A trial of insecticide treated nets for malaria control in Yanomami communities in Amazonas, Venezuela

A community randomized controlled trial was carried out in an area of moderate malaria transmission in the Amazon region, in South East Venezuela, where the indigenous Yanomami population live. The aim of the project was to assess the impact of lambdacyhalothrin treated hammock nets (ITHNs), compared to placebo treated hammock nets (PTHNs), on the malaria incidence rate and on the malaria vector population Anopheles darlingi. In both arms of the study intensive early case detection was performed and prompt malaria treatment administered. Baseline data were collected one year before the intervention and a population of around 924 Yanomami was followed for two years. Despite the recent introduction of nets in the Yanomami villages and the adverse natural conditions in the area, the majority of Yanomami showed high compliance and took good care of the nets. Analysis performed by gas chromatography of samples taken from the nets dried in different ways, i. e. vertically or horizontally, in the sun or in the shade, showed that there were no significant differences between methods with the only exception of drying the nets horizontally and in the sun performing significantly worse then the others. In addition bioassays, with Aedes aegypti, showed that hammock nets treated at village level with lambdacyhalothrin (10 mg/m2) and dried vertically in the sun were effectively killing mosquitoes (87%) after six months of regular use and the mortality rate was 83% after washing the nets twice. The malaria incidence rate per thousand person years at risk was 114.6 in the IHTNs group and 186.8 in the PTHNs group. The adjusted rate ratios indicated that ITHNs prevent 55% (IRR: 0.44, 95% CI: 52-59%) of new malaria cases. ITHNs reduced the prevalence of parasitaemia by 87% (RR: 0.17, 95% CI: 0-45%) in the first cross-sectional survey carried out during the high transmission season, six months after the intervention. The prevalence of splenomegaly and of anaemia was low in both groups, and there was no evidence of reduction due to ITHNs. There was little evidence of a mass killing effect on the density of the vector population, although significant differences between study arms were found when the analysis was carried out adjusting for baseline An. darlingi density. The density of An. darlingi was 62% less in villages with ITHNs than those with PTHNs (density ratio: 0.38, 95% CI 52-70%). The main conclusion of the present study is that ITHNs can reduce malaria incidence in the area and it is the most feasible method of malaria control in a forested area where indigenous villages are scattered over a large territory.