Seasonal variations and other changes in the geographical distributions of different cytospecies of the Simulium damnosum complex (Diptera: Simuliidae) in Togo and Benin
Date
2023
Authors
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Journal ISSN
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Publisher
Elsevier B.V.
Abstract
ABSTRACT
Simulium damnosum s.l., the most important vector of onchocerciasis in Africa, is a complex of sibling species
described on the basis of differences in their larval polytene chromosomes. These (cyto) species differ in their
geographical distributions, ecologies and epidemiological roles. In Togo and Benin, distributional changes have
been recorded as a consequence of vector control and environmental changes (e.g. creation of dams, defores tation), with potential epidemiological consequences. We review the distribution of cytospecies in Togo and
Benin and report changes observed from 1975 to 2018. The elimination of the Djodji form of S. sanctipauli in
south-western Togo in 1988 seems to have had no long-term effects on the distribution of the other cytospecies,
despite an initial surge by S. yahense. Although we report a general tendency for long-term stability in most
cytospecies’ distributions, we also assess how the cytospecies’ geographical distributions have fluctuated and
how they vary with the seasons. In addition to seasonal expansions of geographical ranges by all species except
S. yahense, there are seasonal variations in the relative abundances of cytospecies within a year. In the lower
Mono river, the Beffa form of S. soubrense predominates in the dry season but is replaced as the dominant taxon in
the rainy season by S. damnosum s.str. Deforestation was previously implicated in an increase of savanna
cytospecies in southern Togo (1975–1997), but our data had little power to support (or refute) suggestions of a
continuing increase, partly because of a lack of recent sampling. In contrast, the construction of dams and other
environmental changes including climate change seem to be leading to decreases in the populations of
S. damnosum s.l. in Togo and Benin. If so, combined with the disappearance of the Djodji form of S. sanctipauli, a
potent vector, plus historic vector control actions and community directed treatments with ivermectin, oncho cerciasis transmission in Togo and Benin is much reduced compared with the situation in 197
Description
Research Article
Keywords
Togo, Benin, Onchocerciasis, Simulium damnosum, Simulium sirbanum, Simulium squamosum, Simulium yahense, Simulium sanctipauli, Simulium soubrense, Cytospecies distribution