The Residual Spraying of Dwellings with DDT in the Control of Malaria Transmission in Panama, with Special Reference to Anopheles Albimanus

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Abstract
Summary and Conclusions This paper reports the results of the first fifteen months study of the DDT residual house spray method in the middle Chagres River area of Panama, where the principal malaria vector is Anopheles albimanus. Houses of cane wall and palm-thatch roof construction at the village of Gatuncillo were sprayed, inside and out, with a 5% solution of DDT in kerosene at fourmonth intervals (excepting one trial period of six months). Entomological observations were made at this treated village and two adjacent villages used as controls. It is demonstrated that with this technique anophelines visiting the dwellings are affected in three ways: 1. There is a large reduction in numbers of mosquitoes. 2. Among the mosquitoes which are taken in treated dwellings there is a marked reduction in the per cent engorged, since DDT activates the insects and they lose interest in feeding. 3. Among the engorged mosquitoes the twenty-four hour survival rate is low for three months after treatment. It is thus indicated that the technique tends selectively to reduce the malaria transmission potential by affecting principally those mosquitoes concerned in transmission. Four months is established as the optimum time for retreatment in the area studied. It is demonstrated that with each successive treatment the degree of control improves. (For the period following the third treatment the malaria transmission potential is reduced 99.9%.) Treatment of houses alone was found to produce marked reduction of mosquitoes in the village area outside the houses, and even to some measure in the forest adjacent to the village (for at least three hundred feet), for several weeks. There is evidence of reduction of malaria in the fourteen-month period following the first treatment. The cost of the treatment under conditions of this experiment is calculated as $1.31 per house. Factors affecting the cost of applying the method elsewhere in tropical America are discussed. The place of this method in relation to other malaria control procedures and to the economy of the area is discussed.