antagonists as feeding inhibitors was set up using suramin as test compound for A. gambiae, hence 43 positive controls for this compound. The number of feeding cages used to test other compounds and their mixtures as feeding stimulants or as feeding inhibitors is listed on Table 6. The percentage of fed mosquitoes was estimated by visual inspection of abdominal distention that is indicative of engorgement. [00200] Table 1. Substances tested for characterization of A. gambiae PRs

Substances were purchased from Sigma-Aldrich, Schnelldorf, Germany and *Tocris Bioscience, Bristol, UK. [00201] I. ricinus adults were prefed for 3-4 days and A. hebraeum adults for 10 days on a membrane over bovine blood (Kröber T. and Guerin P.M., 2007, supra). Partly fed I. ricinus and A. hebraeum, freshly detached from the membrane, were tightly fixed by the scutum and alloscutum to double-sided adhesive tape (Tesafix® 4964, Beiersdorf, Hamburg, Germany) on a glass slide and secured with plasticine. Test compounds were fed to these partly-fed female ticks using borosilicate capillary tubes (Clark GC120-10, 0.69 mm i.d., 1.2 mm o.d., 50 mm long; Clark Electromedical Instruments, Pangbourne, Reading, UK) with drawn out tips adapted to fit around the mouthparts of individual ticks. Feeding capillaries were filled with a solution of 100 mM NaCl, 8.9 mM NaHCO ₃ with 1000 µM ATP added, hereafter termed tick feeding solution (TFS) that served as positive control in each feeding experiment. PR antagonists were added to the TFS and pH of the solution was adjusted to 7.2 -7.8. Capillary-fed ticks were kept in a plastic box on a warm plate at 34°C and 80 % RH using wet paper. After 1 h, the amount of solution imbibed by a tick was indicated by the drop of the meniscus in the capillary and measured using Vernier callipers and converted to mg. Numbers of ticks (n) tested per treatment are provided in Table 9. [00202] O. moubata nymphs were fed for 25 min through a membrane (50-75 µm thick) as for hard ticks in glass feeding unit on TFS containing 1000 µM ATP as positive control in each feeding experiment. PR antagonists were added to the TFS as for hard ticks. One compound, TMP was applied onto the membrane in aqueous solution (8 µg TMP in 70 µl) in small drops using a micropipette. Individual nymphs were marked on the dorsum with nail varnish in distinguishable colours and weighed before and after the experiment. Numbers of ticks (n) tested per treatment are provided in Table 9. [00203] Flies of both species were transferred 30 min before feeding experiments to feeding cages as used for mosquitoes at 24°C, 80% RH in an environmental cabinet. Teneral tsetse were fed at 2 to 4 days old during the photophase with between 7 and 13 flies per feeding cage, covered with a black cloth. Horn flies were tested at 1 and 2 days old in the photophase with between 6 and 21 flies per cage. Both fly species were fed using small Hemotek feeding reservoirs (29 mm diameter, 1 ml) equipped with a membrane as for mosquitoes. Horn flies were fed for 15 min with feeding reservoirs placed on the cage. For tsetse, cages were closed with black mosquito netting at the bottom to allow flies to feed through the membrane placed under the cage. Feeding time (1.5 to 6 min) for tsetse was adjusted as a function of age to achieve at least 30 % feeding on the positive control included in each feeding experiment. The positive control for both fly species contained 150 mM NaCl, 8.9 mM NaHCO ₃ with 100 µM ATP added, hereafter termed fly feeding solution (FFS). The pH of the FFS was adjusted as for ticks. The effect of TMP on feeding by G. pallidipes was also tested at 16 µg TMP/cm² dissolved in 140 µl hexane and applied homogeneously to the membrane surface using a micropipette. Hexane was used to treat the membrane with TMP to avoid the test compound being dispersed by fly wing-fanning. Flies of both species were activated just prior to feeding on a treatment by exhalation into