Seasonal fluctuations of the Simulium damnosum Theobald complex and the prevalence of Onchocerca volvulus Bickel in blackflies caught in river systems bordering the Nigeria–Benin border were assessed for their direct impact on the epidemiology of onchocerciasis in South-western Nigeria. Entomological evaluation and heteroduplex assay (HDA) techniques were performed on flies caught in the eight capture points in the Ogun and Yewa river systems between October and December 2007, July and December 2008 and May and December 2009. A total of 5789 blackflies were caught on human bait, of which 727 (12.6%) flies were captured in 2007, 1723 (29.8%) in 2008 and 3339 (57.6%) in 2009. The majority of flies caught during the study were forest flies representing 90.3% of the total catch while savanna flies constituted 9.7%. Proportions of parous to nulliparous flies were low in all the catching points (31.1 and 68.9%, respectively). Of the 5789 flies dissected, 11 (0.2%) flies were infected with Onchocerca parasites with nine of the infected flies having L3 head parasites. The HDA results revealed that the Beffa form of S. soubrense was the dominant cytospecies present (87.1%) in all the capture sites when compared with 12.9% of S. damnosum s.s. The low level of infectivity of flies may therefore indicate a low transmission level of onchocerciasis in the communities along the Ogun and Yewa river systems. However, there is a need for constant surveillance on species composition and fly infectivity in the river systems along the borders of Nigeria–Benin Republic.

The exact taxonomic identity of members of the Simulium damnosum Theobald complex in Ethiopia was not known until recently. A cytotaxonomic analysis of larval chromosomes from the Kulfo River area in southern Ethiopia, from where the ‘Kulfo’ form was reported, and from the Awash River at Sodere in central Ethiopia has revealed the existence of two newly recognized species, namely Simulium kulfoense and Simulium soderense. Although three fixed and 11 floating inversions in the chromosomes of the ‘Kulfo’ form were described in an idiogram, in the present study four fixed and 17 floating inversions were observed, out of which only seven floating inversions were the same as Dunbar's, indicating that the remaining 10 inversions were either missed or misinterpreted. Though both species share many inversions, S. kulfoense sp. nov. has a diagnostic inversion on chromosome II, IIL-E6, with no sex-linked chromosomal arrangement, while S. soderense sp. nov. has a sex-linked inversion, IIS-3, which warrants it as a distinct species. Both of these new species also share many inversions with the recently described new species from south-western Ethiopia, Simulium kaffaense, including the fixed inversions IIL-E1 and E3. Since both the study sites are known to be free of onchocerciasis, and the flies are not anthropophilic, both the species are not presumed to be vectors of human onchocerciasis. The presence of IS-1 and a complex inversion possibly involving IL-3 strongly indicates that the two species belong to the ‘Nile’ phylogenetic group of S. damnosum s.l.