Insecticide Resistance Profile and Mechanisms in An. gambiae s.l. from Ebolowa, South Cameroon

SIMPLE SUMMARY: Monitoring insecticide resistance can help designing strategies to delay or prevent its onset and spread in vector populations. This study aimed at evaluating levels and mechanisms responsible of insecticide resistance in the major malaria vector Anopheles gambiae s.l. from Ebolowa (South Cameroon). Methods: Mosquito were collected in temporary water pools as larvae and reared to adult stages, which were tested against carbamate, organophosphate and pyrethroid insecticides as recommended by the World Health Organization (WHO). The implication of DNA mutations and enzymes that allow mosquitoes to survive insecticide exposure was also investigated. Results: Anopheles coluzzii was the predominant (99%) species in Ebolowa and was fully sensitive to carbamates and organophosphates, but highly resistant to the pyrethroids. Pre-exposure to the piperonyl butoxide (PBO), an inhibitor of detoxification enzymes, allowed deltamethrin to recover its full efficacy, but not permethrin and alphacypermethrin. Only the L1014F (kdr-West) mutation was present, and at a high frequency (75%), likely causing resistance to permethrin and alphacypermethrin, but not deltamethrin. Conclusion: The increased resistance pyrethroids in An. gambiae s.l. from Ebolowa could jeopardize the efficacy of Long-Lasting Insecticidal Nets (LLINs) used for malaria vector control. There is an urgent need to put in place resistance management strategies in this locality. ABSTRACT: Monitoring the trend of insecticide resistance and understanding associated genetic mechanisms is important for designing efficient malaria vector control strategies. This study was conducted to provide temporal data on insecticide resistance status and mechanisms in the major malaria vector Anopheles gambiae s.l. from Ebolowa, Southern Cameroon. Methods: Larvae of An. gambiae s.l. were collected from typical breeding sites throughout the city and reared to adulthood. Emerging adults were morphologically identified and WHO tube assays were performed to determine their susceptibility to carbamate, organophosphate and pyrethroid insecticides at diagnostic doses. When resistance was observed, its intensity was determined by performing WHO tube tests using 5 and 10 times the concentration of the diagnostic dose. Metabolic resistance mechanisms were investigated using insecticide-synergist assays. Sibling species of the An. gambiae complex were identified using SINE-PCR protocol. TaqMan assay was used to genotype the L1014F and L1014S kdr mutations, and the N1575Y mutation, an amplifier of the resistance conferred by the L1014F mutation. Results: Anopheles coluzzii was by far the dominant (99%) member of the An. gambiae s.l. complex in Ebolowa. The species was fully susceptible to carbamates and organophosphates, but resistant to all pyrethroid insecticides tested. Resistance was of moderate intensity for deltamethrin (mortality: 37%, 70% and 99% for 1×, 5× and 10× insecticide concentration, respectively) but rather of high intensity for permethrin (5% for 1×; 62% for 5× and 75% for 10×) and for alphacypermethrin (4.4% for 1×; 57% for 5× and 80% for 10×). Pre-exposure to the synergist PBO resulted in a full recovery of the susceptibility to delthametrin, but this was not observed for the other two pyrethroids tested. L1014S (kdr-East) and the N1575Y mutations were absent, whereas the L1014F (kdr-West) mutation was present at a high frequency (75%), showing a significant association with resistance to permethrin (OR = 3.8; 95%; CI [1.9–7.4]; p < 0.0001) and alphacypermethrin (OR = 3; 95%; CI [1.6–5.4]; p = 0.0002). Conclusion: The increased resistance of An. gambiae s.l. to pyrethroid insecticides as observed in Ebolowa poses a threat to the efficacy of LLINs used to protect populations from the bites of Anopheles mosquitoes that transmit malaria parasites. The present study further highlights the urgent need to implement resistance management strategies in order to maintain the effectiveness of insecticide-based vector control interventions and prevent a rebound in malaria-related mortality.