Comparison of the sensitivity of different geographical isolates of Onchocerca volvulus microfilariae to ivermectin: effects of exposure to drug on development in the blackfly Simulium ornatum

Abstract

Four geographical isolates (Ghana forest, Ghana savannah, Cameroon forest, Guatemala) of Onchocerca volvulus microfilariae (mf) and O. lienalis mf (UK) were examined for their sensitivity to ivermectin by incubation in vitro in drug followed by assessing their ability to develop in the blackfly Simulium ornatum after intrathoracic injection. Parasites were incubated for 30 min in ivermectin (10 −6 to 10 −6 m), which resulted in a concentration dependent decrease in the numbers of parasites surviving and developing in the insect; there were significant reductions in parasite recoveries from all isolates in the 10 −6 m to 5 × 10 −8 m ivermectin groups, but no significant effect was seen following incubation in concentrations of 10 −8 m and below. Experiments consistently demonstrated that the 4 isolates of O. volvulus were similarly sensitive to ivermectin (in the 10 −7 m ivermectin groups there was a reduction of 76.3% to 85.1% in numbers of infective larvae, and 60.9% to 85.5% in numbers of all larval stages, compared to controls); O. lienalis mf were significantly more sensitive (100% reduction in infective larvae, 98.7% reduction in all larval stages). This baseline information on drug sensitivity and techniques should prove useful for examining populations of O. volvulus for possible development of drug resistance in the future.