The development of Onchocerca ochengi in surrogate temperate Simuliidae, with a note on the infective larva

Laboratory reared nulliparous female flies of six temperate species of Simuliidae were examined for their susceptibility to infection with Onchocerca ochengi by intrathoracic injection of cryopreserved skin microfilariae obtained from cattle in Mali. Three species (S. equinum, S. ornatum and S. erythrocephalum) supported development to the infective stage, one species (S. variegatum) allowed partial development and the remaining two species (S. reptans and S. aureum) were insusceptible to infection. The most suitable surrogate vectors were S. equinum and S. ornatum which had survival rates of 44% and 49%, proportions of microfilariae developing to third stage larvae of 6.4% and 3%, and infection rates with infective larvae of 13.5% and 14% respectively. O. volvulus infective larvae, produced by intrathoracic microfilarial injection in S. ornatum, were 586-760 microns (mean 687 microns) long and were significantly shorter (p less than 0.02) than the O. ochengi infective larvae (645-880 microns, mean 756 microns). No constant differences in the posterior or anterior morphology, or in the acid phosphatase staining patterns between O. ochengi and O. volvulus, were seen. These results raise the possibility that the presence of O. ochengi in a population of infective larvae from vector flies in endemic onchocerciasis zones might be identifiable on the basis of their length alone.