Effect of Indoor Residual Spraying on malaria incidence in West Nile, Sub–Region, Uganda, 2017–2024: Time Series Analysis

Background Uganda introduced indoor residual spraying (IRS) in 2006 and gradually expanded it in high-burden regions. In 2022, IRS was extended to the West Nile sub-region, where malaria incidence surpassed 450 cases per 1,000 people, and test positivity rates exceeded 22%, despite ongoing interventions like insecticide-treated net (ITN) distribution. This study assessed the effect of IRS on monthly malaria incidence and test positivity rate (TPR) in nine districts of West Nile from 2017 to 2024. Methods We analyzed routine malaria surveillance data from monthly health facility reports across nine districts in West Nile, extracted from the District Health Information Software version 2 (DHIS2). We compared 70 months of data before IRS implementation (January 2017–October 2022) and 26 months after IRS introduction (November 2022–December 2024). Malaria incidence (cases per 1,000 population) and test positivity rates (TPR) were calculated. Interrupted time series analysis was performed using seasonal autoregressive integrated moving average (SARIMA) models, adjusted for temporal and seasonal variations. Results At the regional level, IRS was associated with a significant downward trend after IRS of − 1.9 cases/1,000 population/month (95% CI: − 3.8 to − 0.14; p = 0.03) and in test positivity rate (–1.6% per month; 95% CI: − 2.3 to − 0.93; p < 0.0001). District-specific analyses showed significant declines in incidence trends in Madi-Okollo, the trend decreased by 2.6 cases per 1,000 population per month − 2.6 (95% CI: − 4.9 to − 0.35; p = 0.024). In Maracha, − 2.23 (95% CI: − 3.60 to − 0.86; p = 0.001). In Terego, − 2.08 (95% CI: − 3.94 to − 0.23; p = 0.028). All nine districts recorded significant downward trends in test positivity rate. Conclusion IRS implementation in West Nile was associated with a significant reduction in malaria transmission over time, as reflected in both incidence and test positivity trends. These findings support the continued use and scale-up of IRS as a key vector control strategy in high-transmission settings, while addressing implementation variability and monitoring for insecticide resistance.