Effects of Three-Month Doses of Ivermectin on Adult Onchocerca Volvulus

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  • American Registry of Pathology, World Health Organization Collaborating Center for the Histopathology of Filarial Diseases in Man, Department of Infectious and Parasitic Disease Pathology, Armed Forces Institute of Pathology, Departamento de Enfermedad de Robles (Oncocercosis), SNEM, Department of Entomology, College of Agriculture, University of Arizona, Dana Center for Preventive Ophthalmology, Wilmer Institute, Johns Hopkins University School of Hygiene and Public Health, Washington, DC, Guatemala

Onchocerca volvulus worms in nodules from Guatemalan patients treated with four, eight, or 11 single doses of ivermectin (150 µg/kg of body weight) that were given once every three months were examined by routine histologic techniques and compared with worms in control nodules from untreated persons living in the same location over the same time periods. All treated nodules were removed four months after the last dose of ivermectin, i.e., 13, 25, or 34 months after the start of the trial. At the 25th and 34th months, i.e., after the eighth or eleventh doses of ivermectin, there were excess mortalities in female worms of 25.5% and 32.6%, respectively, over and above the levels in controls. Furthermore, the proportions of live females still producing scanty embryos up to the gastrula stage were only 7.7% and 18.2%, and no females were producing microfilariae. Ivermectin given at 3-month intervals also reduced significantly the mean numbers of live male worms in nodules, as well as the proportions of inseminated females. This regimen was effective in preventing embryogenesis to the microfilarial stage while, at the same time, it caused a slow but steady attrition of the adult worms.