After more than thirty years of malaria control in northern Transvaal with residual insecticides, malaria prevalence has been reduced to a low level. However, low-grade transmission of Plasmodium falciparum continues, with periodic focal outbreaks after abnormally high rainfall. From October 1973 to September 1976, the operational and epidemiological factors involved in this residual transmission were studied in over 17 000 people of an area of northern Transvaal. Incidence surveys based on the screening of fever cases revealed 42 autochthonous cases of malaria in 1974-75 and 10 cases in 1975-76. Parasite prevalence surveys were not sensitive enough to assess the malaria situation, and serological testing indicated different levels of infection according to the method used. One of the two principal vectors of malaria in Africa—Anopheles funestus Giles—was not detected in the project area, and the A. gambiae group (species A and B) was found in extremely low numbers, so that it could not have accounted for the low-grade transmission in the area. A recently discovered member of the A. funestus group somewhat resembling A. aruni Sobti, and a species hitherto undiscovered in the Transvaal, which is abundant in the area and is indistinguishable from A. flavicosta Edwards, may be involved. Both were found biting man—mostly outdoors during the four hours following dusk, when people frequently gather outside their houses and are thus vulnerable to mosquito bites
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