Abstract
BackgroundSimulium damnosum s.l., the most important vector of onchocerciasis in Africa, is a complex of sibling species that have been described on the basis of differences in their larval polytene chromosomes. These (cyto) species differ in their geographical distributions, ecologies and epidemiological roles. In Ghana, distributional changes have been recorded as a consequence of vector control and environmental change (e.g. deforestation), with potential disease consequences. We review the distribution of cytospecies in southern Ghana and report changes observed with reference to historical data collated from 1971 to 2005 and new identifications made between 2006 and 2011.Methods/ResultsLarvae were collected from riverine breeding sites, fixed in Carnoy’s solution and chromosome preparations made. Cytotaxonomic identifications from 1,232 samples (including 49 new samples) were analysed. We report long-term stability in cytospecies distribution in the rivers Afram, Akrum, Pawnpawn and Pru. For the rivers Oda, Ofin and Tano we describe (for the first time) patterns of distribution. We could not detect cytospecies composition changes in the upper Pra, and the lower Pra seems to have been stable. The elimination of the Djodji form of S. sanctipauli in the Volta Region seems to have had no long-term effects on the distribution of the other cytospecies, despite an initial surge by S. yahense. There has been a recent increase in the occurrence of savannah cytospecies in the river Asukawkaw, and this might be related to continuing deforestation.ConclusionsCytospecies’ distributions have not been stable from 1971 to 2011. Although there are no obvious causes for the temporary appearance and subsequent disappearance of cytospecies in a particular location, a major influence has been vector control and migration patterns, probably explaining observed changes on the Black Volta and lower Volta rivers. Deforestation was previously implicated in an increase of savannah cytospecies in southern Ghana (1975–1997). Our data had little power to support (or refute) suggestions of a continuing increase, except in the Asukawkaw river basin.
Highlights
Simulium damnosum s.l., the most important vector of onchocerciasis in Africa, is a complex of sibling species that have been described on the basis of differences in their larval polytene chromosomes
There are no obvious causes for the temporary appearance and subsequent disappearance of cytospecies in a particular location, a major influence has been vector control and migration patterns, probably explaining observed changes on the Black Volta and lower Volta rivers
Deforestation was previously implicated in an increase of savannah cytospecies in southern Ghana (1975–1997)
Summary
Simulium damnosum s.l., the most important vector of onchocerciasis in Africa, is a complex of sibling species that have been described on the basis of differences in their larval polytene chromosomes. These (cyto) species differ in their geographical distributions, ecologies and epidemiological roles. Vector surveys in West Africa (including Ghana) up to 1968 have been listed [2], but by 1966 it had already become clear that the morphotype known as S. damnosum was not a single species, but rather a complex of sibling species which were morphologically very similar or identical, they could be separated on the basis of differences in their larval polytene chromosomes [3]. The recent strategic shift from control to elimination has changed this perception, and the strategy developed by the WHO African Programme for Onchocerciasis Control (APOC) for verification of elimination includes entomological evaluation with regular collection of cytotaxonomic data in all foci [14]
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