The development of Onchocerca spp. in Simulium decorum Walker and Simulium pictipes Hagen

Two species of Nearctic black fly, Simulium decorum and S. pictipes, exhibit partial susceptibility to infection with the bovine parasite, Onchocerca lienalis, when inoculated intrathoracically with microfilariae. In addition, a proportion of S. decorum females will support the development of the human parasite, O. volvulus, to the third larvae stage. Infection rates with second- or third-stage larvae seven or more days after varied among geographic strains of S. decorum, ranging from 6.5% in a strain from northern New York State to 48.7% in a strain from Georgia. The average number of larvae per infected female ranged from 1.00 to 1.78 in the three strains examined. Partial susceptibility to O. lienalis was found to persist in colonized strains of s. decorum, and cryopreserved microfilariae of this parasite retained their infectivity. Seven or more days after inoculation with microfilariae of the Guatemalan strain of O. volvulus, 16.7% of the females of S. decorum harbored second- or third-stage larvae. Development of O. lienalis and O. volvulus proceeded normally in these black flies, and moderate increases in susceptibility rate and number of infective larvae were noted in response to increased microfilarial dosages.