Chemoprophylaxis of Onchocerca infections: in a controlled, prospective study ivermectin prevents calves becoming infected with O-ochengi

Onchocerciasis ('River Blindness'), caused by the filarial nematode Onchoce rca volvulus is of major public health importance in West Africa. Ivermecti n, a drug originally developed for veterinary use, is non being incorporate d in control strategies but whilst it has potent efficacy against L-1 larva e (microfilariae), ivermectin is not lethal to adult (L-5) O. volvulus, nor to adults of the related cattle parasite O. ochengi. We have exploited thi s model to determine if ivermectin has prophylactic activity against natura lly transmitted, O. ochengi infections in a controlled, prospective study i n northern Cameroon. Calves were treated monthly with ivermectin at either 200 mu g/kg or 500 mu g/kg for 21 months. None of 15 treated calves develop ed adult worm infection, whereas 5/6 untreated controls became infected (P < 0.001) with a total of 54 O. ochengi nodules, and all 5 developed patent microfilaridermia. These results have significant implications for the use of ivermectin in humans, and suggest that strategic chemotherapy at times o f maximal transmission will confer prophylactic as well as therapeutic bene fits.