Plant based insect repellent and insecticide treated bed nets to protect against malaria in areas of early evening biting vectors: double blind randomised placebo controlled clinical trial in the Bolivian Amazon.

OBJECTIVE: To determine the effectiveness in reducing malaria of combining an insect repellent with insecticide treated bed nets compared with the nets alone in an area where vector mosquitoes feed in the early evening. DESIGN: A double blind, placebo controlled cluster-randomised clinical study. SETTING: Rural villages and peri-urban districts in the Bolivian Amazon. PARTICIPANTS: 4008 individuals in 860 households. INTERVENTIONS: All individuals slept under treated nets; one group also used a plant based insect repellent each evening, a second group used placebo. MAIN OUTCOME MEASURE: Episodes of Plasmodium falciparum or P vivax malaria confirmed by rapid diagnostic test or blood slide, respectively. RESULTS: We analysed 15,174 person months at risk and found a highly significant 80% reduction in episodes of P vivax in the group that used treated nets and repellent (incidence rate ratio 0.20, 95% confidence interval 0.11 to 0.38, P<0.001). Numbers of P falciparum cases during the study were small and, after adjustment for age, an 82% protective effect was observed, although this was not significant (0.18, 0.02 to 1.40, P=0.10). Reported episodes of fever with any cause were reduced by 58% in the group that used repellent (0.42, 0.31 to 0.56, P<0.001). CONCLUSIONS: Insect repellents can provide protection against malaria. In areas where vectors feed in the early evening, effectiveness of treated nets can be significantly increased by using repellent between dusk and bedtime. This has important implications in malaria vector control programmes outside Africa and shows that the combined use of treated nets and insect repellents, as advocated for most tourists travelling to high risk areas, is fully justified. REGISTRATION: NCT 00144716.

Are mosquitoes diverted from repellent-using individuals to non-users? Results of a field study in Bolivia.

Outside sub-Saharan Africa, Anopheline mosquito exophagic and/or crepuscular behaviour patterns imply that insecticide-treated nets may provide incomplete protection from malaria-infective mosquito bites. Supplementary repellent treatment has been recommended in such circumstances, especially where vectors are exophilic and so are not susceptible to residual insecticide spraying. As maintaining complete usage of repellents in a community is unrealistic, the potential negative impact on non-users of repellent usage by 'neighbours' in the same community needs to be addressed in the context of health policy promoting equity. This study quantifies diversion of host-seeking mosquitoes, from repellent wearing to unprotected individuals, 1 m apart under field conditions in Bolivia. Each of the six volunteer-pairs sat >20 m apart from other pairs. Volunteers were allocated di-ethyl toluamide (DEET) or mineral oil in ethanol control. Treatments were rotated, so that during the trial, both pair-members wore repellent on 72 occasions; both pair-members wore control on 72 occasions; and on 36 occasions, one pair-member wore repellent and the other control. Unprotected (control) pair-members received 36.4% [95% confidence interval (CI): 8.1-72.0%] more Anopheles darlingi landings (P = 0.0096) and 20.4% (95% CI: 0.6-44.0%) more mosquito landings (P = 0.044), when their 'partner' wore repellent than when their partner also wore control. A second, smaller Latin-square trial using 30% lemongrass (Cymbopogon citratus) repellent, with control, obtained 26.0% (95% CI: 5.2-51.0%) more mosquito landings when controls sat with repellent-wearers rather than other controls (P = 0.0159). With incomplete community repellent usage, non-users could be put at an increased risk of malaria. The results also have implications for repellent-efficacy assay design, as protection will appear magnified when mosquitoes are given a choice between repellent-users and non-users.