West Nile virus's spread becoming easier with warming climate

Key Points:

• A new Climate Central study finds that 95% of 239 U.S. cities have experienced an increase in "mosquito disease days"—days with temperatures ideal for West Nile virus transmission—averaging 18 more days annually from 1970 to 2025.
• San Francisco and Reno, Nevada, are the most affected cities, with an average increase of 52 and 50 mosquito disease days per year, respectively, while regions like the Southeast, Northeast, and Northwest have seen the largest regional increases.
• The optimal temperature range for West Nile virus transmission is approximately 73 to 79 degrees Fahrenheit, with variations by city and mosquito species, as warmer temperatures speed up virus replication but shorten mosquito lifespans.
• To reduce risk, experts recommend using insect repellents (especially those with DEET or oil of lemon eucalyptus), wearing protective clothing during peak mosquito hours, eliminating standing water, and using natural mosquito-repelling plants or fans outdoors.
• Most West Nile virus infections are asymptomatic, but about 20% develop flu-like symptoms; less than 1% experience severe neurological effects, which can be life-threatening and require immediate medical attention.