BLACK FLY (DIPTERA, SIMULIIDAE) SALIVARY SECRETIONS - IMPORTANCE IN VECTOR COMPETENCE AND DISEASE

When blood-feeding, black flies introduce secretions into the feeding lesion that act in a coordinated manner on the 3 arms of the vertebrat e hemostatic system (platelet aggregation, coagulation, and vasoconstr iction). Apyrase activity inhibits platelet aggregation and is ubiquit ous in the saliva of black flies, although activity per gland varies b y species and has a positive association with anthropophagy. Anticoagu lants target components in the final common pathway of the coagulation cascade, including factors V, Xa, and II (thrombin). The antithrombin salivary protein may exert a redundant effect by inhibiting the role of thrombin in platelet aggregation. Antithrombin presence and activit y also varies among black fly species, supply to the feeding wound app ears to be an important requirement for Simulium spp., because substan tial erythema-inducing activity has been demonstrated in salivary glan ds of all New World species examined. Salivary glands of Simulium ochr aceum (Walker), a highly anthropophilic vector of Onchocerca volvulus (Leuckhart), contain greater vasodilator activity than several other s pecies, including S. metallicum Bellardi, a secondary zoophagic vector of human onchocerciasis, Simulium vittatum Zetterstedt saliva affects immune cell responses and cytokine production. The ability of the sal iva to modulate components of the host immune system provides an oppor tunity for enhancing transmission of pathogens during bloodfeeding. Th us, the likely possibility that effective pathogen transmission relies on vector saliva may complement present efforts aimed at target epito pes of O. volvulus or identify additional molecules to be investigated as part of a ''river blindness'' vaccine cocktail. Components in sali va also may enhance the transmission of other microbial agents either by a cofeeding process similar to that observed in ixodid ticks or thr ough rupture of the labrum during escape of Onchocerca infective stage larvae. In a few instances, saliva of some Simulium spp. also has bee n associated with extensive tissue and organ pathology, including hemo rrhagic shock and death. Pathologic signs associated with this syndrom e indicate an enhanced antihemostatic activity in saliva.