The impact of climate change on transmission season length: West Nile virus as a case study

Abstract

BackgroundClimate change is accelerating the spread of temperature-sensitive vector-borne diseases such as West Nile virus (WNV), the most widespread mosquito-borne disease in the United States, with 2,400 reported cases in 2024. In New York State (NYS), where WNV first emerged in the U.S., mean temperatures have increased by 1.4{degrees}C since the early 1900s. Although temperature is a well-established driver of WNV transmission, its effect on transmission season length remains poorly quantified. This study examines whether WNV transmission seasons have lengthened in NYS and whether these changes are associated with increased disease risk, shifts in seasonal timing, and anthropogenic climate change. MethodsWe integrated daily county-level observational data from 1999-2024, including temperature, mosquito surveillance, and human case data, with climate model simulations spanning 1850-2024 to assess trends in transmission season length. FindingsBased on observed temperature suitability, the WNV transmission season in NYS has lengthened by an average of 20 days over the past 25 years, beginning 3.8 days earlier and ending 16.3 days later. Longer transmission seasons are positively associated with higher WNV prevalence in both mosquito vectors and humans. While such changes could occur in the absence of global warming, climate model analyses indicate that the observed increase in season length is 6.4 times more likely under historical climate forcing since 1900 than under pre-industrial conditions. InterpretationThese findings demonstrate that climate change is reshaping the phenology and burden of WNV transmission. FundingSupported by NIH, NSF, Stanford University, and the Fogarty International Center. Research in ContextO_ST_ABSEvidence before this studyC_ST_ABSWe searched Google Scholar for publications from 1999 to 2024 using the term "West Nile virus and season length," yielding 18,200 results. Previous studies focused on how climatic factors affect transmission and its seasonality in the United States, but none assessed how climate change influences transmission season duration. We analyzed over two decades of data to link extended WNV transmission seasons with increased vector and human prevalence. Additionally, we evaluated the causal role of climate warming using simulated climate data and counterfactual analyses. Added value of this studyUsing daily temperature and WNV case data from 1999- 2024, we found that the WNV transmission season in NYS has lengthened by an average of 20 days. The season now begins 3.8 days earlier and ends 16.3 days later. Longer seasons were associated with higher prevalence in mosquitoes and humans. Climate model simulations indicate that human-driven greenhouse gas emissions have significantly extended the WNV season, making observed trends in season length, onset, and conclusion 6, 2, and 16 times more likely, with implications for mosquito infection and human disease risk. Implications of all available evidenceOur findings suggest that climate change is lengthening WNV transmission seasons, a phenomenon likely relevant to other temperature-sensitive vector-borne diseases. As warming continues, extended transmission seasons may amplify risks for WNV and other vector-borne diseases, underscoring the need for climate-adaptive public health surveillance and intervention strategies.