Host Defenses

Humans and other animals have developed elaborate means to defend themselves against infestation by arthropods and infection by pathogens they may transmit. Both behavioral and immunological responses are used to resist infestation by arthropods. Behavioral defenses include evasive, offensive, or defensive action against biting flies such as mosquitoes, black flies, ceratopogonids, stable flies, and horse flies. Grooming and preening by animals (e.g., biting, scratching, or licking) are defensive behaviors used to reduce or prevent infestations by ectoparasites and other potentially harmful arthropods. Host immunological defenses against arthropods vary with different arthropods and with respect to previous exposure to the same or antigenically related taxa. Details concerning such host immune responses are beyond the scope of this book, but some general trends are noteworthy. Repeated feeding attempts by the same or antigenically cross-reactive arthropods often lead to fewer arthropods being able to feed successfully, reduced engorgement weights, greater mortality, and decreased fecundity of female arthropods. Widespread arthropod mortality rarely results. For more information concerning the types of host immune responses and cell types involved against various ectoparasites, see Wikel (1996b) and other works listed at the end of this chapter.

Many blood-feeding arthropods partially or completely counteract the host immune response by inoculating immunomodulators or immunosuppressive compounds into the bite site. In fact, a wide range of pharmacologically active compounds is known to be released at the bite site by various arthropods (Ribeiro 1995). These compounds range from anticoagulants to prevent the blood from clotting, local analgesics to reduce host pain, apyrase to prevent platelet aggregation and promote capillary location, and various enzymes and other factors for promoting blood or tissue digestion. Some of these compounds are perceived by the host as antigens and may elicit an immune response, whereas others can cause localized or systemic toxic responses and itching.

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