Assessment of insecticidal impact on the malaria mosquito's vectorial capacity, from data on the man-biting rate and age-composition.

The reduction in average age of a vector population after application of a residual insecticide is commonly interpreted under the implicit assumption that the vector population is uniformly exposed to the insecticide. This assumption maximizes the calculated impact of the insecticide on the vectorial capacity. An alternative assumption, namely that the vector population is composed of two subpopulations, one endophilic and exposed, the other exophilic and not exposed, leads to a much smaller calculated impact of insecticide on the vectorial capacity. This is illustrated with data collected before and after application of propoxur in the Garki District, Kano State, Nigeria. These data were also used to estimate, on the one hand, before spraying, the proportion of blood meals taken by Anopheles gambiae s.l. on man that are followed by rest indoors, and on the other hand, after spraying, the proportion exposed after the first blood meal, according to the model of non-uniform exposure; the two estimates agree fairly well. The discussion compares the models of uniform and non-uniform exposure in terms of plausibility and concludes that the latter is more realistic. It is therefore prudent to take it into account when evaluating the impact of insecticides on the transmission of malaria.