the cage. Tsetse that had fed were identified with swollen abdomens and counted at the end of a feeding experiment. Since it was difficult to ascertain by eye whether a horn fly had fed on a treatment, 10 µl/ml of blue food colorant (No.4866908, Trawosa AG, St.Gallen, Switzerland) was added. This facilitated the detection of ingested liquid when the abdomen of flies, killed by freezing, was squashed onto filter paper. Only experiments in which the feeding rate by flies was >30% on the positive control were included in analyses. Numbers of flies tested per treatment (n) are provided in Table 10. Data analysis [00204] Significance level for all tests was set at P =0.05. Asterisks on bar graphs represent significance levels at * P <0.05, ** P <0.01, and *** P <0.001; NS not significant. [00205] Table 2. P1 PR and P2 PR antagonists tested for characterization of A. gambiae PRs

Substances were purchased from Sigma-Aldrich Schnelldorf, Germany; * Tocris Bioscience, Bristol, UK; and **Chemical Point (Deisenhofen, Germany) [00206] Spike counts elicited by 200pM doses of nucleotides, nucleosides, purines, pyrimidines and water were log(x + 1.25)-transformed (optimal constant numerically established using a maximum likelihood procedure) and analysed using a linear mixed effects model (function 'lme' in “nlme” R package) with mosquito number as random effect. To model inhomogeneous variance in residuals (near normal distribution, P = 0.045, Shapiro-Wilk test, W=0.98604), the variance function 'varIdent' in the "nlme" package that allows for different variance per treatment was used. Statistical differences were identified between treatments using post-hoc-Tukey test with contrasts adjusted by Holm’s family wise error compensation (FWEC). Spike counts elicited by ATP, AMP, adenosine and their binary mixtures were analysed using a generalised linear model (LM) of log (x + 10) transformed data and differences were identified using post-hoc-Tukey test with Holm’s FWEC. [00207] Log transformed spike amplitudes generated in response to ATP, adenosine and sucrose were compared using linear mixed effects models using the 'lme' function of the “nlme” R package, with spike amplitude as the dependent variable, treatment as the fixed factor and mosquito number within treatment as random effect. Residuals were normally distributed (Shapiro-Wilk test P > 0.0563) and their variances homogeneous (Bartlett test P >0.9799); P – values were corrected using Holm’s FWEC. [00208] Pairwise comparisons of A. gambiae electrophysiological (EP) responses to ATP and adenosine alone and combined with a given dose of PR antagonist, EP responses to ATP alone and combined with a given dose of a metal modulator, and EP responses to ATP prior to and after exposure of a sensillum to a given dose of a PR antagonist were made with a one-tailed Wilcoxon signed-rank test for data showing a non-normal distribution, with a paired Student’s t-test for normally distributed data showing homogeneous variance and with a Welch’s t-test for normally distributed data showing non-homogeneous variance. Effects on feeding of adding ATP, AMP-CPP, adenosine, ATP + adenosine, AMP and ADP-βS, CPT with ATP or adenosine, TMP with ATP + adenosine, TMP + CPT with ATP + adenosine to the FS for A. gambiae, and of adding PR antagonists with ATP to the FS for A. stephensi were compared using one- tailed Mann-Whitney U-test or Welch’s t-test, depending on data distribution; Holm’s FWEC was applied where treatment comparisons involved the same positive control. Percent feeding inhibition induced by PR antagonists in A. stephensi was calculated using Abbott’s formula. [00209] Dose-response models fitted to mosquito data were generated using the ‘drm’ command in the “drc” R package (version 2.5-12). Best fitting models (goodness of fit) and effects of dose of a compound were assessed using, respectively, ‘modelFit’ and ‘noEffect’ functions in the same R package. A 3-parameter logistic model (L3) was fitted to spike-counts recorded to increasing doses of adenosine, AMP, ATP and AMP- CPP. Spike numbers recorded in response to 200 pM ATP with increasing doses of PR antagonists added were analysed using a 4-parameter log-logistic model (LL.4) for A- 317491 and spinorphin, using a 4-parameter Weibull model (W1.4) for PPADS, RB-2 and TMP, and using a 3-parameter log-logistic model (LL.3) for suramin. An L3 model was fitted to proportions of females feeding on increasing doses of ATP. Feeding rates on 20 µM ATP with increasing doses of a PR antagonist added were analysed using a 4- parameter log-logistic model (LL.4) with the median of the positive control (20 µM ATP) fixed as the upper asymptote. Response dynamics in 3-parameter models are related to the slope b, the upper asymptote, and inflexion point of the model e (the ED ₅₀ value), whereas 4 parameter models also include the lower asymptote parameter. The upper asymptote d in EP responses represents the maximum response, which is insurmountable, i.e., the response in spike count does not rise regardless of how much the dose of agonist increases. [00210] Effective ED ₅₀ and ED ₉₅ doses of agonists and inhibitory ID ₅₀ and ID ₉₅ doses of antagonists were estimated using the ‘ED function’ of the “drc” package. An EDx value is the effective dose of a compound at x percent of the total response encompassed between the upper and lower asymptotes estimated by the “drc” model. Statistical differences between ED ₅₀ values of compounds were compared using the ‘EDcomp’ and ‘comParm’ functions in the “drc” package. In EP experiments an IDx value is the inhibitory dose of an antagonist inducing x percent of its maximum effect on the response to 200 pM ATP. Reduction in spike count in the response to 200 pM ATP induced by a PR antagonist at a dose approximating its ID ₉₅ value was compared with the response to the corresponding positive control (200 pM ATP) presented before and after the antagonist treatment using one-tailed Wilcoxon rank-sum test, Students t-test or Welch’s t-test. In feeding experiments an ID _x value is the dose of an antagonist reducing the feeding rate by x percent on a solution of a phagostimulant relative to the upper and lower limits of the drc estimated for the phagostimulant presented alone. The effect of a PR antagonist at a dose approximating its ID ₅₀ value on feeding by A. gambiae on 20 µM ATP was compared to the positive control using the Mann Witney U-test or Student t-test, depending on the data distribution; P – values were corrected using Holm’s FWEC for paroxetine and A-438079 that shared the same positive control. [00211] Uptake of TFS containing PR antagonists, converted to mg/h for hard ticks and weight gains in mg/25 min for soft ticks, was compared to TFS uptake in the corresponding positive controls. For soft ticks, 3.5 mg was added to all measurements to compensate for negative values when ticks did not feed and lost weight through evaporative water loss. For tsetse and horn flies the percentage flies that fed per antagonist treatment served as the criterion for inhibition of feeding. As horn fly sexes emerged within 24 h, both sexes were assumed to show the same degree of appetence. In tsetse, males emerged first so a gender-related appetence could not be excluded. No sex-related feeding differences in feeding were found for testse (GLM, P >0.05) so data for the sexes were pooled. Comparisons of percentage fed flies were made using a Bayesian GLM — the most appropriate model considering the zero- inflated distributions shown by the data sets. As measurements of reduced feeding on TFS containing PR antagonists by hard ticks and weight gain by soft ticks did not fit to a particular distribution type, values were compared with corresponding positive controls using the Kruskal-Wallis test for soft tick data, the one-tailed Wilcoxon test for A. hebraeum and the one-tailed Mann-Whitney U tests for I. ricinus. P – values were