Patterns of insecticide resistance in Aedes aegypti and Aedes albopictus in different cities across Cameroon

The increasing frequency of arboviral diseases' outbreaks in Cameroon necessitates evidence-based vector control. However, the efficacy of chemical control interventions is seriously threatened by the rapid evolution of insecticide resistance in Aedes vector populations. This study investigated the insecticide resistance profiles of Aedes aegypti and Aedes albopictus across ecologically diverse cities in Cameroon. Aedes larvae were collected from April to November 2024 in eight major cities representing diverse ecological landscapes across Cameroon: Yaoundé (Congo-Guinean equatorial forest); Douala and Kribi (coastal areas); Nkongsamba, Bafoussam, Mbouda, and Maroua (highland areas); and Bertoua (sub-tropical/savanna transition zone). World Health Organization (WHO) insecticide susceptibility bioassays were conducted on F1 adults against pyrethroids (0.75% permethrin, 0.05% deltamethrin), carbamates (0.1% propoxur, 0.1% bendiocarb), and organophosphates (0.05% pirimiphos-methyl). Molecular mechanisms potentially driving insecticide resistance were characterized using multiplex TaqMan qPCR to genotype kdr mutations and RT-qPCR to quantify the expression of detoxification genes such as cytochrome P450s, Glutathione S-transferases, and Carboxylesterases. Ae. aegypti displayed widespread resistance to pyrethroids, with mortality as low as 17.4% (permethrin, Kribi). This resistance could possibly be attributed to distinct geographic distribution of target-site mutations, with F1534C ranging from absent in Bafoussam to 95% in Kribi and Bertoua (alongside high frequencies in Yaoundé and Douala), and V1016I peaking at 46.3% in Bertoua while remaining absent in Bafoussam and Maroua. Furthermore, double-mutant homozygous haplotypes (CC-II) reached up to 15%, alongside massive metabolic upregulation, notably a 184.8-fold increase in GSTD4 expression in Bafoussam. In contrast, Ae albopictus populations appeared generally more susceptible across most sites, though localized resistance to permethrin and deltamethrin emerged in Douala and Nkongsamba. Susceptibility to bendiocarb and pirimiphos-methyl was observed across all sites for both species. In contrast, most of these populations were found to be resistant to propoxur. The continued susceptibility of Ae. albopictus in most regions, contrasted with its emerging resistance in urban centers, underscores the need for species-specific and geographically tailored management interventions. Overall, these findings emphasize the urgent need to deploy targeted larviciding approaches in breeding sites and integrate routine resistance monitoring to mitigate the spread of arboviral vectors across Cameroon.