First West Nile Virus-Positive Mosquitoes of 2025 Reported in Chicago

CHICAGO - The Chicago Department of Public Health (CDPH) has confirmed the first West Nile virus (WNV)-positive mosquitoes in Chicago in 2025. The Illinois Department of Public Health previously confirmed the first mosquitoes to test positive for WNV in the state on May 13, 2025. No human cases of WNV have been reported in Illinois this year.

CDPH has a robust program to prevent and control WNV, which includes treating over 80,000 catch basins with larvicide to prevent adult mosquitoes, collecting and testing mosquitoes every week for WNV, spraying to kill adult mosquitoes in specific areas if indicated, and monitoring human WNV infections.

"As the weather gets warmer, all Chicago residents are encouraged to Fight the Bite and protect themselves against mosquito bites so they can remain safe and healthy this summer," said CDPH Commissioner Dr. Olusimbo Ige.

The best way to prevent WNV is for Chicagoans to protect themselves from mosquito bites by:

• Using EPA-registered insect repellent according to label instructions, especially when applying to children.
• Wearing long-sleeved shirts and pants when outside between dusk and dawn.
• Controlling mosquitoes indoors by making sure that window and door screens do not have holes and by using air conditioning if possible.
• Controlling mosquitoes outdoors by keeping grass and weeds short to eliminate hiding places for mosquitoes and emptying items that hold water like flowerpots or birdbaths once weekly.
• Checking on neighbors regularly, especially those who are older, live alone or need additional assistance.

WNV is typically spread to humans via the bite of infected mosquitoes. Most mosquitoes do not carry the virus. While most people infected with WNV do not feel sick, about 1 in 5 people develop a fever and flu-like symptoms. Severe illness can occur in about 1 in 150 people and is most likely in people over age 55 or who have weakened immune systems. Because there are no specific medications to treat WNV in people, the best way to keep yourself from getting sick is to prevent mosquito bites.

For more information about mosquito-borne diseases, including symptoms and prevention, visit https://www.chicago.gov/mosquitoes. To report standing water, dead birds or high grass/weeds, call 311.

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