Mosquito pools in Oak Park and River Forest have tested positive for West Nile virus as Illinois' case count climbs to 13. Suburban Cook County's infection risk was rated high as of the week ending Aug. 22, and the transmission season stretches until the first hard frost.

The statewide total, reported by Patch on Tuesday, Sept. 8, is up from the four suburban Cook County cases the Cook County Department of Public Health (CCDPH) announced on Aug. 28. West Nile activity has been detected in 50 Illinois counties through positive mosquitoes, horses or humans, according to the Illinois Department of Public Health (IDPH).

Chicago has also confirmed its first West Nile death of 2026, along with the city's first two human cases, Yahoo News reported. IDPH Director Dr. Sameer Vohra called the death "a sobering reminder to take mosquito-borne illness seriously," according to that report.

Oak Park, River Forest in the zone

Both communities had mosquito batches test positive for West Nile as of the week ending Aug. 22, according to a CCDPH social media post on Sept. 2. Oak Park's Public Health Department had first confirmed positive mosquito batches in the village in late June.

"In light of high West Nile virus activity in Cook County, it is incredibly important for people to protect themselves from mosquito bites," Dr. Kiran Joshi, chief operating officer of CCDPH, said in the department's Aug. 28 announcement.

The Des Plaines Valley Mosquito Abatement District sprayed for adult mosquitoes in Oak Park on Aug. 26 and Aug. 27. The district treats all street catch basins with larvicide every seven to 10 days during breeding season and will treat private-property catch basins by request at 708-447-1765.

A severe mosquito year

The numbers reflect a national trend. The Centers for Disease Control and Prevention (CDC) has tallied 407 cases across 40 states. By the end of June, the U.S. had recorded its earliest start to the West Nile season and the most cases by that point since 2004.

Weather has fueled the surge. Illinois recorded one of its 10 wettest Junes, according to the National Oceanic and Atmospheric Administration. July was the warmest month in the contiguous U.S. since recordkeeping began in 1895. Warm, wet conditions speed mosquito development and shorten the time the virus needs to become transmissible.

Suburban Cook County reported 44 human West Nile cases and four deaths in 2025. Statewide, Illinois confirmed 150 cases last year, more than double the 69 in 2024, with 10 deaths.

What comes next

The transmission window lasts until the first hard frost. Near Chicago, that averages around Nov. 1, according to the Old Farmer's Almanac.

No vaccine or antiviral treatment exists for West Nile. CCDPH's "Fight the Bite" campaign recommends using EPA-registered insect repellent, eliminating standing water around the home, repairing damaged window and door screens, and taking extra precautions at dawn and dusk.

Oak Park residents can sign up for spray notifications at oak-park.us/notifyme.