Deltamethrin resistance mechanisms in Culex quinquefasciatus populations in Thailand

Abstract

Resistance to pyrethroids in Culex quinquefasciatus, a lymphatic filariasis vector, has been reported in many countries including Thailand. Our previous study revealed that deltamethrin resistance in the Cx. quinquefasciatus mosquito is caused by at least two major mechanisms: L1014F knock down resistance (kdr) mutation in the voltage gated sodium channel (VGSC) gene and cytochrome P450s mediated-metabolic mechanism. This study aimed to develop a real-time PCR with melt curve analysis to evaluate the distribution of the L1014F mutation in Cx. quinquefasciatus throughout Thailand and to characterize the expression profiles of cytochrome P450s genes from mosquito strains of Cx. quinquefasciatus in response to deltamethrin phenotypes. A total of 3,760 females from 18 localities across five regions of Thailand were bio-assayed by exposure to 0.05% deltamethrin WHO papers, showing mortality rates ranging from 2.4% to 83.0%. Genotyping of 753 dead and surviving mosquitoes using our novel real-time PCR assay with melt curve analysis and tetra-primer AS-PCR revealed the mutant F1014 allele is closely associated with the deltamethrin resistance phenotype. The L1014F mutation was found at high frequency throughout Thailand, particularly in the North. However, some survivors were homozygous for wild type L1014 allele, which were further sequenced for the IIP-IIS6 region of VGSC gene, indicated the presence of other possible kdr alleles/resistance mechanisms at play including two novel mutations, V977E and D992E. The finding of new putative kdr alleles and widespread distribution of the F1014 allele emphasizes the significant role of kdr mutations in pyrethroid resistance in Thai Cx. quinquefasciatus populations. Monitoring kdr variations and phenotypic resistance is critical for managing resistance in Cx. quinquefasciatus.