More Mosquito Disease Days in 95% of Major U.S. Cities

Climate Central's analysis of 239 major U.S. cities from 1970 to 2025 found that 95% of these cities experienced an increase in 'mosquito disease days'—days with temperatures suitable for West Nile virus transmission. On average, these cities saw an increase of 18 days per year. The largest increases occurred in the Southeast, Northeast, and Northwest regions, with San Francisco, CA, seeing the highest increase of 52 days. The report also notes that while West Nile virus is the most common mosquito-borne disease in the U.S., warming is expanding the risk and range of other viruses like dengue and Zika, particularly along the Gulf Coast and into the Ohio Valley and Upper Midwest by 2050.

• Between 1970 and 2025, 95% (227) of the 239 analyzed U.S. cities saw an increase in annual mosquito disease days, which are days with average temperatures suitable for the transmission of West Nile virus. These cities experienced an average increase of 18 days compared to the early 1970s.
• The most significant increases in annual mosquito disease days were recorded in San Francisco, CA (52 days), Reno, NV (50 days), Santa Maria, CA (49 days), Albany, GA (41 days), and Salinas, CA (40 days). Regionally, the Southeast, Northeast, and Northwest saw the largest average increases, each adding 20 days since 1970.
• From 1996 to 2025, the U.S. regions with the highest average annual mosquito disease days were the South (262 days, 72% of the year) and the West (261 days, 71% of the year).
• Warming is projected to expand the risk of Aedes-transmitted viruses, such as dengue and Zika, from the South and Southeast into the Ohio Valley, Upper Midwest, and East Coast by 2050. Specifically, significant dengue risk expansion is projected throughout Gulf Coast states by 2080, and a 2026 study suggests 4 million more California residents could face dengue transmission risks.
• Globally, rising temperatures are expected to lengthen transmission seasons and expand the geographic range of malaria and dengue. An estimated 18% of dengue cases in Asia and the Americas between 1995 and 2014 are attributed to warming that has already occurred.

APA

Climate Central (2026). More Mosquito Disease Days in 95% of Major U.S. Cities. https://www.climatecentral.org/climate-matters/mosquito-disease-days

Chicago

Climate Central. More Mosquito Disease Days in 95% of Major U.S. Cities. 2026. https://www.climatecentral.org/climate-matters/mosquito-disease-days.

Wikipedia

{{cite report |author=Climate Central |title=More Mosquito Disease Days in 95% of Major U.S. Cities |date=22 July 2026 |url=https://www.climatecentral.org/climate-matters/mosquito-disease-days |access-date=23 October 2026 |via=Climate Insights Directory}}

BibTeX

@techreport{climatecentral2026more, author = {{Climate Central}}, title = {{More Mosquito Disease Days in 95\% of Major U.S. Cities}}, institution = {Climate Central}, year = {2026}, month = jul, url = {https://www.climatecentral.org/climate-matters/mosquito-disease-days}, urldate = {2026-10-23}, note = {Indexed by Climate Insights Directory} }