The effect of experimental Onchocerca infections on the fecundity and oviposition of laboratory reared Simulium sp. (Diptera, Simuliidae).

Two species of Simulium were used to examine the effect of experimental Onchocerca lienalis infections on their fecundity and oviposition rates. S. ornatum s.l. was chosen as a natural vector of bovine onchocerciasis in Britain, and S. lineatum was selected because of its suitability as an experimental model for oviposition studies. Infection was either by feeding with blood containing microfilariae, or by intrathoracic injection of the parasites. Using the blood feeding technique, reductions in the fecundity of S. lineatum were observed at rates of 21 to 76%, depending on the concentration of parasites. A reduction of 21% in the fecundity of S. ornatum s.l. (P less than 0.05) was achieved when flies were fed on microfilariae at 69,000 per ml. Although individual flies feeding on infected blood may take as much as those in control groups, the feeding rates were reduced in several instances. S. lineatum also showed a depression of oviposition rate when infected by O. linenalis larvae. Fecundity was also significantly reduced when the route of infection was intrathoracic. Reductions for S. lineatum depended on the inoculum, but were either 36% (10 microfilariae per fly) or 54% (50 microfilariae per fly). S. ornatum s.l. showed a 13% reduction in fecundity when given 20 microfilariae per fly. It was concluded that similar experimental studies could be usefully performed with S. damnosum s.l. infected with O. volvulus microfilariae.