First West Nile Virus-Positive Mosquitoes Reported in Chicago Area

Health officials detected the first West Nile virus-positive mosquitoes of 2026 in Chicago, Northbrook, and McHenry County on June 12, 2026, according to reports from Block Club Chicago, Patch, and Shaw Local. These detections serve as an early warning for residents to take precautions against mosquito-borne illness as the virus spreads across the Chicagoland area.

Block Club Chicago and the Chicago Sun-Times reported that the city’s first positive mosquito tests of the year have been confirmed. This urban detection coincides with findings in the surrounding suburbs, indicating the virus is present in multiple regional mosquito populations simultaneously.

In the northern suburbs, Patch reported that mosquitoes in Northbrook tested positive for the virus. Further northwest, the Lake and McHenry County Scanner reported that the health department issued warnings after the first positive tests of 2026 were recorded in McHenry County. Shaw Local specifically identified Fox River Grove as a location where positive mosquitoes were found.

Where has West Nile virus been detected in Illinois?

As of June 12, 2026, West Nile virus has been identified in mosquitoes within the city limits of Chicago and several suburban jurisdictions. Verified reports place the virus in Northbrook, Fox River Grove, and broader McHenry County.

The simultaneous reporting across these diverse geographic areas—ranging from a dense urban center to rural parts of McHenry County—suggests a widespread presence of the virus in the regional ecosystem. Typically, West Nile virus activity peaks in late summer, but these June detections indicate an early start to the 2026 transmission season.

What are the symptoms and risks of West Nile virus?

West Nile virus is a flavivirus transmitted to humans through the bite of an infected mosquito, according to the Centers for Disease Control and Prevention (CDC). The CDC states that most people infected with the virus do not develop any symptoms.

For those who do experience symptoms, the CDC categorizes them into two primary groups:

• Mild illness: This includes fever, headache, body aches, joint pains, vomiting, diarrhea, or rash.
• Severe illness: A small percentage of people develop severe neurological diseases, such as encephalitis or meningitis, which can lead to death, according to the CDC.

The risk of severe illness is higher for adults older than 60 and people with certain medical conditions, such as cancer, diabetes, or high blood pressure, according to CDC guidelines.

How can residents prevent mosquito-borne infections?

Public health departments in McHenry County and Chicago advise residents to focus on two primary prevention strategies: avoiding bites and reducing mosquito populations.

The Environmental Protection Agency (EPA) and the CDC recommend using insect repellents containing active ingredients such as DEET, picaridin, or oil of lemon eucalyptus. They also suggest wearing long-sleeved shirts and long pants to minimize exposed skin.

To reduce the number of mosquitoes near homes, health officials recommend removing standing water. The CDC notes that mosquitoes breed in small amounts of water, including:

• Clogged rain gutters
• Birdbaths and flowerpot saucers
• Old tires
• Uncovered buckets or trash cans

The McHenry County health department warned residents that these preventative measures are critical following the first positive tests of the year to limit the potential for human transmission.

What happens next in the monitoring process?

Health departments typically continue “surveillance trapping” throughout the summer. This involves collecting mosquitoes from various traps across the city and suburbs to monitor the prevalence of the virus in the insect population, according to standard public health protocols.

The early detection in June 2026 allows health agencies to initiate public awareness campaigns before the peak mosquito season. While the virus is common in the Midwest, the timing and geographic distribution of these initial reports will help officials determine if the 2026 season will be more aggressive than previous years.