Studies on the reinvasion of the Onchocerciasis Control Programme in the Volta River Basin by Simulium damnosum s.I. with emphasis on the south-western areas.

The aerial larvicidng operation of the Onchocerciasis Control Programme of the World Health Organization which began in February 1975 resulted in a sharp reduction in Simulium damnosum numbers. However, at the onset of the rainy season the fly population increased in certain areas. Detailed surveys both on the ground and by helicopter did not reveal any significant failures of treatments that could account for the fly densities observed, and it was concluded that the flies must originate from sources outside the controlled zone. This reinvasion proved to be an annual occurrence which has been studied in detail between 1975 and 1978 in the south-western parts of the control zone. The methods used included full day catches by vector collectors carried out every day throughout the season, cytotaxonomic determination of larvae, detailed morphological examination of reared and biting adults and treatment of suspected source rivers with insecticide. The results indicate that the invasion takes place in a SW-NE direction across country for distances of 300 km or more along the track of the monsoon winds. It involves mainly the savanna cytospecies S. damnosum s. str. and S. sirbanum. The invading populations are composed of older parous flies, many of which carry infective 3rd stage larvae indistinguishable from those of Onchocerca volvulus. These females tend to bite close to the rivers and do not disperse as far as normal populations with a higher proportion of younger flies.