A large-scale field trial of malathion as an insecticide for antimalarial work in Southern Uganda.

Malathion shows promise as a substitute for chlorinated-hydrocarbon insecticides in the control of malaria whenever the latter are unsuitable because of Anopheles resistance or other reasons. A field trial of malathion was carried out in 1963-64, covering an area of about 500 km(2) with a population of about 26 000, in Masaka District, southern Uganda. All houses and animal shelters were sprayed with malathion at 2 g/m(2) at roughly 4-month intervals. The average combined densities of the females of the two main malaria vectors, Anopheles funestus and An. gambiae, fell from an average of 66 per shelter per day in a pre-trial survey in 1960-61 to 0.0011 at the end of 1964 in the sprayed area; no significant changes were noted in unsprayed comparison areas. The transmission of the infection in humans was apparently interrupted when allowance was made for imported cases.The presence of unsprayed surfaces in houses which had recently been built or altered interfered somewhat with complete coverage. Case detection was reliable and achieved excellent coverage.No toxic effects of malathion in humans were noted, while the effect on mosquitos was considerable even in the absence of direct contact. This effect of malathion lasted for a considerably shorter period of time in houses roofed with corrugated iron than with thatch; this should be borne in mind in the design of spraying programmes.