The relative importance of the 3 known vectors of Bancroftian filariasis in East Africa was evaluated on an annual basis at 3 contrasting localities in north-eastern Tanzania. In the urban parts of Tanga, a coastal town, the only significant filariasis vector was C. p. fatigans. This mosquito was responsible for about 10,000 bites per person per annum, with an infectivity rate of about 0·23%. In the rural district around Muheza, some distance inland, all 3 filariasis vectors were active. Their annual mean biting densities per person at the village of Ubwari were 1,591 A. funestus, 2,971 A. gambiae A. and 3,058 C. fatigans, with calculated infectivity rates of 6·1%, 2·4% and 0·54% respectively. The third locality, further inland at Gonja Maore, receives only about two-thirds as much rainfall annually as do Tanga and Muheza. At Gonja, A. funestus and A. gambiae (mainly species B) were found to be filariasis vectors, with annual mean biting densities of 1,250 A. funestus and 5,963 A. gambiae per person, and infectivity rates of 3·5% and 0·9%. Densities of C. fatigans at Gonja averaged 3,603 bites per person per annum, but infective filarial larvae were not observed in any of 360 specimens dissected. From these data it was calculated that during 1969 the mean personal exposure to the risk of infection with 3rd-stage larvae of Wuchereria bancrofti was due to the bites of 23 ± 15 infective mosquitoes at Tanga, to the bites of 185 ± 84 infective mosquitoes at Muheza Ubwari and to the bites of 95 ± 66 infective mosquitoes at Gonja Maore. The apparent risks of filariasis transmission at Muheza were due 52% to A. funestus, 40% to A. gambiae A. and 8% to C. fatigans. At Gonja the risks of transmission were due 46% to A. funestus and 54% to A. gambiae (principally species B). These data are discussed. Finally it is pointed out that although the anopheline vectors of filariasis in rural areas present an intractable problem, it should be relatively easy to reduce risks of filariasis transmission in urban areas through control of C. fatigans. © 1971.
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