Characterization of deltamethrin resistance in field populations of Aedes aegypti in Thailand.

Five field collections of adult Aedes aegypti mosquitoes from different areas in Bangkok and Pathum Thani provinces were subjected to susceptibility tests against deltamethrin. Low levels of resistance were detected among all populations tested (RR50 = 8-17.2) compared to the susceptible strain, Bora (French Polynesia). Among the five populations tested, the BKH (Bang Khen, Bangkok) and PSC (Phasicharoen, Bangkok) populations showed a higher level of deltamethrin resistance than the other three populations (RR50 of BKH = 17.2, and of PSC= 13.6) and cross-resistance to DDT was observed in these strains. Biochemical analysis showed a significant elevation of mixed function oxidases enzyme activity in all populations. There was an elevation of non-specific esterases in all populations except BKL, and there was no consistent association of glutathione S-transferases with deltamethrin and DDT resistance, although not all populations were bioassayed for DDT. The partial cDNA sequence of the para-type voltage-dependent sodium channel (IIS4-IIS6) was determined for BKH and PSC populations. Common amino acid substitution, leucine to phenylalanine in the IIS6 region, found for insects including Anopheles gambiae was not found in either the BKH or the PSC populations. However, two other amino acid substitutions (proline substituted with serine at position 64 in the PSC population and leucine with phenylalanine at position 69 in the BKH population) were found in the IIS5-IIS6 inter-segment region sequenced. The role these substitutions play in target site resistance is uncertain at this time.