Entomological surveillance of onchocerciasis in Burkina Faso: Progress towards interrupting transmission in blackflies in the main river basins of the country

Current guidelines for onchocerciasis elimination rely heavily upon assessment of the presence of Onchocerca volvulus in the vector Simulium damnosum (sensu lato). This entomological study was conducted over four years in several regions of Burkina Faso to determine the progress made towards interrupting onchocerciasis transmission. Larvae and adult blackflies were collected in eight river basins (Comoé, Léraba, Dienkoa, Mouhoun, Bougouriba, Bambassou, Nakambé, Nazinon and Sissili). Larvae were analysed by cytotaxonomy, and the adult blackflies analysed for the presence of infective larvae of O. volvulus by PCR. Blackfly infectivity rates were first determined by year for each basin, then compared to the thresholds established by the WHO. The results indicate that the blackflies collected belong to the savannah group Simulium damnosum (sensu stricto) and Simulium sirbanum. Hybrids of the two species were also identified. Overall, the prevalence of flies carrying infective larvae was below the threshold of 0.05% established by the WHO indicating important progress towards the interruption of onchocerciasis transmission in Burkina Faso, though hotspots with infectivity rates well above the WHO’s thresholds remain. Onchocerca volvulus continues to be transmitted in six of the nine basins evaluated, all of which border neighbouring countries. These data indicate that it will be necessary to maintain entomological surveillance in these hotspot areas until transmission is interrupted throughout the region.