Climate Change Is Giving West Nile Virus More Days to Spread

Global warming is contributing to the increased spread of the West Nile virus across a large swath of the U.S. Read on for where the mosquito-borne disease is most prevalent, as well as what you can do to cut your risk.

West Nile Virus Spreads More Readily Because of Global Warming

Warmer temperatures at the hands of climate change are resulting in a growing threat of West Nile Virus in the U.S., according to a new study from Climate Central. The disease is the most transmissible when temperatures are warm but not sweltering. With summer starting earlier and lasting longer in recent years, it makes sense that many parts of the country are seeing a surge in this transmission.

The Climate Central study looked at how the average number of days with temperatures capable of spreading the virus has changed from 1970 to 2025. Known as "mosquito disease days," this is when transmission is at its peak. The researchers analyzed data from 239 cities in the U.S., finding that 95% of the communities saw more disease days on average each year over the 55-year period.

San Francisco saw the biggest jump, coming in with an average of 52 mosquito disease days per year. Reno, Nevada, was in second place at 50 days.

Only a dozen of the studied cities did not see an increase in mosquito days. These included the scorching cities of Phoenix, Las Vegas, and Austin.

Looking at specific regions, the Southeast, the Northeast, and the Northwest came in with the largest average increase in mosquito disease days dating back to 1970.

Scientists believe that West Nile virus transmission is highest between 73 and 79 degrees. There is a bit of variation in the range depending on the prominent mosquito species and other influential environmental factors.