Entomological evaluation of ivermectin mass treatment against onchocerciasis.

To evaluate the impact of onchocerciasis control programmes on the annual transmission potential, it is customary to monitor the man-biting rate of vector Siniuliuin (Diptera: Simuliidae) and the number of Oizclzocerca (Nematoda: Onchocercidae) infective larvae found in them (Walsh et al., 1978). In addition to the transmission of Onchocerca volvulus (Leuckart) causing human onchocerciasis, Simuliunz females may also be the vectors of various other species of Onchocerca which parasitize wild or domestic animals. The larvae of other Oizclzocerca spp. are difficult to distinguish from those of O. volvulus. Therefore, using routine methods in the field, it is not easy to determine the efficiency of a mass host chemotherapy campaign targeting only one Oizclzocerca species. To evaluate the impact of ivermectin on natural transmission of O. volvulus, we tried to improve the usual entomological index of infection by measuring lengths of Onchocerca larvae found in female blackflies (Simuliidae) collected on human bait. Annual mass treatment with ivermectin has been carried out since 1987 in the Vina Valley, a savanna region of North Cameroon, involving about 20,000 people (Prod'hon et al., 1991). Ivermectin was given to eligible people at the target dosage of 150 kg per kilogram of body weight. Coverage with the treatment was higher than 60% (Prod'hon et al., 1991). According to Traoré-Lamizana & Lemasson (1987), the principal vectors of onchocerciasis in Vina valley are Simulium damnosunz Theobald sensu stricto and Xsirbaizurn (Vajime & Dunbar). Entomological surveys of man-biting Simuliidae were performed 1 month before treatment; then done again during the 2 months following the campaign. Simiilium females were dissected and larvae of Onchocerca were observed and counted as described by Philippon (1977). All third stage larvae found in the blackfly head, considered to be infective larvae, were stretched by heat exposure, then measured using an eye-piece micrometer. Infective larvae were recorded per 1000 parous blackflies. Infective larva lengths were repre-