The effect of repeated doses of ivermectin on adult female Onchocerca volvulus in Sierra Leone.

The effects of single and multiple doses of ivermectin on mortality and morphology were assessed in over 700 female Onchocerca volvulus worms and the effects on embryogenesis were assessed in 490. Nodules were surgically removed from Sierra Leoneans recruited from a double-blind placebo controlled study of ivermectin given at six-monthly intervals. Nodules were digested in collagenase to isolate whole adult worms. After four or five doses of ivermectin there were significant increases in the numbers of discoloured and calcified worms and possibly a trend towards increased mortality, but this was not seen consistently. There was no evidence of a prophylactic effect of the drug. Worms were then homogenised and embryograms constructed. A single dose of ivermectin produced large numbers of degenerating intrauterine microfilariae, but embryonic development occurred normally. After multiple doses we observed almost complete cessation of embryogenesis, with a highly significant decrease in the numbers of viable multicellular embryonic stages, while oocytes appeared to be produced normally. Development is probably impeded at the single cell stage, possibly because of reduced fertilization. In planning the future role of ivermectin as a control measure for onchocerciasis it is crucial to determine if these effects on embryogenesis are reversible.