Evaluation of existing spatial repellents for the control of malaria vectors in rural Tanzania

Background: Current malaria vector control programmes rely on insecticides with rapid contact toxicity. However, spatial repellents can also be applied to reduce man-vector contact, with the ultimate goal of reducing malaria transmission. Objectives and methods: The overall goal of my PhD thesis was to evaluate existing spatial repellents as potential tools for malaria control. This thesis focused on characterizing the effect of pyrethroid spatial repellents on mosquito behaviour indoors and outdoors. Emphasis was placed on the effect on entomological parameters that influence malaria transmission. Experiments were conducted in experimental huts in a malaria endemic village in rural south eastern Tanzania and in a semi-field system against laboratory reared Anopheles gambiae sensu stricto mosquitoes. Results and conclusions: Transfluthrin and Metofluthrin coils and DDT reduced human vector contact through deterrence, irritancy/excito-repellency and blood-feeding inhibition. Pyrethroid coils were shown to cause excitation and increased activity of mosquitoes in the presence of humans. Transfluthrin coils did not hinder attraction of mosquitoes to humans but prevented mosquitoes from biting and blood feeding. This way coils provided area wide protection for up to 15m and prolonged anti-feeding for 12 hours. There was no evidence of Transfluthrin induced repellency (directional movement of mosquitoes away from humans) under outdoor conditions. Locally developed Transfluthrin hessian strips also prevented mosquitoes from biting. This thesis elucidates the mode of action of spatial repellents: spatial repellents reduce human – vector contact and induce mortality, hence directly affect ma: human biting rate, m: mosquito density and p: mosquito survival which are among the most important parameters of the vectorial capacity of a mosquito population. This information is critical for the development of target product profiles for spatial repellent products. This study shows that spatial repellents may be a suitable complementary option where mosquitoes feed in the early evening and rest outdoors.