The Simulium damnosum complex in western Uganda and its role as a vector of Onchocerca volvulus

The status of onchocerciasis vectors in the former Ruwenzori focus in weste rn Uganda was re-examined some 15 years after control measures against Simu lium damnosum s.l. were suspended. The four cytoforms S. kilibanum, 'Sebwe' , 'Nkusi' and S. pandanophilum were found. While the nonanthropophilic 'Seb we' was still widely distributed in rivers north, east and south of the Ruw enzori, the only anthropophilic species and vector, S. kilibanum, had disap peared from most of its former habitats and was now restricted to two limit ed foci, where high biting densities were encountered. It was still a vecto r south of the Ruwenzori (Kasese focus), where 15.4% of the parous flies we re infected with larval stages of Onchocerca volvulus and 34 infective larv ae were found in the heads of 1000 parous flies. In the second focus along the Mahoma and Nsonge rivers, a chromosomally highly polymorphic population of S. kilibanum had replaced the former vector S. neavei, but does not act as a vector. Only 2.3% of the parous females were infected and just 1 infe ctive larva was found in the heads of 1000 parous flies.