Seasonal variation in onchocerciasis transmission by Simulium squamosum at perennial breeding sites in Togo

Analyses of data collected routinely by the World Health Organization Onchocerciasis Control Programme in West Africa showed that the transmission of onchocerciasis by Simulium squamosum at Amou-Oblo, in Togo, was maximal in the dry season. Mean monthly transmission potentials ranged from 10·4 in August to 519·6 in February. Infectivity in terms of numbers of L3 larvae in the head per 1000 parous flies was highest in March. Additional, more detailed, studies involving the staining of flies from Amou-Oblo, Djodji and Tinkiro confirmed these seasonal trends but also revealed significantly higher rates with L1/L2 larvae in the dry season. The results are discussed in relation to seasonal changes in fly numbers, fly longevity, fly size and man-fly contact rates.