West Nile Virus Detected in St. Croix County Mosquitoes

St. Croix County Public Health is encouraging residents to take steps to prevent illness after West Nile virus (WNV) was detected in mosquitoes collected in St. Croix County.

A mosquito pool, collected by St. Croix County Public Health‘s mosquito surveillance program between August 12 and 13, tested positive for WNV. This is the first group of mosquitoes in St. Croix County to test positive for WNV this year and the second detection of WNV activity in the County in 2026. A dead bird previously tested positive for the virus. No human infections have been reported in the County this year as of August 21. Residents can find reported WNV activity across Wisconsin on the Wisconsin Department of Health Services (DHS) West Nile Virus Data webpage.

St. Croix County Public Health‘s mosquito surveillance program collects mosquitoes around the County and submits them to the state for testing and identification. Public Health also performs tick surveillance in the County. Surveillance helps Public Health be proactive and to identify when and where mosquitoes and ticks are carrying disease and provide timely information to residents. 

“This detection is a good reminder that West Nile virus is active in our area, and residents should continue taking precautions to prevent mosquito bites,” said Elle Klasen, St. Croix County Public Health Administrator. “Simple steps such as using insect repellent and removing standing water around your home can help reduce your risk.”

WNV can spread to people through the bite of an infected mosquito. Mosquitoes become infected by feeding on infected birds. The virus is not spread directly from person to person.  

According to DHS, most people who are infected with WNV do not get sick. Those who do become ill usually experience mild symptoms such as fever, headache, muscle aches, rash, and fatigue. However, a small percentage of people who become infected with the virus become seriously ill, with symptoms that can include high fever, muscle weakness, stiff neck, disorientation, confusion, tremors, paralysis, and coma. People over age 60 and those with weakened immune systems are more likely to develop severe illness if infected. It is important that people contact a health care provider if they suspect they have WNV illness.

The risk of contracting WNV and other mosquito-borne illnesses such as Jamestown Canyon virus, La Crosse encephalitis virus, or eastern equine encephalitis virus, is present anytime mosquitoes are active. Mosquito activity and the risk of WNV will continue until a hard frost occurs (temperatures below 28 degrees Fahrenheit for at least four consecutive hours). The best way to avoid illnesses spread by mosquitoes is to reduce exposure to mosquitoes and eliminate mosquito breeding sites. DHS offers these tips to protect yourself and your family against mosquito bites:

Avoid Mosquito Bites:

• Apply an insect repellent with DEET, picaridin, oil of lemon eucalyptus, or IR3535 to exposed skin and clothing.
• Before heading outdoors, treat clothing with permethrin; do not apply permethrin directly to skin.
• Consider rescheduling outdoor activities that occur during evening or early morning hours, when mosquitoes that spread WNV are most active.
• Wear long sleeves, long pants, and socks when outdoors to help keep mosquitoes away from your skin.

Mosquito-Proof Your Home:

• Prevent mosquitoes from breeding around your home by removing stagnant water from items around your property.
  • Empty standing water that has collected in tin cans, plastic containers, flower pots, discarded tires, roof gutters, and downspouts. 
  • Turn over wheelbarrows, kiddie pools, buckets, and small boats such as canoes and kayaks when not in use. 
  • Change the water in bird baths and pet dishes at least every three days. 
  • Clean and chlorinate swimming pools, outdoor saunas, and hot tubs; drain water from pool covers.
• Make sure window and door screens are intact and tightly fitted to prevent mosquitoes from getting into your home. 
• Trim or mow tall grass, weeds, and vines since mosquitoes use these areas to rest during hot daylight hours.

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