The relationship between innate susceptibility to Onchocerca, and haemolymph attenuation of microfilarial motility in vitro using British and west African blackflies.

The effect of haemolymph from individual flies of differing species of Simulium were compared in their ability to attenuate Onchocerca microfilariae in vitro. Small maintenance systems, each of 3-4 microliters total volume, were set up in Terasaki plates. Three British species of greatly differing susceptibilities to Onchocerca lienalis (Simulium ornatum, S. equinum and S. vernum) were compared in their effect on O. lienalis. Three forms of the S. damnosum complex from Liberia (S. yahense and 2 geographical variants of S. soubrense) were similarly compared in their effects on O. volvulus microfilariae. The more refractory species in both sets of experiments were found to significantly reduce the in vitro motility of the microfilariae more than the susceptible species. It is proposed that this innate variation in in vitro attenuation, may be caused be humoral haemolymph proteins. Such an assay, perhaps in a modified form could be used in predicting vector susceptibility. It could also be used to analyse mechanisms responsible for variation in susceptibility, of different S. damnosum complex members, to the geographical forms of O. volvulus.