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

Climate 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 US, 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. We integrated daily county-level observational data from 1999--2024, including temperature, mosquito surveillance, and human case data in a mixed-effects model, with climate model simulations spanning 1860--2024 to assess trends in transmission season length. Based on observed temperature suitability, the WNV transmission season has lengthened by an average of 19.7 days over the past 25 years and 29.6 days over the past 45 years, averaged across NYS. 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 7.1 times as likely under observed climate forcing since 1900 than under pre-industrial conditions.