Experimental hut and field evaluation of a metofluthrin based spatial repellent against pyrethroid resistant Anopheles funestus in Siaya County, western Kenya

Abstract

Abstract Background Sustained transmission of malaria, despite high coverage of indoor-based interventions (including long-lasting insecticidal nets and indoor residual spraying of insecticides), may be attributable to exposure of people to infectious bites outdoors or at times other than when people are sleeping under bed nets, or to insecticide resistance. Spatial repellents (SR) may complement current vector control tools and provide coverage under these conditions of residual transmission. Here we assessed the efficacy of a metofluthrin based SR in reducing exposure to pyrethroid-resistant Anopheles funestus in Siaya County, western Kenya. Methods The active ingredient, metofluthrin, was vaporized into the air by heat generated from an emanator configured to a liquid petroleum gas (LPG) canister, placed inside experimental huts (Phase 1) or outdoors (Phase 2). Phase 1 evaluated effects of combustion of LPG gas with no metofluthrin, as in use of an LPG cook stove indoors; or vaporization by LPG combustion of metofluthrin for 1, 2, 4, or 12 hours; on indoor mosquito density as measured by landing rate on humans and aspiration of mosquitoes from hut walls, as well as mosquito knockdown and mortality rates. Phase 2 evaluated the protective range of the SR product while emanating from the centre of a team of mosquito collectors sitting outdoors in north, south, east, and west directions at 1.5, 3 and 6 meters from the emanating device. Results Combustion of LPG with a cook stove increased density of Anopheles funestus indoors by 51% over controls with no cook stove. In contrast, huts with metofluthrin vaporized with LPG combustion had lower indoor densities of Anopheles funestus (99.3% less than controls), with knockdown and mortality rates of 95.5 and 87.7% respectively in the mosquitoes collected in the treated huts. In the outdoor study (Phase 2), the outdoor landing rate was significantly lower at 1.5 and 3 m compared to 6 m away from the emanator. Conclusion Vaporized metofluthrin almost completely prevented An. funestus landing indoors and led to 10 times lower landing rates within 10ft of the emanator outdoors, the first product to demonstrate such potential. Cooking with LPG inside the house could increase exposure to Anopheles mosquito bites but the use of the metofluthrin canister eliminates this risk.