corrected using Holm’s FWEC in multiple comparisons of data for each tick and fly species. Examples Evidence for P1 and P2X PRs on malaria mosquito mouthparts [00212] The labellum, used for probing substrates on which the malaria mosquito feeds, is located at the tip of the mouthparts, and bears long (30 – 35 µm) terminal- pore trichoid sensilla housing four chemosensory cells. One of these cells responds to sugars for nectar feeding. The inventors set out to establish whether other cells respond to stimuli and identified a cell which is particularly sensitive to ATP (detection threshold <100 pM). ATP is present in blood. The inventors therefore investigated if the capacity to sense the presence of ATP is associated with blood-feeding behaviour in the mosquito using a series of electrophysiological (EP) assays. [00213] EP responses to a set of pertinent nucleotides, nucleosides, purines and pyrimidines (1 – 18, Table 1) each at 200 pM indicate how labellum sensory cells show strongest responses to adenine-containing ATP, AMP-CPP, adenosine and AMP, all at 23 to 26 spikes/2s (Figure 1A and B). ADP, a P2Y PR agonist, showed no activity, suggesting that biological activity of nucleotides is limited to one or three phosphates. Loss of OH groups on the ribose moiety, as in dATP and ddATP, strongly reduced activity. This led to prioritising P2X ₂ , P2X ₃ and P2X _2/3 PR agonists ATP and AMP-CPP, and P1A ₁ PR agonists adenosine and AMP as best stimulants. [00214] Action potential frequencies generated as a function of dose for adenosine, AMP and ATP indicate an increase in spike frequency from 100pM reaching an unsurmountable asymptote, typical of PRs, at 250 pM where approximately 30 spikes/2 s were recorded for all three compounds (Figure 2A and B). No difference was found between dose-response models (drc) generated for adenosine, AMP and ATP (all P >0.2) with similar ED ₅₀ (~120 pM) and ED ₉₅ values (~200 pM, P >0.05 in all cases). AMP-CPP (α,β-methylene ATP), a stronger P2X PR agonist than ATP (Coddou et al., 2011 in "Activation and Regulation of Purinergic P2X Receptor Channels", Pharmacological Reviews 63, 641-683; doi 10.1124/pr.110.003129), proved the most potent agonist. This nonhydrolysable ATP phosphonic analogue, with a methylene group substituting for oxygen between the α- and β-phosphates, shifted the EP response curve to lower doses with a steep increase in spike frequency from 10 pM to an asymptote at 80 pM (ED ₉₅ of 50 pM) and an ED ₅₀ at 20 pM, significantly lower than for adenosine, AMP and ATP (all P < 0.001; Figure 2A and B). The drc – slope computed for AMP-CPP was steeper compared to AMP and adenosine (P < 2E-16), but no different from that estimated for ATP (P = 0.1563), suggesting different kinetics in the binding of AMP and adenosine to a P1 PR than by AMP-CPP and ATP to a P2 PR. Superior activity of AMP-CPP and failure by the P2Y PR agonists ADP, ADP-βS and uracil nucleotides to elicit responses (Figure 1A) suggested presence of a P2X PR activated by ATP and AMP-CPP accompanied by a P1A ₁ PR responding to adenosine and AMP on A. gambiae mouthparts. [00215] Whereas ATP and adenosine alone at 50 pM hardly elicited a response, respectively 2.5 ± 0.7/2s and 1.25 ± 0.62/2s, an equimolar mixture containing 50 pM of each agonist elicited a spike frequency of 16.28 ± 4.68/2s, higher than to 100 pM of either compound presented alone (Figure 2C and D). Equimolar amounts of the two agonists were necessary since adding either 20 pM adenosine to 200pM ATP or 20 pM ATP to 200 pM adenosine did not significantly enhance the response. Presenting adenosine admixed with ATP at increasing equimolar doses to the sensillum shifted the ED ₅₀ value to 86.09 ± 10.2 pM, significantly lower than for either adenosine or ATP alone (Figure 2D). Despite this, the drc models and ED ₉₅ values estimated for ATP, adenosine and their combination are not different. The insurmountable asymptote reached for adenosine, AMP, ATP