Weld County Culex Mosquitoes Test Positive for West Nile Virus

Published on July 07, 2021

Mosquitoes in a petri dish

Weld County, CO — West Nile virus (WNV) has been detected in mosquitoes in Colorado for the first time this season in Weld County, and Culex mosquitoes are more prevalent than in previous years. Compared to a five-year average, the 2021 trapped Culex mosquito count is more than 3 times higher this mosquito season.

“The increasing hot weather and afternoon thundershowers have created favorable conditions for the Culex mosquito that carries West Nile virus,” said Mark Lawley, Executive Director of the Weld County Health Department.

The Health Department continues to take proactive measures to monitor for WNV. A Health Department contractor monitors mosquito traps on a weekly basis in 3 zones of Weld County: the Greeley, Evans, Kersey, LaSalle area (Zone 1); the Johnstown, Milliken, and Platteville area (Zone 2); and the Firestone, Dacono and Fort Lupton area (Zone 3). Mosquitoes trapped in Zone 1 tested positive for WNV.

Currently, there are no human cases of WNV in Weld County. However, health officials expect this to change as the Culex mosquito season continues. “The public needs to be vigilant because West Nile virus has a permanent summer presence in Colorado,” Lawley said.

West Nile virus symptoms can appear 3-14 days after infection. Initial symptoms can include fever, headache, nausea and vomiting, muscle aches and weakness, and rash, but most infected people don’t show any symptoms. If a person develops symptoms, they should see a health care professional immediately. There are no medications to treat or vaccines to prevent WNV infection. Less than 1% of infected people develop a serious, sometimes fatal, neuroinvasive illness.

Health officials recommend following 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 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 and pants, and a hat, in areas where mosquitoes are active.

For more information about preventing mosquito bites and WNV in Weld County, see the Health Department's Zoonotic Disease Seasonal Update

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