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

Climate change is accelerating the spread of vector-borne diseases like West Nile virus (WNV), which is highly temperature-sensitive. WNV is the most widespread mosquito-borne disease in the continental United States, with over 2,400 reported cases in 2024. In New York State (NYS), where WNV first emerged in the US, temperatures have risen over 1.4°C since the early 1900s. While temperature's role in WNV transmission is established, its effect on season length is less clear. This study asks: (1) Has the WNV season lengthened in NYS over the past 25 years? (2) Is a longer season linked to higher WNV incidence in mosquitoes and humans? and (3) Are these changes associated with shifts in the timing of infection onset and termination in hosts? We integrated daily county-level temperature, mosquito surveillance, and human case data from 1999-2024. Our results show that based on temperature suitability, the WNV season has extended by an average of 24.8 days-starting 4 days earlier and ending 20 days later. Longer seasons are positively associated with greater WNV prevalence in both mosquitoes and people. These findings underscore how climate change is reshaping the phenology and burden of vector-borne disease. Many vector-borne diseases may face amplified risks as transmission seasons lengthen, highlighting the need for adaptive public health responses.