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 extends from late spring through early fall. The risk of transmission is highest between July and September. The presence of the virus in mosquitoes indicates that residents should take preventive measures for the remainder of the season.
Most people infected with West Nile virus do not develop symptoms. However, some may experience severe illness that can be life-threatening. Individuals over 60 and those with certain medical conditions face a higher risk of complications. Health authorities advise seeking medical attention 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.”
The public is encouraged to use insect repellent containing DEET or other approved ingredients and follow label instructions; limit outdoor activities at dawn and dusk when mosquitoes are most active; wear protective clothing; remove standing water from yards and gutters weekly; treat undrainable water with Bti larvicide; and ensure window and door screens are installed or repaired.
Colorado has seen significant outbreaks in previous years. In 2023, there were 634 reported human cases statewide—more than any other state—and 51 deaths linked to West Nile virus (https://cdphe.colorado.gov/diseases-a-to-z/west-nile-virus). The number of cases fluctuates each year.
For ongoing updates and historical data on West Nile virus activity in Colorado, visit the CDPHE West Nile virus webpage (https://cdphe.colorado.gov/diseases-a-to-z/west-nile-virus).
