Long-lasting residual efficacy of Actellic®300CS and Icon®10CS on different surfaces against Anopheles stephensi, an invasive malaria vector.

Background: Anopheles stephensi, a malaria-transmitting mosquito species, has developed resistance to various insecticides such as DDT, Dieldrin, Malathion, and synthetic pyrethroids. To combat this issue, the World Health Organization (WHO) suggests using Actellic®300CS and Icon®10CS for Indoor Residual Spraying to tackle pyrethroid-resistant mosquitoes. The aim of this research project was to evaluate the susceptibility of An. stephensi to certain insecticides at the diagnostic concentration + intensity 5x diagnostic concentration (5XDC) assays in Iran and to study the lasting effectiveness of Actellic®300CS and Icon®10CS against this particular malaria vector.

Methods: This study assessed the susceptibility of An. stephensi populations in southern Iran to various insecticides, including deltamethrin 0.05%, DDT 4%, malathion 5%, bendiocarb 0.1%, a synergist assay with PBO 4% combined with deltamethrin 0.05%, and an intensity assay using 5x the diagnostic concentration of deltamethrin (0.25%) and bendiocarb 0.5%. Laboratory cone bioassay tests were conducted to determine the residual effectiveness of Actellic®300 and Icon®10CS insecticides on different surfaces commonly found in households, such as cement, mud, plaster, and wood. The tests were carried out following the WHO test kits and standard testing protocols.

Results: The An. stephensi populations in Bandar Abbas were found to be susceptible to malathion 5% and deltamethrin 0.25% (5XDC), but exhibited resistance to DDT, standard concentration of deltamethrin, and both standard and intensity concentrations of bendiocarb. In laboratory cone bioassay tests, An. stephensi mortality rates when exposed to Actellic®300CS and Icon®10CS on different surfaces remained consistently more than 80%. Actellic®300CS achieved more than 80% mortality on all substrates for the entire 300-day post-spraying period. Conversely, Icon®10CS maintained mortality rates more than 80% on plaster and wood surfaces for 165 days and on mud and cement surfaces for 270 days post-spraying. Both Actellic®300CS and Icon®10CS demonstrated 100% mortality within 72 h of each test on all surfaces throughout the entire 300-day post-spraying period.

Conclusion: The study shows the varying levels of resistance of An. stephensi Bandar Abbas population to different insecticides and demonstrates the consistent performance of Actellic®300CS in controlling these mosquitoes on various surfaces. The findings suggest that long-lasting CS formulations may be more effective for malaria vector control compared to the current options. Further research is needed to validate these findings in field settings and assess the impact of these insecticides on malaria transmission.