LOW-LEVEL IVERMECTIN COVERAGE AND THE TRANSMISSION OF ONCHOCERCIASIS

Regular collections of biting Simulium damnosum s.l, were carried out during a community-based trial of doses of ivermectin every 6 months f or onchocerciasis in Sierra Leone. Over 64 000 blackflies were caught at 4 sites close to treated villages and one site near an untreated vi llage. More than 17 000 of these blackflies were dissected during the 31 months of the study and 5 doses of ivermectin were distributed to a bout 30% of the human population in the treated villages. High annual biting rates (about 100 000 bites per year) and transmission potential s (about 5000 larvae per year) were found at all catching sites. Appro ximately 30% of parous blackflies carried Onchocerca volvulus larvae, and 8% had infective stage larvae. None of these indices to be affecte d by the distribution of ivermectin. However, the mean number of larva e per infected fell from 8.7 to 5.8 during the study period in the tre ated villages, equivalent to a 21% decrease per year. No such reductio n was seen in the control village. This study demonstrates that in are as where high capacity vectors predominate, the effect on transmission of even a low coverage of the human population with repeated doses of ivermectin may be detectable using the sensitive entomological index of intensity of infection in infected flies. Statistical analysis of o nchocerciasis transmission data is a complex issue and ways of improvi ng the design of trials and applying appropriate statistical methods a re discussed.