and AMP-CPP show no difference, suggesting presence of PRs that saturate in the same dose range. Yet adding either 200pM adenosine or 200pM AMP to 200pM ATP (approximately asymptote doses of each) elicited spike frequencies higher than either agonist presented alone, whereas adenosine combined with AMP had not such effect (Figure 2C and D, and Table 3). Taken together, these data indicate a P1A ₁ PR activated by adenosine and AMP, and a P2X PR activated by ATP and AMP-CPP on A. gambiae mouthparts. One sensory cell with two PR types on malaria mosquito mouthparts [00216] The inventors next addressed the question of where the PRs reside in labellum sensilla of A. gambiae females may be located. Action potentials recorded in response to purinergic compounds and sucrose were typically biphasic (Fig.3A and B). Spike amplitudes elicited by ATP (2.49 mV ± 0.030, mean ± s.e.m.) and adenosine (2.57 mV ± 0.031) were the same whereas sucrose systematically elicited higher amplitude spikes (4.44 mV ± 0.047; Figure 3A and B). Firing of sensory cells with distinct receptor types in response to stimulation by two agonists generate doublets from near simultaneous firing by the cells. No doublets were found in responses recorded for ATP combined with adenosine whereas responses to ATP combined with sucrose did (Figure 3A and B). This suggests that ATP and adenosine affect PRs on the same sensory cell within mouthpart sensilla of the malaria mosquito. [00217] Table 3. Comparisons of mean spike frequencies (± s.e.m.) recorded from A. gambiae T1 labellum sensilla in response to stimulation with adenosine, AMP and ATP alone and combined.

* Each compound was tested at 200 pM alone and in binary mixtures. Treatments with spike frequencies that are significantly different have P – values in bold. Recording were made from between 9 and 25 sensilla per compound and binary mixtures of compounds. Malaria mosquito PR responses are inhibited by P1 PR and P2 PR antagonists [00218] Xanthines selectively block adenosine-sensitive PRs. One µM of the non- selective P1 PR antagonist theophylline (27, Table 2) reduced the EP response elicited by 200 pM adenosine by almost half from 30.23 ± 3.02 spikes/2s to 17.63 ± 4.48 for the adenosine theophylline combination (paired t-test, P = 0.0259). As ATP complexes with theophylline in solution the inventors used the high affinity and selective P1A ₁ PR antagonist and theophylline-related compound 8-cyclopentyl-1,3-dimethylxanthine (CPT, 28, Table 2) as an alternative. CPT inhibited the EP response to 200pM adenosine but not that to ATP (Figure 4A and C). Conversely, 1nM suramin (19, Table 2), a non-selective P2 PR antagonist, inhibited the response to ATP but not to adenosine (P > 0.05; Figure 4B and D). Inhibition persisted for at least 2min in both cases (Figure 4 A and B, and Figure 7 A). Use of these PR inhibitors confirmed presence of P1A ₁ and P2X PRs on A. gambiae mouthparts. [00219] The P2X PR expressed by the A. gambiae mouthpart sensory cell was further characterised using non-selective P2 PR and selective P2X PR antagonists (19 – 26, Table 2), some tested over several doses (Figures 5 and 6). PPADS (P2 PR antagonist) inhibited the response to ATP at an ID ₅₀ of 1.42 nM, close to that of suramin (0.55 nM; Table 4). EP responses to ATP were inhibited by the P2X ₂ PR antagonist RB-2, the P2X ₃ PR antagonists TMP and spinorphin, and by the P2X ₃ / P2X _2/3 PR antagonist A-317491 but not by antagonists for other P2X PRs (Figures 5, 6 and 7, Table 4). The most effective antagonists were spinorphin and A-317491 (matching ID ₉₅ values at ca.0.4 nM; Table 1), suggesting a P2X ₃ / P2X _2/3 PR on A. gambiae mouthpart sensilla. P2, P2X ₃ and P2X _2/3 PR antagonists, but not the P2X ₂ PR antagonist RB-2, induced persistent antagonism to ATP presented to the sensillum approximately 2 min later (Figures 5 and 7). Response curves for these antagonists, except that for suramin, indicate resistance to inhibition, characteristic of PRs, when spike counts reach ca. 10/2 s (Figures 5 and 6). [00220] Table 4. Neuropharmacological effects of P2 PR antagonists on responses of A. gambiae mouthpart ATP-sensitive receptor. 