Bolivian Amazon study finds using insect repellent between dusk and bedtime makes insecticide-treated bednets more effective

Using insect repellent in addition to insecticide treated bednets (ITNs) has been shown to provide greater protection against malaria in areas where mosquitoes feed in the early evening.

The findings of a study carried out by the team based at the London School of Hygiene & Tropical Medicine and working in collaboration with the National Bureau of Malaria Control at the Ministry of Health in La Paz, Bolivia, are published today in the British Medical Journal.

36% of the population of the Americas live in areas with a risk of malaria. This includes 293 million people in 21 endemic countries. Of the 1.14 million cases of malaria reported in the Americas during 2000, 87% were recorded in the Amazonian subregion of South America.

The primary malaria vector in the Amazon in anopheles Darlingi. Its peak biting activity is between 8 and 10pm, and more than 80% of feeding occurs before most local people to go bed, where they can be protected by an insecticide treated bednet. Tourists visiting the area are already advised to use repellents as well as bed nets. The researchers sought to determine whether this advice was justified, and whether the combination of repellents and ITNs is indeed more effective in reducing malaria than using ITNs alone.

4008 individuals, based in 860 households in rural villages and peri-urban districts in the Bolivian Amazon, were divided into two groups; all participants already slept under ITNs, but one group also used a plant-based insect repellent, and the second a placebo.

The result was an 80% reduction in episodes of p.vivax (the malaria parasite) in the group that used both ITNs and repellent.

Nigel Hill, a Medical Entomologist at the London School of Hygiene & Tropical Medicine, and lead author of the study, comments: 'Prior to this study, it was already known that ITNs are a highly effective way of reducing morbidity and mortality from malaria in Africa, where local vectors bite indoors, late at night. Insect repellents were also known to reduce bites, but the extent to which they were able to protect against insect-borne disease was less clear.

'The findings from this study tell us that treated bednets should not be used as the only means of treating malaria in areas where vectors feed mainly in the evening. Use of an insect repellent can significantly reduce the risk of malaria. The combined use of a repellent and a treated bednet should be advocated to those travelling to malaria risk areas'.