The unusual occurrence of savanna members of the Simulium damnosum species complex (Diptera: Simuliidae) in southern Sierra Leone in 1988

All available identifications of sibling species of the Simulium damnosum complex in southern Sierra Leone for the years 1983–1987 have been analysed for any evidence that they might provide on whether the appearance of S. sirbanum in this area in 1988 is indicative of a gradual southerly extension of range, or a unique occurrence. No evidence was found to suggest a long-term immigration trend over the five years prior to 1988, and the latter of the two possibilities is the more likely. No meteorological or other factors are known that might account for the sudden appearance of S. sirbanum in southern Sierra Leone in 1988, but the possible causes of such immigration into forested parts of West Africa and their epidemiological implications for human onchocerciasis in the region are briefly discussed.