Combined benefits of annual mass treatment with ivermectin and cattle zooprophylaxis on the severity of human onchocerciasis in northern Cameroon

We studied the effect of 10 annual distributions of ivermectin for control of human onchocerciasis on the reduction of Onchocerca volvulus transmissio n by Simulium vector flies. Prevalence and infection load in the human popu lation decreased, whilst the annual biting rates (ABR) of the vector remain ed unchanged. The annual transmission potential of infective larvae of O. v olvulus fell to 40% of the preintervention level, but was still sufficient to maintain endemicity. However, recent immigration of herds of nomadic cat tle into the study area has diverted the flies from man to cattle, creating an additional zooprophylactic effect. The predominant transmission of the bovine filaria O. ochengi to man also provides some concomitant cross-prote ctive immunity against further infestations with O. volvulus. The effects o f ivermectin on O. volvulus, combined with the zooprophylactic effects of t he increased cattle population, have produced a complex beneficial influenc e on the transmission of human onchocerciasis.