Deltamethrin resistance intensity and synergistic effect of piperonyl butoxide for combating Aedes aegypti in dengue fever endemic areas in Thailand

Insecticide resistance is threatening the effectiveness of efforts to control dengue vectors in Thailand. This study explores the levels of insecticide resistance in Aedes aegypti mosquitoes to deltamethrin. A study was carried out in dengue-endemic areas of Thailand to find the situation of insecticide susceptibility status of dengue vectors. Female Ae. aegypti mosquitoes were collected from 8 districts in different provinces and were exposed to various discriminating concentrations of deltamethrin (1×, 5×, and 10×) using the WHO insecticide resistance intensity bioassay. All tested populations of Ae. aegypti were found to be resistant to the WHO discriminating concentration of deltamethrin (0.03%). Mosquito populations collected from Surat Thani (SUT), Nakhon Si Thammarat (NST) and Ayutthaya (AYU) Provinces were resistant to deltamethrin exposure up to 10× the discriminating concentration suggesting the high resistant intensity. The effect of pre-exposure to the insecticide synergist, piperonyl butoxide (PBO), was also tested at the discriminating concentration of 0.03% deltamethrin. A significant increase in susceptibility to deltamethrin and knockdown percentage was observed in all populations of mosquitoes that were pre-exposed to PBO. These findings indicate that dengue control in Thailand with deltamethrin only could be threatened by high resistance in Ae. aegypti populations. It is recommended that vector control strategies incorporating PBO as an effective synergist can enhance the susceptibility of mosquitoes in the presence of deltamethrin resistance.