West Nile virus activity confirmed in Colorado — first human cases reported

Mosquitoes carrying the virus detected in five counties; Coloradans urged to take precautions

Denver (July 22, 2025) — West Nile virus activity has officially begun in Colorado. Mosquitoes carrying the virus have been identified in Adams, Boulder, Denver, Larimer, and Weld counties. State health officials have confirmed the first human cases of West Nile virus this year in two residents of Adams County.

Mosquito season in Colorado typically runs from late spring through early fall, with the highest risk of West Nile virus transmission from July through September. When the virus shows up in mosquitoes, it signals that it’s time for people in the area to use precautions for the rest of the season.

While most people infected with West Nile virus don’t develop symptoms, the virus can cause serious, potentially life-threatening illness. People over 60 and those with certain medical conditions are at greater risk of complications. Seek medical care if you experience fever, severe headaches, or confusion 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 protect yourself:

• Use insect repellent with DEET, picaridin, IR3535, oil of lemon eucalyptus, or para-menthane-diol. Follow label instructions.
• Limit time outside at dawn and dusk, when mosquitoes that spread the virus are most active.
• Wear long sleeves, pants, and socks in areas with mosquito activity and apply repellent to clothing.
• Remove standing water from yards and gutters weekly. Treat water that can’t be drained with Bti larvicide. Install or repair window and door screens.

Colorado has experienced significant West Nile virus seasons in the past. In 2023, the state reported 634 human cases, including 51 deaths — the highest number of cases in the nation. Although 2024 was a milder year, the number of cases can vary widely from season to season, and human cases are a signal to stay alert.

Weekly data updates and additional information, including historical trends, are available on the CDPHE West Nile virus webpage.

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