First positive case of West Nile Virus in Weld County

Published on August 31, 2026

Close-up of a mosquito perched on a green leaf with a blurred natural background.

The Weld County Department of Public Health and Environment (WCDPHE) has been notified of a case of West Nile Virus (WNV) in a Weld County resident. This is the first case of the 2026 mosquito season in Weld County and comes a month later than in 2025. Last year there were 34 cases of WNV investigated in Weld County.

This year, the number of culex mosquitos (those that transmit WNV) trapped is lower than the 5-year average. However, culex mosquitoes are currently making up approximately 80% of mosquitoes that have been trapped in the three sentinel zones in Weld County, which includes the Greeley, Evans, Kersey, LaSalle area, the Johnstown, Milliken, and Platteville area, and the Firestone, Dacono, and Fort Lupton area. These mosquito traps are monitored on a weekly basis and mosquitoes in all three zones have tested positive for WNV. 

Symptoms of WNV can appear three to 14 days after infection. While most infected people do not show any symptoms, initial symptoms can include fever, headache, nausea and vomiting, muscle aches and weakness, and rash. If a person develops symptoms, they should see a health care professional immediately. There are no medications to treat or vaccines to prevent West Nile virus infection. Health officials recommend the public stay vigilant as WNV has a permanent summer presence in Colorado and follow the four “D’s” to prevent mosquito bites:

  • DRAIN standing water around your house weekly. Remember to drain water from tires, cans, flowerpots, clogged rain gutters, rain barrels, toys, and puddles.
  • DUSK and DAWN are when Culex mosquitoes are most active. Limit outdoor activities and take precautions to prevent mosquito bites during these times.
  • DEET is an effective ingredient to look for in insect repellents. Other repellents containing picaridin, IR3535, oil of lemon eucalyptus, or para-menthane-diol are also effective against mosquitoes. Always follow the product label instructions carefully.
  • DRESS in long sleeves, pants, and a hat in areas where mosquitoes are active.

For more information about preventing mosquito bites and WNV, go to weld.gov/go/zoonosis.

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