The frequencies of knockdown resistance mutations in phlebotomine sandflies under different degrees of indoor residual spraying

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The emergence of pyrethroid resistance in Phlebotomus sandflies is an urgent issue for vector control using indoor residual spraying (IRS). Two amino acid substitutions at codon 1014 (L1014F and L1014S) in the voltage-gated sodium channel (VGSC) gene have been reported in Phlebotomus argentipes, a major vector of visceral leishmaniasis (VL). Known as “knockdown resistance (kdr),” these substitutions confer pyrethroid resistance in various insect species. The frequency of the VGSC mutant allele was investigated in Phlebotomus and Sergentomyia species at two different IRS regimes: “long-term treated,” 12 rounds for seven years in Mymensingh, Bangladesh; “short-term treated,” four rounds for two years in Pabna, Bangladesh. In Mymensingh, the L1014F/S allele frequency was 100% in P. argentipes and 98% in S. babu babu. In Pabna, the frequency was 41% in P. argentipes. At other kdr sites (codons 1011, 1016, and 1020), the genotypes of all specimens in Bangladesh were wild-type homozygotes. This study showed that a long and frequent exposure to IRS is crucial for the development of genetic mutations in VGSCs, a higher kdr frequency, and pyrethroid resistance in Phlebotomus.