Effect of semiannual treatments of ivermectin on the prevalence and intensity of Onchocerca volvulus skin infection, ocular lesions, and infectivity of Simulium ochraceum populations in southern Mexico

The effect of semiannual ivermectin treatment along with nodulectomy on filarial transmission levels were estimated during the three dry seasons of 1991-1993 in a hyperendemic village in southern Mexico. Parasitologic and ophthalmologic examinations were carried out every six months until five drug treatments were completed. Ivermectin mass treatment with a coverage of approximately 80% had a significant impact (P < 0.05) on the prevalence of skin infection and the mean microfilarial skin density (CMFL), which were reduced 38% and 89%, respectively. A gradual and significant (P < 0.05) decrease in the mean microfilariae number in the anterior chamber of the eye and in corneal opacities was also observed as the CMFL was reduced. After three treatments, these were reduced 84% and 69%, respectively. However, after two years of continuous intervention, no significant differences (P > 0.05) were observed in either the daily mean infective biting density and the daily mean transmission potential. This was probably due to the remaining microfilarial load provided by the untreated resident population and migrant groups. On the whole, our results confirm both the efficacy of ivermectin to alleviate the clinical manifestations of the disease and its minimal impact on Onchocerca volvulus transmission, and indicate the need both to achieve higher levels of drug coverage and to incorporate other measures to stop transmission until a macrofilaricide drug is found.