Profile and Characterization of Insecticide Resistance Mechanisms in the Anopheles gambiae s.l. Population of Sierra Leone

Background Information on the status of insecticide resistance in malaria vectors is critical for implementing effective malaria vector control. The Sierra Leone National Malaria Control Program, in collaboration with the PMI VectorLink project, assessed the resistance status to insecticides commonly used in public health, and associated resistance mechanisms in Anopheles gambiae s.l., the main vector of malaria in Sierra Leone. Methods The susceptibility of An. gambiae s.l. against pyrethroids with and without piperonyl butoxide (PBO), chlorfenapyr, clothianidin, bendiocarb and pirimiphos-methyl was evaluated in four districts of Sierra Leone in 2018 and 2019 using WHO and CDC bottle bioassay protocols. A subset of samples that were exposed to the insecticides were screened for molecular markers of insecticide resistance, knock-down resistance (kdr) L1014F, 1014S and N1575Y, and ( Ace-1 -G119S). Results Anopheles gambiae s.l. from all sites were resistant to the diagnostic doses of three pyrethroids: deltamethrin, permethrin and alpha-cypermethrin. Intensity of resistance to all three pyrethroids was high, with less than 95% mortality at 10X concentration. However, pre-exposure of An. gambiae s.l. to PBO increased overall mortality by 41.6%, 50.0% and 44.0% for deltamethrin, permethrin and alpha-cypermethrin, respectively. The vector was susceptible to chlorfenapyr, clothianidin and pirimiphos-methyl, while bendiocarb showed possible resistance. The frequency of kdr alleles was 98.2% for L1014F, 2.1% for 1014S and 8.9% for N1575Y, while the frequency of the Ace-1 G119S allele was 13.6%. Significant deviation from the Hardy-Weinberg equilibrium and deficiency of heterozygotes was detected only at the G119S locus of An. gambiae s.l. (p < 0.0001). Of the 191 An. gambiae s.l. that were molecularly identified to the species level, 81.7% were An. gambiae (95% CI 75.3–86.7), followed by An. coluzzii (17.8%, 95% CI (12.8–24.1) with one hybrid of An. gambiae/An. coluzzii 0.5%, 95% CI (0.03–3.3). Conclusion Malaria vectors were highly resistant to pyrethroids but exposure to PBO partially restored susceptibility in An. gambiae s.l. in Sierra Leone. Malaria vectors were susceptible to chlorfenapyr, clothianidin and pirimiphos-methyl with possible resistance to bendiocarb. These data informed the selection and distribution of ITN PBO in Sierra Leone’s mass campaigns in 2020 and selection of clothianidin for indoor residual spraying in 2021.