Study links warming climate to more days favorable for West Nile virus transmission in U.S. cities
Research by Climate Central shows that 95% of 239 U.S. cities experienced an increase in days with temperatures optimal for West Nile virus spread, averaging 18 additional days per year since 1970.
A Climate Central analysis found that the number of days with temperatures conducive to West Nile virus transmission has risen in most U.S. cities as the climate warms. The study examined 239 cities from 1970 to 2025 and determined that 95% of them now experience, on average, 18 more "mosquito disease days" each year. San Francisco and Reno, Nevada, saw the largest increases, with about 52 and 50 additional days respectively. Only 12 cities, including Austin, Las Vegas and Phoenix, did not show a rise.
On a regional level, the Southeast, Northeast and Northwest each recorded an average increase of roughly 20 days since 1970, while the Southwest and south-central United States recorded the highest total numbers of favorable days. The optimal temperature range for West Nile virus transmission is estimated at 73 °F to 79 °F, though the study used city-specific ranges from 53.8 °F to 94.8 °F.
Public health officials advise using insect repellent containing DEET or oil of lemon eucalyptus, wearing protective clothing during dawn and dusk, and eliminating standing water to reduce mosquito breeding sites. Most West Nile infections are asymptomatic, but about 20% develop flu-like illness and less than 1% progress to severe neurological disease.
"The increase in temperature-suitable days underscores how climate change can amplify the risk of mosquito-borne diseases," a Climate Central spokesperson said.