The incidence of malaria in Solomon Islands has been declining since 1992, but there is a large geographical variation between areas in the incidence level and the rate of decline. The authors used a mix of control interventions, including DDT residual house spraying and insecticide-treated mosquito nets. Data on monthly incidence and control activities performed from January 1993 to August 1999 were gathered for 41 out of the 110 malaria zones in the country. Monthly reports on the number of fevers seen at outpatient health clinics in the same zones over the same period were also extracted from the clinical health information system. The authors used multivariate random effects regression, including calendar month as an instrumental variable, to investigate the relationship between the number of malaria or fever cases and the control measures applied by month and zone, while adjusting for rainfall and proximity to water. The results showed that DDT house spraying, insecticide treatment of nets, and education about malaria were all independently associated with reduction in incident cases of malaria or fever, while larviciding with temephos was not. This was true for confirmed malaria cases even when a variable representing the passage of time was included in the models. The results show how much each method used was contributing to malaria control in Solomon Islands and how it can be used to design the most cost-effective package of interventions. The evidence suggests that impregnated bednets cannot easily replace DDT spraying without substantial increase in incidence, but impregnated nets do permit a substantial reduction in the amount of DDT spraying.
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