Resistance of Anopheles gambiae s.s. against commonly used insecticides and implication of cytochrome P450 monooxygenase in resistance to pyrethroids in Lambaréné (Gabon)

Abstract

Background: Insecticides are a crucial component of vector control. However, resistance constitute a threat on their efficacy and the gains obtained over the years through malaria vector control. In Gabon, little data on phenotypic insecticide resistance in Anopheles vectors are published, compromising the rational implementation of resistance management strategies. We assessed the susceptibility to pyrethroids, carbamates and organophosphates of Anopheles gambiae sensu lato (s.l.) and discuss the mechanisms involved in the pyrethroid resistance-phenotype.

Methods: A. gambiae s.l. larvae were collected from breeding sites in Lambaréné. Emerging adults were used in WHO tube assays at an insecticide concentration that defines resistance (diagnostic concentration). Subsequently, deltamethrin and permethrin were used at 5x and 10x diagnostic concentrations and after preexposure with the cytochrome p450 (and glutathione S-transferase) inhibitor piperonyl butoxide (PBO). A subset of mosquitoes was typed by molecular methods and screened using Taqman assays for mutations conferring target site resistance at the Voltage-gated sodium channel 1014 (Vgsc-1014) locus and the acetylcholinesterase (Ace-1) gene.

Results: All mosquitoes were A. gambiae sensu stricto (s.s.) and resistant to permethrin, deltamethrin and alphacypermethrin (mortality less than 98%). However, mosquitoes were susceptible to malathion but resistant to bendiocarb. The level of resistance was high for permethrin and at least moderate for deltamethrin. Pre-exposure to PBO significantly increased the mortality of resistant mosquitoes (P < 0.0001). They became fully susceptible to deltamethrin and permethrin-induced mortality increased 4-fold. The G119S Ace-1 resistance allele, which confers resistance to both organophosphates and carbamates, was not present. All sampled mosquitoes were either homozygous for the Vgsc-L1014F or heterozygous for Vgsc-L1014F/L1014S, a marker for resistance to pyrethroids and organochlorides.

Conclusion: These findings demonstrate a role of cytochrome P450 monooxygenases in the pyrethroid-resistance of A. gambiae s.s. from Lambaréné. Combining PBO with pyrethroids, as done in second generation bednets, may be used to revert resistance. In addition, malathion could also be used in combination with pyrethroids-based methods for resistance management.

Keywords: A. gambiae s.s.; Cytochrome p450; Gabon; Pyrethroids; Resistance intensity.

Fig. 1

Map of the study area

Fig. 2

Susceptibility profile of A. gambiae s.s. to permethrin only and after preexposure to PBO. A Proportion of mosquitoes knocked down after exposure to various concentrations. B Mean mortality after exposure to various concentrations. The horizontal green line represents the 98% WHO threshold for susceptibility. Error bars represent standard error of mean mortality after the 24h recovery period following insecticide exposure

Fig. 3

Susceptibility profile of A. gambiae s.s. to deltamethrin only and after preexposure to PBO. A Proportion of mosquitoes knocked down after exposure to various concentrations. B Mean mortality after exposure to various concentrations. The horizontal green line represents the 98% WHO threshold for susceptibility. Error bars represent standard error of mean mortality after the 24h recovery period following insecticide exposure. Proportion of mosquitoes knocked down after exposure to various concentrations of deltamethrin

Fig. 4

Mean mortality of A. gambiae s.s. after exposure to diagnostic concentrations of malathion and bendiocarb. The horizontal green line represents the 98% WHO threshold for susceptibility. Error bars represent standard error of mean mortality after the 24h recovery period following insecticide exposure