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 also reported the first human cases of West Nile virus this year in two residents of Adams County.
Mosquito season in Colorado generally spans from late spring through early fall. The risk of transmission is highest between July and September. When infected mosquitoes are found, health authorities recommend that people in affected areas take precautions for the remainder of the season.
Most individuals infected with West Nile virus do not develop symptoms; however, the infection can lead to serious illness or be life-threatening. People over 60 years old and those with certain medical conditions face a higher risk of complications. Medical attention should be sought if symptoms such as 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.”
Health officials advise using insect repellents containing DEET or other approved ingredients and following label instructions. Limiting outdoor activities during dawn and dusk—when mosquitoes are most active—wearing protective clothing treated with repellent, removing standing water weekly from yards and gutters, treating undrainable water with Bti larvicide, and maintaining window and door screens are recommended preventive measures.
Colorado has seen notable outbreaks of West Nile virus before; in 2023 there were 634 human cases—including 51 deaths—which was the highest case count nationwide according to state data (https://cdphe.colorado.gov/diseases-a-to-z/west-nile-virus). The number of cases fluctuates each year; although 2024 saw fewer infections than previous years, any occurrence of human cases serves as a reminder for continued vigilance.
The Colorado Department of Public Health and Environment provides weekly updates on case numbers along with historical trends on its official West Nile virus webpage (https://cdphe.colorado.gov/diseases-a-to-z/west-nile-virus).