1 ID ₅₀ and ID ₉₅ values in nM (unless otherwise stated) for PR antagonists were estimated by testing them at increasing doses admixed to 200 pM ATP that served as positive control (see Figures 5 and 6). The maximum dose tested is provided when an antagonist failed to inhibit. ² Mean spike frequencies provided for 200 pM ATP + antagonist is for an antagonist dose near its estimated ID ₉₅ value (column 4); P – values indicating significant reductions in spike frequency compared to 200 pM ATP are indicated in bold. Treatment repetitions (n) are reported in Materials and Methods. Malaria mosquito P2X PR responses are modulated by metal ions [00221] Essential trace metals like Zn ²⁺ and macrometals like Mg ²⁺ modulate vertebrate P2X PRs via extracellular binding sites (Coddou et al., supra). The crystal structure and EP responses of AmP2XR from the Gulf Coast tick Amblyomma maculatum indicate it functions as an ATP-gated cation channel with two extracellular metal-binding sites, M1 and M2 (Kasuya et al., 2016 in "Structural Insights into Divalent Cation Modulations of ATP-Gated P2X Receptor Channels", Cell Reports 14, 932-44; doi 10.1016/j.celrep.2015.12.087). The M1 site located at the trimer interface permits allosteric potentiation by Zn ²⁺ of AmP2XR responses to ATP. LsP2X of the pond snail Lymnaea stagnalis with a similar M1 site is likewise potentiated by Zn ²⁺ . The inventors established through EP recordings from A. gambiae labellum sensilla that 200pM Zn ²⁺ admixed with 100pM ATP induced a 5-fold increase in spike frequency but 200pM Zn ²⁺ had no such effect at the asymptote dose of 200 pM ATP (Table 5). The M2 site on AmP2XR is associated with the ATP binding site and subject to modulation by Mg ²⁺ , reducing ATP receptor affinity at higher Mg ²⁺ levels (Kasuya et al., supra). The inventors found that adding 200pM Mg ²⁺ to 200pM ATP reduced spike frequency recorded from A. gambiae labellum sensilla by half compared to 200pM ATP alone (Table 5). These findings suggest the P2X PR of A. gambiae shares characteristics with vertebrate, tick, and snail P2X PRs. [00222] Table 5. Effects of metals ions on electrophysiological responses of A. gambiae labellum ATP-sensitive receptor.

* Treatment repetitions (n) are reported in Materials and Methods. Feeding by biting insects and ticks is inhibited by P1 PR and P2 PR antagonists [00223] Considering the sensitivity shown by A. gambiae peripheral neurones for adenosine, adenine nucleotides and PR antagonists, the inventors set out to test the effects of these compounds on feeding by mosquitoes, biting flies and ticks. Adding ATP to the feeding solution induced dose dependent feeding from 1 µM ATP with an ED ₅₀ at 6.54 µM and an asymptote at 45 µM where 70.25 % mosquitoes fed (the ED ₉₅ at 22.5 µM for 68.15 % feeding was rounded to 20 µM for feeding assays; Figure 8A and Table 6). AMP-CPP at 20 µM proved a better feeding stimulant, eliciting 84.35 % feeding though not significantly different to ATP (Table 6). AMP and adenosine each presented at 20 µM elicited increased feeding albeit to a significantly lower extent than 20 µM ATP, whereas 1mM adenosine induced an equivalent level of feeding as on 20 µM ATP. No effect on feeding higher than on 20 µM ATP was recorded when 20 µM ATP was admixed with 200 µM adenosine (Table 6). The increase in feeding induced by the P2Y PR agonist ADP-βS was not significantly different to that of the feeding solution (Table 6). [00224] Table 6. Proportion of A. gambiae feeding on the feeding solution, on nucleotides and adenosine in the feeding solution, and on the P1A ₁ PR antagonist CPT and the P2X ₃ PR antagonist TMP admixed with ATP and adenosine in the feeding solution. Different experimental series are separated by a blank line.

