Health officials said Wednesday they are warning the public after a batch of mosquitoes tested positive for West Nile virus for the first time this year in McHenry County.
A mosquito “pool,” also known as a batch of mosquitoes, was sampled on Friday in Fox River Grove and later tested positive for West Nile virus.
The McHenry County Department of Public Health (MCDH) said this mosquito pool is the first confirmed indicator of West Nile virus presence in McHenry County in 2026.
No human cases of West Nile virus have been reported in McHenry County this year so far.
“Culex mosquitoes, which are the primary carriers of WNV, typically breed in stagnant water with high organic content, often found in artificial containers like rimless tires, abandoned swimming pools, buckets, bird baths, and clogged gutters,” the health department said.
“The first and best defense against mosquito-borne illnesses is to eliminate potential mosquito breeding sites by emptying standing water from containers around your home and ensuring that doors and windows have tight-fitting screens,” the department added.
Human West Nile virus illness typically occurs two to 14 days after the bite of an infected Culex mosquito.
Most people exposed to West Nile virus do not develop symptoms due to the belief that past exposure to West Nile virus results in lifelong immunity, health officials said.
About 20% of those infected experience mild fever illness with other symptoms such as headaches, body aches, joint pains, vomiting or a rash.
Severe symptoms are very rare and include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis.
Those who are 60 years of age or older or those with serious underlying health conditions such as diabetes, kidney disease, hypertension, cancer or who have received organ transplants are at greater risk for developing severe symptoms.
There is no vaccine or treatment for West Nile virus. Mild symptoms can be relieved with rest, fluids and over-the-counter medications for pain and fever, the health department said. “Seek medical attention if you or an individual you know develops severe symptoms.”
When outdoors, wear long-sleeved shirts, pants, shoes, and socks, especially during dawn and dusk when Culex mosquitoes are most active. EPA-registered insect repellents are recommended to be applied.
From May through mid-October, McHenry County health department staff conduct West Nile surveillance throughout McHenry County. Mosquito batch collection and testing are conducted weekly.