Follow-up studies after withdrawal of deltamethrin spraying against Anopheles culicifacies and malaria incidence.

Follow-up studies were carried out from 1989 to 1998 after withdrawal of deltamethrin indoor spraying to evaluate the recovery rate of a population of Anopheles culicifacies resistant to dichlorodiphenyltrichloroethane (DDT) and hexachlorocyclohexane (HCH) in selected villages in Uttar Pradesh State, India. The study revealed 82.4-96.5% reduction in adult density of An. culicifacies and 72.7-96% reduction in malaria incidence in the area sprayed with deltamethrin at 20 mg/m2 as compared to a control area sprayed with HCH, for 5 successive years even after withdrawal of deltamethrin spray. The impact was very clear when the annual falciparum incidence was compared with that of the control area. The vector population gradually started recovering after 5 years. However, the slide falciparum rate remained below 4 even after 10 years of withdrawal of spraying. The study revealed that indoor residual spraying of deltamethrin would be cost-effective, at least in areas where malaria is transmitted by An. culicifacies, which is primarily a zoophilic species and associated with malaria epidemics. In view of this, a review of the insecticide policy and strategy of vector control is urgently needed because of the possible risks associated with the presence of nonbiodegradable insecticide in the environment, as well as to minimize the costs of operation and to enhance the useful life of insecticides.