Pyrethroid resistance reduces the efficacy of space sprays for dengue control on the island of Martinique (Caribbean).

BACKGROUND: Dengue fever is reemerging on the island of Martinique and is a serious threat for the human population. During dengue epidemics, adult Aedes aegypti control with pyrethroid space sprays is implemented in order to rapidly reduce transmission. Unfortunately, vector control programs are facing operational challenges with the emergence of pyrethroid resistant Ae. aegypti populations. METHODOLOGY/PRINCIPAL FINDINGS: To assess the impact of pyrethroid resistance on the efficacy of treatments, applications of deltamethrin and natural pyrethrins were performed with vehicle-mounted thermal foggers in 9 localities of Martinique, where Ae. aegypti populations are strongly resistant to pyrethroids. Efficacy was assessed by monitoring mortality rates of naturally resistant and laboratory susceptible mosquitoes placed in sentinel cages. Before, during and after spraying, larval and adult densities were estimated. Results showed high mortality rates of susceptible sentinel mosquitoes treated with deltamethrin while resistant mosquitoes exhibited very low mortality. There was no reduction of either larval or adult Ae. aegypti population densities after treatments. CONCLUSIONS/SIGNIFICANCE: This is the first documented evidence that pyrethroid resistance impedes dengue vector control using pyrethroid-based treatments. These results emphasize the need for alternative tools and strategies for dengue control programs.

Reduced efficacy of pyrethroid space sprays for dengue control in an area of Martinique with pyrethroid resistance.

In the Caribbean, insecticide resistance is widely developed in Aedes aegypti and represents a serious obstacle for dengue vector control. The efficacy of pyrethroid and organophosphate ultra-low volume space sprays was investigated in Martinique where Ae. aegypti has been shown to be resistant to conventional insecticides. In the laboratory, a wild-field caught population showed high levels of resistance to deltamethrin, organophosphate (naled), and pyrethrum. Simulated-field trials showed that this resistance can strongly reduce the knock-down effect and mortality of deltamethrin and synergized pyrethrins when applied by thermal-fogging. Conversely, the efficacy of naled was high against insecticide-resistant mosquitoes. Chemical analyses of nettings exposed to the treatments showed a decrease in residues over distance from release for the pyrethroids, and naled was not detected. This finding has important implications for dengue vector control and emphasizes the need to develop innovative strategies to maintain effective control of resistant Ae. aegypti populations.