* n =number of cages for each experimental situation;** FS =feeding solution (buffered NaCl/dextrose). [00225] PR antagonists identified by EP recordings could act on PRs to inhibit feeding.10 µM CPT, the selective P1A ₁ PR antagonist, significantly inhibited A. gambiae feeding on 1 mM adenosine but not on 20 µM ATP added to the feeding solution (Table 6). Testing P2 PR antagonists over several doses shows how suramin, PPADS, RB-2, TMP, spinorphin and A-317491 inhibited feeding on a solution containing the feeding stimulant ATP (Figures 8B and 9, Table 7). A. gambiae feeding proved most sensitive to TMP (ID ₅₀ 15 nM) and A-317491 (ID ₅₀ 0.24 nM; Figures 8B and 9, Table 7). Except for the P2X7 PR antagonist A-438079, antagonists for other P2X PR subtypes were less effective (Table 7). [00226] Table 7. Effect of P2 PR antagonists on A. gambiae feeding on a feeding solution containing ATP. * ID ₅₀ values in nM and ID ₉₅ values µM (unless otherwise stated) for antagonists were estimated by testing their effects at increasing doses on A. gambiae feeding on 20µM ATP (positive control) in the feeding solution (cf. Figures 8 and 9). ģ The log logistic LL.4 model did not permit a reliable estimate of the ID ₉₅ for PPDAS, RB-2 and A-317491. Data for the maximum dose tested is provided when an antagonist failed to inhibit feeding. For spinorphin and A438079 (not tested over several doses) the % reduction in feeding is provided for the dose indicated. The four columns on the right present, respectively, the proportion of mosquitoes feeding on 20 µM ATP (cf. Table 6), the dose of antagonist tested nearest the ID ₅₀ value, the proportion of mosquitoes feeding on that dose of antagonist admixed with 20 µM ATP, and the probability level for the comparison of the two proportions (significant values are in bold). Treatment repetitions (n) are reported in Materials and Methods. [00227] 20 µM CPT with 7.5 µM TMP added to the feeding solution inhibited the feeding stimulant effect of the 20 µM ATP plus 200 µM adenosine combination to a level no different to that of the feeding solution, and this combination of P1A ₁ PR and P2X ₃ PR antagonists proved a better feeding inhibitor for the 20 µM ATP plus 200 µM adenosine combination in the feeding solution than 7.5 µM TMP alone (Figure 10, Table 6). It is noteworthy how even the highest doses of PR antagonists tested only reduced feeding to levels corresponding to that on the feeding solution tested as negative control, allowing the inventors to conclude that the action of PR antagonists is on purine-sensitive cells. [00228] Another anopheline mosquito, A. stephensi, also responded as a function of dose to ATP as a feeding stimulant with an ED ₉₅ value at 160 µM ATP (Table 8). The P2 PR antagonists suramin and PPADS, the P2X ₃ PR antagonist TMP and the P2X ₃ / P2X _2/3 PR antagonist A-317491 all inhibited feeding by A. stephensi on the feeding solution containing ATP in a pattern like that recorded for A. gambiae where the P2X ₃ / P2X _2/3 PR antagonists TMP and A-317491 again induced the strongest inhibition (Table 8). [00229] Table 8. Effect of P2 PR antagonists on A. stephensi feeding on a feeding solution containing ATP.

