Salivary apyrase in New World blackflies (Diptera: Simuliidae) and its relationship to onchocerciasis vector status

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Abstract
Salivary gland apyrase is believed to be critical to blood-feeding in arthropod vectors. This enzyme was measured in six New World blackflies representing three taxonomic pairs of non-vectors and vectors of Onchocerca volvulus. In Simulium (Psilopelmia) ochraceum, a highly anthropophilic vector in Mexico and Guatemala, apyrase exhibited maximum activity between pH 8.0 and 9.0, mean 39.8 pM 4.7 milliUnits/pair of gland equivalents (mU), and was enhanced when ATP was used as a substrate. In the zoophilic non-vector Simulium (Psilopelmia) bivittatum maximum activity was significantly less (5.1 pM 0.7 mU) under all conditions examined. Preference for ADP or ATP as substrate was a function of the pH of the reaction for this species. Apyrase activity in Simulium (Simulium) metallicum Bellardi (29.5 pM 11.5 mU), a zoophilic secondary vector in Mexico and Guatemala, resembled that of S.(Ps.)ochraceum (24.8 pM 13.7 mU at pH 8.5) with ADP as substrate, but showed reduced activity with ATP. Both these Central American vectors had higher apyrase activity than found in Simulium (Notolepria) exiguum, a vector of O. volvulus in Ecuador and Colombia. However, maximum apyrase activity, measured at pH 8.0 with ADP as substrate, was greater in S.(N.) exiguum (10.9 pM 0.6mU) than in Simulium (Notolepria) gonzalezi (5.9pM1.9mU), a non-vector species widespread in Central America. Therefore, for the consubgeneric species pairs examined, a positive association was detected between higher concentrations of apyrase activity and their vector status for O. volvulus.