Personal protection of long lasting insecticide-treated nets in areas of Anopheles gambiae s.s. resistance to pyrethroids

Background: The development of mosquito nets pre-treated with insecticide, Long Lasting Impregnated Nets (LLINs) that last the life span of the net, is a solution to the difficulty of the reimpregnation of conventional nets. Even if they showed a good efficacy in control conditions, their efficacy in the field, particularly in areas with resistance of Anopheles gambiae to pyrethroids, is not well documented. This study compares wide (OlysetR)) and small (Permanet(R)) mesh LLINs in field conditions, using entomological parameters. Methods: The two LLINs were tested in a rice-growing area of south-western Burkina Faso ( West Africa) with year around high density of the main malaria vector An. gambiae s.s. In the study village (VK6), there is a mixed population of two molecular forms of An. gambiae, the S-form which dominates during the rainy season and the M-form which dominates the rest of the year. The two LLINs Olyset(R) and Permanet(R) were distributed in the village and 20 matched houses were selected for comparison with four houses without treated nets. Results: Mosquito entrance rate was ten fold higher in control houses than in houses with LLINs and there was no difference between the two net types. Among mosquitoes found in the houses, 36% were dead in LLIN houses compared to 0% in control houses. Blood feeding rate was 80% in control houses compared to 43% in LLIN houses. The type of net did not significantly impact any of these parameters. No mosquitoes were found inside Permanet(R), whereas dead or dying mosquitoes were collected inside the Olyset(R). More than 60% of mosquitoes found on top or inside the nets had had blood meals from cattle, as shown by ELISA analysis. Conclusion: The percentage of blood-fed mosquitoes in a bed net study does not necessarily determine net success. The efficacy of the two types of LLINs was comparable, during a period when the S-form of An. gambiae was carrying the kdr gene. Significantly higher numbers of mosquitoes were collected in control houses compared to intervention houses, indicating that the LLINs provided an additional deterrent effect, which enhanced their expected prevention capacity.