* Treatment effects were compared to the positive control (feeding solution with 160 µM ATP added). Treatment repetitions (n) are reported in Materials and Methods. [00230] In feeding experiments, suramin inhibited feeding on a solution containing ATP by two hard tick spp. Ixodes ricinus and Amblyomma hebraeum and by the soft tick Ornithodorus moubata, and A-317491 inhibited feeding by I. ricinus and O. moubata (Table 9). Suramin and A-317401 likewise inhibited feeding by the tsetse fly Glossina pallidipes and horn fly Haematobia irritans, and TMP inhibited feeding by the horn fly on solutions containing ATP (Table 10). TMP in solution failed to inhibit feeding by the soft tick and tsetse, yet when applied onto the feeding membrane it significantly inhibited feeding by both blood feeders (Tables 9 and 10), suggesting that antagonism of peripheral PRs is sufficient to inhibit feeding. [00231] Table 9. Effect of P2 PR antagonists on feeding by three tick species on a feeding solution containing ATP.

* TFS: tick feeding solution containing 100 mM NaCl + 8.9 mM NaHCO3 + 1000 µM ATP as positive control; + Feeding solution intake in mg/h for hard ticks and weight gain in mg/25 min for soft ticks; # Within species, treatments effects significantly different to the TFS are in bold. [00232] Table 10. Effect of P2 PR antagonists on feeding by two biting fly species on a feeding solution containing ATP. * FFS: fly feeding solution containing 150 mM NaCl + 8.9 mM NaHCO3 + 100 µM ATP as positive control; # Within species, treatment effects significantly different to the FFS are in bold. [00233] Evidence is presented herein for P1A ₁ and P2X ₃ /P2X _2/3 PRs on mouthparts of blood-feeding arthropods. The P2X PR is particularly sensitive to selective P2X ₃ / P2X _2/3 PR antagonists A-319471, spinorphin and TMP that inhibit feeding by all blood- feeding insects and ticks tested herein. The characteristics of the P2X ₃ / P2X _2/3 PRs described by the inventors show similarities to vertebrate homomeric P2X ₃ PRs and heteromeric P2X _2/3 PRs found on unmyelinated C-fibres and on thinly myelinated Aδ- sensory fibres that transmit pain. Such nociceptive fibres and their first relay neurons in dorsal root ganglia (DRG) show high expression levels of P2X ₃ PRs (Burnstock and Verkhratsky, 2012 in "Purinergic signalling and the nervous system", Springer-Verlag Berlin Heidelberg; doi 10.1007/978-3-642-28863-0). In contrast to P2X ₃ PRs, slowly desensitising P2X _2/3 PRs maintain currents in response to repeated stimulation (Coddou et al., 2011, supra). The inventors recorded little adaptation in response to repeated presentation of ATP to A. gambiae mouthpart sensilla at intervals varying from tens of seconds to 6 minutes. Vertebrate P2X ₃ and P2X _2/3 PRs show high affinity for AMP-CPP and ATP, at <1 nM for the latter, and are readily antagonised by A- 317491 (Kd 0.9 nM; Jarvis et al., 2002, supra). The antinocicepive spinorphin, an endogenous heptapeptide inhibitor of encephalin-degrading enzyme, antagonises human P2X ₃ PR at an ID ₅₀ of 8.3 pM. [00234] During blood feeding, ATP and ADP are hydrolysed by A. gambiae salivary apyrase to prevent platelet aggregation for wound healing. This results in adenosine being presented to mouthpart PRs. The inventors report how equimolar combinations of ATP and adenosine elicit spike frequencies significantly higher than either agonist presented singly. This shifts the threshold to lower doses of the agonist mix and the response surpasses the unsurmountable asymptote reached at higher doses of either agonist alone, leading to a wider dose range over which the PRs respond. Mouthpart PRs of A. gambiae are eminently equipped to detect physiologically operating levels of both ATP and adenosine, and as disclosed herein, can be readily inhibited by PR antagonists. [00235] Even though the inventors tested and confirmed the usefulness of PR antagonists in the protection against arthropod parasites using only a number of arthropode parasite species, this invention is not limited to the arthropod species tested herein.