Mosquitoes In McHenry County Test Positive For West Nile Virus

MCHENRY COUNTY, IL — McHenry County health officials have confirmed that a mosquito pool collected in Fox River Grove on June 5 has tested positive for West Nile virus, marking the first positive mosquito detection in the county for the 2026 season.

No human cases have been reported to date, local health officials said.

Officials with the McHenry County Department of Health said it will continue routine West Nile virus surveillance through ongoing mosquito trapping and laboratory testing throughout the season, which runs from May 1 through Oct. 15. Officials note that the risk of West Nile virus persists until the first hard frost, according to a news release from the health department.

Residents are encouraged to take preventive measures to reduce exposure to mosquito-borne illness, including wearing long-sleeved shirts, long pants, socks, and light-colored clothing when outdoors and using EPA-registered insect repellents, according to a news release.

Health officials also reminded residents that Culex mosquitoes—the primary carriers of West Nile virus—breed in stagnant water with high organic content. Common breeding sites include items such as discarded tires, abandoned swimming pools, buckets, bird baths, and clogged gutters.

Eliminating standing water around homes and ensuring doors and windows have tight-fitting screens are key steps in reducing mosquito populations.

Most people infected with West Nile virus do not develop symptoms. When illness does occur, symptoms typically appear 2 to 14 days after a bite from an infected mosquito. Approximately 20 percent of infected individuals may experience mild symptoms, including fever, headache, body aches, joint pain, vomiting or rash, health officials said.

In rare cases, the virus can cause severe illness, including high fever, neck stiffness, disorientation, tremors, seizures, muscle weakness, vision loss, numbness, or paralysis. Individuals age 50 and older and those with underlying health conditions—such as diabetes, kidney disease, hypertension, cancer or organ transplants—are at higher risk for serious complications.

There is currently no vaccine or specific antiviral treatment for West Nile virus. Mild cases are typically managed with rest, fluids, and over-the-counter medications for pain or fever relief, while severe symptoms require immediate medical attention.

Health officials also provided guidance on EPA-registered insect repellents, which should contain active ingredients such as DEET, picaridin, IR3535, oil of lemon eucalyptus, para-menthane-diol or 2-undecanone and be used according to label instructions.

For more information on approved repellents, residents can visit the U.S. Environmental Protection Agency’s “Find the Right Repellent for You” page at: https://www.epa.gov/insect-repellents/find-repellent-right-you.