A guide to the Simulium damnosum complex (Diptera: Simuliidae) in Nigeria, with a cytotaxonomic key for the identification of the sibling species

Abstract

Although approximately 40% of all the people blinded by Onchocerca volvulus are Nigerians, almost nothing was known about the various cytospecies of the blackfly vectors present in Nigeria until 1981. The activation of the Nigerian National Onchocerciasis Control Programme in 1986 (and that programme’s initiation of mass distributions of ivermectin in 1991) provided a significant stimulus to understand the biology of the Nigerian vectors but the exploration of any possible differences between the cytospecies has been hampered by a lack of accessible taxonomic information. This review attempts to satisfy that need. There are nine different cytoforms reliably recorded from Nigeria (Simulium damnosum s.s. Nile form, S. damnosum s.s. Volta form, S. sirbanum Sirba form, S. sirbanum Sudanense form, S. soubrense Beffa form, S. squamosum A, S. squamosum B, S. squamosum C and S. yahense typical form), and three more are known from surrounding countries and might be reasonably expected to occur in Nigeria. All of these cytospecies are presumed to be vectors, although there have been almost no identifications of the vectors of O. volvulus in Nigeria. The biogeographical distribution of the cytoforms is broadly similar to that known in other parts of West Africa (although many of the cytoforms remain insufficiently studied). The physico–chemical hydrology of the Nigerian breeding sites of the cytospecies does not, however, correspond to that seen elsewhere in West Africa, and it is not clear whether this might be related to differences in the cytoforms. An illustrated cytotaxonomic key is presented to facilitate and encourage future studies.

This work was partly funded by the British Medical Research Council (via project grant 77615) and partly by TDR — the United Nations Children’s Fund/United Nations Development Programme/World Bank/World Health Organization’s Special Programme for Research and Training in Tropical Diseases — via a research institute strengthening grant (ID900314), a research training grant (ID880037), a re-entry grant (ID930332) and a research project grant (ID950612). The authors thank Dr M. Traore-Lamizana, for Figures 11 and 12, and Z. Adams, for her technical and statistical assistance.

Figure 11. 1C long split from a male Simulium squamosum ‘A’.

Figure 12. 1C short split from a male Simulium squamosum ‘B’.