Malaria in the pare area of Tanzania III The course of malaria transmission since the suspension of an experimental programme of residual insecticide spraying

Abstract

Malarious communities in South Pare, in northern Tanzania, were protected during a 3-year experimental programme of residual insecticide spraying which ended early in 1959. On the conclusion of the spraying programme a simple treatment organization was set up to soften the clinical impact of the inevitable resurgence of malaria. The results obtained by this organization are briefly reviewed. In addition, regular malaria parasite surveys have been carried out in an attempt to follow the course of malaria transmission in South Pare since 1959. Although much of the area was very malarious in former times, the resumption of intense malaria transmission there has been unexpectedly delayed. This delay has been attributed largely to changes brought about in the original vector mosquito populations, changes originating during the residual spraying campaign but persisting well beyond the interval during which the mosquitoes were exposed to the direct activity of the residual insecticide. For instance, the once important local vector, A. funestus, had failed to re-establish itself completely in the area as recently as mid-1966; furthermore, it now seems probable that the South Pare A. gambiae underwent selection during the residual spraying campaign, resulting in the ascendancy of a relatively exophilic population of the species which lingered for several years after selection pressure was lifted. A further factor that appears to be restraining any overall increase in malaria parasite rates, despite a recent rise in malaria risk, is the increased popularity of modern antimalarial drugs for the treatment of fever. In addition, there is a general public awareness of the value of these remedies in coping with the current seasonal epidemic waves of malaria transmission. This report covers a period of 7 years since the final round of residual insecticide spraying. As the epidemiological situation in South Pare is not yet stable, it is considered that further regular surveys there should prove worth while.