Efficacy of ivermectin against Onchocerca volvulus in Ghana

Abstract

Mike Osei-Atweneboana and colleagues1Osei-Atweneboana MY Eng JK Boakye DA Gyapong JO Prichard RK Prevalence and intensity of Onchocerca volvulus infection and efficacy of ivermectin in endemic communities in Ghana: a two-phase epidemiological study.Lancet. 2007; 369: 2021-2029Summary Full Text Full Text PDF PubMed Scopus (281) Google Scholar conclude that ivermectin-resistant parasite populations are emerging in some areas. We are not convinced that their results provide conclusive evidence. Their study shows that ivermectin retains its full microfilaricidal effectiveness, but Osei-Atweneboana and colleagues base their conclusion on the microfilarial repopulation dynamics after treatment. In four villages, repopulation was faster than in five other villages, and Osei-Atweneboana and colleagues took this as “evidence that ivermectin resistance is developing”. However, comparable repopulation dynamics have also been seen in situations where incomplete control led to renewed transmission and the introduction of young worms into an ageing worm population. The critical question is why these four villages had relatively high infection levels after 12–17 treatment rounds. One possible explanation is insufficient treatment coverage. Reports available to us are incomplete but suggest that treatment coverage has been highly variable, sometimes as low as 20–40%, and with no treatment reported for some years. Osei-Atweneboana and colleagues even report on a hyperendemic village that never received treatment. Such untreated villages, and the untreated members of villages with poor treatment coverage, will contribute to transmission and infections with new onchocercal worms, also in neighbouring villages and among those who received treatment. We believe that inadequate coverage provides a plausible explanation for the observations. We do not dismiss the possibility of resistance and the African Programme for Onchocerciasis Control will coordinate further investigations. Furthermore, WHO is working with partners towards the establishment of comprehensive systems for monitoring drug use and efficacy in large-scale anthelmintic treatment programmes. We declare that we have no conflict of interest. Efficacy of ivermectin against Onchocerca volvulus in Ghana – Authors' replyEd Cupp and colleagues suggest that an ivermectin resistance mechanism involving reduced suppression of reproduction by adult female Onchocerca volvulus would be unique. Although ivermectin resistance in trichostrongylids of livestock is manifested by the survival of the parasitic stages, including adult worms, the action of ivermectin against adult filariae is unique in that ivermectin mainly suppresses reproduction rather than killing adult filariae. Nevertheless, it is of interest that Caenorhabditis elegans unc-9 mutants share phenotypes of ivermectin resistance and an egg-laying defect. Full-Text PDF