VECTOR-PARASITE TRANSMISSION COMPLEXES FOR ONCHOCERCIASIS IN WEST-AFRICA

Background In West Africa, there are two strains of the filarial paras ite Onchocerca volvulus, which differ in their ability to induce ocula r disease. Transmission studies have suggested that six sibling specie s of the parasite vector, the black fly Simulium damnosum sensu late, allow development of the two strains of O volvulus with varying effici ency. We aimed to test the hypothesis of parasite-vector complexes, wh ereby the two parasite strains, known as forest and savanna, are prefe rentially transmitted by distinct groups of the species of S damnosum s I. Methods During 1993 and 1994, wild black flies were collected fro m 11 river basins within the area covered by the Onchocerciasis Contro l Programme (OCP). The flies were dissected and filarial larvae, ovari es, and malpighian tubules removed. Genomic DNA was extracted from lar vae, and PCR amplification was used to classify O volvulus parasites a s forest or savanna strains. PCR-amplified DNA from ovaries and malpig hian tubules was used to distinguish sibling species of S damnosum s I . S yahense and S squamosum were distinguished by body colour. Finding s 214 of 105105 flies dissected were infected with filarial larvae; 84 of these were infected with mature O volvulus parasites. Of the 35 sa vanna-dwelling infected flies, 17 carried forest-strain parasites and 18 savanna-strain parasites. Of the 45 infected flies identified as th e forest dwelling sibling species, 20 carried savanna-strain parasites and 25 forest-strain parasites. No significant differences were found in the numbers of mature larvae of each strain carried by the forest- dwelling species of fly or in the number of forest and savanna larvae in savanna-dwelling vector species. Interpretation Vector-parasite tra nsmission complexes do not currently play a part in the biology of O v olvulus transmission in the area of the OCP in West Africa. This findi ng has important strategic implications for the future of efforts to c ontrol onchocerciasis in West Africa.