Association of knockdown resistance mutations with pyrethroid resistance in Aedes aegypti, a major arbovirus vector in Cameroon

Background: The development of insecticide resistance in Aedes mosquitoes has been reported in several African countries. However, information about the mechanisms involved remains scarce. This study aimed to address this issue by updating the resistance profile of A. aegypti and evaluating the role of known knockdown resistance (kdr) mutations in the observed phenotypic resistance in Ae. aegypti in Cameroon.

Methods: Larvae and pupae of Aedes were collected in 2022 in four sites in Cameroon and reared to adulthood. Adult mosquitoes were tested using World Health Organization (WHO) tube bioassays for pyrethroids, bendiocarb and fenitrothion, synergist assays with piperonyl butoxide (PBO) and WHO bottle tests for clothianidin following WHO recommendations. Dead and live mosquitoes after exposure to deltamethrin and permethrin insecticides were used for the genotyping of the F1534C, V1016I and V410L mutations, sequencing of fragments of the voltage-gated sodium channel (VGSC) gene and assessment their association with observed resistance.

Results: The analyses revealed that A. aegypti exhibited high resistance to all of the tested pyrethroids. Mortality rates ranged from 0% for alphacypermethrin 0.05% in Douala to 63.57% for deltamethrin 0.3% in Yaoundé. An increase in resistance was also observed for 0.1% bendiocarb, with mortality rates ranging from 50.54% in Douala to 68.31% in Garoua. Full susceptibility was observed with 1% fenitrothion. Partial or full recovery of mortality was reported following pre-exposure to a synergist. This suggests the involvement of cytochrome P450 genes in the observed resistance, although other mechanisms may also be involved. The F1534C, V1016I and V410L mutations were found in live and dead mosquitoes in Douala, Yaoundé and Bertoua. However, the V1016I and V410L mutations were more prevalent in alive mosquitoes than in dead ones, indicating an association between pyrethroid resistance and these mutations. After a 1 h exposure, clothianidin showed full susceptibility in samples from Bertoua, Douala and Garoua after 7 days of observation. In Yaoundé, probable resistance was observed with a mortality rate of 94.3%.

Conclusions: These findings provide evidence that metabolic and kdr resistance are both involved in A. aegypti resistance to insecticides in Cameroon. This should be considered when implementing arbovirus vector control strategies and insecticide resistance management in the country.