Background Bed nets are the main tools used in vector control of malaria. However, insecticide resistance is a looming threat on their efficacy and the gains obtained over the years. Thus, a better understanding of the resistance profile of vectors is a prerequisite towards the implementation of vector control measures adapted to local settings. We therefore aimed to evaluate the resistance of mosquitoes to various insecticides and the effect of the synergist piperonyl butoxide (PBO) on pyrethroid resistance.

Methods Anopheles gambiae s.l larvae were collected in Lambaréné and reared until adult emergence. The susceptibility of adult mosquitoes to deltamethrin, permethrin, bendiocarb and malathion was tested using the WHO protocol with additional testing performed for permethrin and deltamethrin with mosquitoes pre-exposed to PBO.

Results An. gambiae s.l. mosquitoes were resistant to permethrin 0.75% and deltamethrin 0.05% with mortalities of 11% and 72% respectively, after 24 hours. Resistance to permethrin was of high intensity with mortality of 47% with permethrin 3.75% and 88% with permethrin 7.5%. The combination PBO+permethrin 0.75% resulted in a 4-fold increase in mortality to 44%. The intensity of resistance to deltamethrin is considered moderate with a mortality with deltamethrin 2.5% of 86% in the tests performed. The combination PBO+deltamethrin 0.05% resulted in a complete recovery of susceptibility with a mortality of 100%. Finally, mosquitoes were resistant to bendiocarb and susceptible to malathion with mortalities of 75% and 100% respectively.

Conclusion The results obtained in this study confirm the high intensity of resistance of Anopheles to pyrethroids. However, the improvements observed with the use of PBO in terms of mortality rates suggest that second generation bed nets which are impregnated with PBO could be useful tools for vector control. These results also allow us to consider the use of malathion in combination with other insecticides to mitigate resistance.