Decision-making for indoor residual spraying in the post-elimination phase of visceral leishmaniasis in Nepal

BACKGROUND: The effectiveness of indoor residual spraying (IRS) during the post elimination phase of visceral leishmaniasis (VL) and when to stop its application is uncertain. This study investigated the relationship between VL occurrence and frequency of IRS on vector density, infection rates, and insecticide susceptibility. METHODOLOGY/PRINCIPAL FINDINGS: Four villages in the Sarlahi district served as sentinel surveillance sites for sandfly density measurement and xeno-monitoring, selected based on VL endemicity levels of high, moderate, low and non-endemic (no reported cases in the past 10 years). A random sub-sample of households from each village was selected for sandfly surveillance. The sample size of 380 was determined to detect a 1% infection rate of sandflies with 95% confidence interval. Ecological and epidemiological data were collected and IRS activity data between 2012-2023 was analysed. Sandflies were collected using CDC light traps and mouth aspirators for 12 months from March 2023 to February 2024 and tested with PCR for kDNA of Leishmania donovani. Monthly density of female Phlebotomus argentipes sandflies varied across endemicity levels, with a peak in November, with fluctuations observed throughout the year. Village wise sandfly pools positivity with parasite DNA varied with level of endemicity (66.7% in high VL endemicity villages, 62.1% in moderate, 36.8% in low, and 26.3% in non-endemic villages). Overall, among the total 91 pools of sandflies tested, 50.5% were positive for parasite DNA. P. argentipes showed high susceptibility to insecticides alpha-cypermethrin, bendiocarb, deltamethrin, and malathion. There were differences in IRS applications with variations in coverage and frequency and programmatic factors across municipalities, with no IRS conducted in some villages. Occupational distribution varied across endemicity levels, and there were differences in sleeping habits during warm weather. Non-impregnated bed nets were available across all endemicity levels. CONCLUSIONS/SIGNIFICANCE: IRS should be continued in the villages based on surveillance of sandfly density and reports of VL cases in the post elimination phase of VL.