210 - Evaluating the Impact of a Larvicide Product Change on Culex Mosquito Abundance and West Nile Virus Vector Index in the Chicago Area

Abstract: Effective larval control is a cornerstone of integrated vector management programs designed to reduce the public health burden of West Nile virus. In 2021, the North Shore Mosquito Abatement District in the Chicago area implemented a major operational change by switching its larvicide product. This study evaluates the impact of this intervention on Culex mosquito populations and West Nile virus transmission risk. Weekly surveillance data from the District during 2017-2024 were analyzed using an Interrupted Time Series Seasonal AutoRegressive Integrated Moving Average with Exogenous Variables model. The model accounted for seasonal variation and key meteorological covariates, including temperature, precipitation, humidity, and wind speed. Preliminary results indicate that following the 2021 product change, nightly Cx. mosquito abundance decreased by approximately 11.6% relative to model-based counterfactual forecasts. The West Nile virus Vector Index—calculated as the product of infection rate and mosquito abundance—also declined by 7.8%, though this metric showed greater variability due to fluctuations in infection rate estimates. The results showed: higher precipitation four weeks prior was associated with reduced mosquito abundance, while increased humidity and lower wind speed favored mosquito activity. These findings provide strong preliminary evidence that the larvicide product change effectively reduced Cx. mosquito abundance in this District. The analysis is being expanded to include data from the Northwest and Desplaines Valley Mosquito Abatement Districts using Difference-in-Differences and Synthetic Control frameworks to improve causal inference and evaluate broader regional impacts on West Nile virus risk.