Insecticide Susceptibility of Phlebotomus argentipes and Residual Action of Alpha-Cypermethrin in India: A Multicenter Study

Visceral leishmaniasis (VL), a potentially fatal disease caused by Leishmania donovani and transmitted by Phlebotomus argentipes sand flies, remains a public health issue in India, as evidenced by 385 cases reported in 2024. Despite low incidence, sporadic fluctuations necessitate vigilant vector control. Indoor residual spraying (IRS) is the principal strategy for controlling Ph. argentipes. After resistance to dichloro-diphenyl-trichloroethane (DDT) became widespread, alpha-cypermethrin IRS replaced it in 2016 for VL control in India. Although prior studies have assessed sand fly susceptibility to insecticides at WHO-recommended discriminatory concentrations, the present study is the first to simultaneously evaluate the residual efficacy of alpha-cypermethrin 5% wettable powder on common household substrates across three VL-endemic Indian states. From 2023 to 2024, three Indian Council of Medical Research institutes in Bihar, Jharkhand, and Uttar Pradesh assessed the susceptibility of wild Ph. argentipes to DDT (4.0%), malathion (5.0%), alpha-cypermethrin (0.05%, 0.1%), and deltamethrin (0.05%) using WHO tube tests. Residual efficacy of alpha-cypermethrin (25 mg/m2) was tested on cement concrete, limewashed cement, mud, and wood. Ph. argentipes were resistant to DDT but susceptible to malathion, deltamethrin, and alpha-cypermethrin. Residual efficacy of alpha-cypermethrin dropped below 80% 1-month post spray on cement and limewashed cement, and after 3 months on mud. Mortality remained 100% for 4 months on wood. These results support the use of one to two rounds of alpha-cypermethrin IRS to interrupt VL transmission in Indian villages, given wall substrate types and seasonal transmission patterns.