Efficacy of ivermectin against Onchocerca volvulus in Ghana - Authors' reply

Abstract

Ed 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.1 It has been suggested that low treatment coverage might explain the observed rapid skin repopulation with microfilariae, and Charles Mackenzie and Jan Remme and colleagues suggest treatment coverage of less than 50%. In fact, the average treatment coverage in the 5 years before the study, in the communities that showed poor parasitological responses, were: Jagbenbendo 71·5%, New Longoro and Kyingakrom 66·5% each, and Wiae 68·7%. These levels of coverage are in excess of the target of 65% set out by the African Programme for Onchocerciasis Control.2 Furthermore, the almost complete removal of skin microfilariae by 30 days after treatment confirms that treatment coverage was 100% for all patients in our study. A low treatment coverage cannot explain the rapid repopulation of skin microfilariae in these patients in the communities that showed poor parasitological response to ivermectin. Remme and colleagues point to the ivermectin-naive community and suggest that incoming infections could cause newly maturing adult worms to rapidly repopulate the skin with microfilariae. Ivermectin-naive communities are rare in Ghana and the naive community found in this study was in the East Gonja district. This area is remote from Kyingakrom and New Longoro (Kintampo district), two of the communities in which skin repopulation with microfilariae was alarming.