Phenotypic and Genotypic Resistances Associated with Pyrethroid and Organophosphate in Aedes aegypti (Diptera: Culicidae)

Background For decades, reliance on insecticides for vector control has been a common approach in combating the yellow fever mosquito, Aedes aegypti (L.), and this approach has led to the development of insecticide resistance. This study investigates the phenotypic and genotypic resistance of Ae. aegypti to pyrethroid (permethrin and deltamethrin) and organophosphate (malathion and pirimiphos-methyl) across Malaysia, Thailand, Indonesia, and the USA. Methods Adult female Aedes aegypti were subjected to WHO-recommended insecticide bioassays to assess susceptibility to pyrethroids and organophosphates. Molecular analyses were performed to detect kdr mutations, while biochemical assays quantified metabolic enzyme activities. Results High resistance levels were observed in Malaysian and the US strains to both pyrethroids and organophosphates, with intermediate resistance in Thailand and susceptibility in Indonesia. Notably, new mutations T1520I and I1011M were detected in Ae. aegypti Malaysian populations, marking the first report of T1520I in the region. Additionally, V1016I was newly identified in Indonesian strains, highlighting emerging resistance trends. The coexistence of multiple kdr mutations (S989P, V1016G, F1534C, and T1520I) in Malaysian strains poses a significant challenge to vector control efforts. Interestingly, the Riverside strain from the USA exhibited up to a 10-fold increase in β-EST metabolic enzyme activity compared to the VCRU reference strain, indicating substantial metabolic resistance. In contrast, despite high phenotypic resistance, the Malaysian Hamna strain showed no significant increase in detoxifying enzymes, suggesting that kdr mutations alone may drive resistance in these populations. Furthermore, resistance in Thai strains was not associated with kdr mutations but rather with altered acetylcholinesterase and elevated GST activities, highlighting the diversity of resistance mechanisms. The study also identified multiple-loci mutations (triple and quadruple haplotypes) in Malaysian strains, suggesting an advanced stage of resistance evolution. Conclusions These findings highlight the importance of continuous surveillance and targeted vector control strategies in mitigating the spread of resistance. The detection of novel mutations and diverse resistance mechanisms emphasizes the adaptability of Ae. aegypti to insecticide pressure and the need for innovative approaches to maintain the efficacy of vector control measures.