Effect of repeated community-based ivermectin treatment on the intensity of onchocerciasis in Nigeria

Abstract

Ivermectin (Mectizan) is an effective and well-tolerated microfilaricidal drug that has emerged as the drug of choice for large-scale treatment of onchocerciasis. Repeated doses of ivermectin is reported to lower the incidence of microfilardermia, while a single dose will markedly reduce skin microfilarial loads for up to 12 months with a transient fall in microfilarial level. The aim of the study was to evaluate the effect of repeated community-based ivermectin treatment on the intensity of onchocerciasis in 10 onchocerciasis-endemic communities of Lade District, Pategi Local Government Area of Kware State, Nigeria. The participants, a cohort of 330 men from whom parasitologic pre-treatment data were collected before mass ivermectin distribution commenced in 1992, were evaluated just before the fifth round of annual ivermectin treatment in August-September 1996, and 12 months after the fifth round in August-September 1997. Two bloodless skin snips were obtained from participants' left and right iliac crests and incubated for 24 h in an 0.5 mL microfuge tube containing 0.2 mL normal saline, before microscopic examination and enumeration of microfilariae. The community microfilarial load (CMFL), the most sensitive parasitologic indicator, was computed. Following four rounds of repeated annual treatments of onchocerciasis with ivermectin in the 10 communities studied, varying degrees of reduction in CMFL were observed in six of the 10 communities. These reductions were significant in three of the six communities at the first evaluation carried out just before the fifth round of annual ivermectin treatment (p = or <0.05). There was an increase in the remaining four communities, mainly attributed to poor annual acceptance and compliance with annual ivermectin treatment due to the occurrence of severe adverse reactions and absenteeism. Post-treatment evaluation (12 months after the previous evaluation), just before the sixth round of annual ivermectin treatment was preceded by community mobilization (enlightenment on the usefulness of ivermectin an assurance on its safety), especially in the four affected communities. This brought about reductions in the CMFL in the four communities that were not previously affected. Although the CMFL of these four communities was reduced, the CMFL of three of the communities did not fall below the 1992 pre-treatment level but was below the level recorded 12 months earlier. One of these affected communities had its CMFL reduced below the 1992 pre-treatment CMFL. This study confirms the efficacy of ivermectin in the reduction of CMFL but exposes the danger of failing to monitor annual ivermectin treatment where community ivermectin-based control efforts are ongoing. The results obtained from this study are not as impressive as results reported from closely monitored scientific evaluations of ivermectin efficacy studies.