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 for onchocerciasis in Sierra Leone. Over 64,000 blackflies were caught at 4 sites close to treated villages and one site near an untreated village. More than 17,000 of these blackflies were dissected during the 31 months of the study and 5 doses of ivermectin were distributed to about 30% of the human population in the treated villages. High annual biting rates (about 100,000 bites per year) and transmission potentials (about 5000 larvae per year) were found at all catching sites. Approximately 30% of parous blackflies carried Onchocerca volvulus larvae, and 8% had infective stage larvae. None of these indices appeared to be affected by the distribution of ivermectin. However, the mean number of larvae per infected blackfly fell from 8.7 to 5.8 during the study period in the treated villages, equivalent to a 21% decrease per year. No such reduction was seen in the control village. This study demonstrates that in areas 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 onchocerciasis transmission data is a complex issue and ways of improving the design of trials and applying appropriate statistical methods are discussed.