West Nile virus activity has been confirmed in Colorado, with mosquitoes carrying the virus detected in Adams, Boulder, Denver, Larimer, and Weld counties. State health officials have reported the first human cases of West Nile virus this year in two residents of Adams County.
Mosquito season in Colorado generally lasts from late spring through early fall. The risk of West Nile virus transmission is highest between July and September. Health authorities advise that when the virus is found in local mosquito populations, people should begin taking precautions for the remainder of the season.
Most individuals infected with West Nile virus do not develop symptoms. However, some may experience serious or potentially life-threatening illness. Those over 60 years old and people with certain medical conditions face a higher risk of complications. Officials recommend seeking medical attention if fever, severe headaches, or confusion occur after a mosquito bite.
“Mosquitoes are more than just an annoyance — they can pose a real danger,” said Dr. Ned Calonge, Chief Medical Officer at the Colorado Department of Public Health and Environment. “We want people to get outside this summer — it’s Colorado, after all. Spend time outdoors, connect with others, and enjoy the season. But we also want everyone to take simple steps to stay safe from West Nile virus. A little prevention goes a long way.”
To reduce risk of infection, state health officials recommend using insect repellent containing DEET or other approved ingredients as directed on product labels; limiting outdoor activities at dawn and dusk when mosquitoes are most active; wearing long sleeves, pants and socks; applying repellent to clothing; removing standing water weekly from yards and gutters; treating undrainable water with Bti larvicide; and installing or repairing window and door screens.
Colorado has seen significant outbreaks of West Nile virus before. In 2023, there were 634 reported human cases statewide—more than any other state—and 51 deaths linked to the disease. While case numbers were lower in 2024, officials caution that totals can fluctuate greatly each year.
The Colorado Department of Public Health and Environment provides weekly updates on West Nile virus data along with historical trends on its official webpage (https://cdphe.colorado.gov/diseases-a-to-z/west-nile-virus).
