First positive case of West Nile Virus reported in Weld County

Published on July 25, 2025

A Culex mosquito sitting on a person's hand.

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 2025 mosquito season. Last year, 15 cases of WNV were investigated in Weld County.

This year, the number of Culex mosquitoes (those that transmit WNV) trapped is lower than the five-year average. However, Culex mosquitoes currently make up approximately 80% of those trapped in the three sentinel zones in Weld County: the Greeley, Evans, Kersey and LaSalle area; the Johnstown, Milliken and Platteville area; and the Firestone, Dacono and Fort Lupton area. These mosquito traps are monitored weekly, 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 signs can include fever, headache, nausea and vomiting, muscle aches and weakness, and rash. Anyone who develops symptoms should see a health care professional immediately. There are no medications to treat or vaccines to prevent West Nile virus infection. Fewer than 1% of infected people develop a serious, sometimes fatal, neuroinvasive illness. 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 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 www.weld.gov/go/zoonosis.