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.  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 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. 

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