West Nile Virus Cases Grow In Active Mosquito Year: What To Know In IL

West Nile virus rates are higher than they've been in more than two decades, and Illinois is among 40 states where public health officials have reported cases of the mosquito-borne illness.

The Centers for Disease Control and Prevention reports 407 West Nile virus cases nationwide in its latest report. The Illinois Department of Public Health currently reports 13 human cases in Illinois. IDPH notes that state totals can differ from CDC numbers because of the timing of reports.

Illinois confirmed its first human case of 2026 in late August in a Piatt County resident. At the time, IDPH also identified three probable cases in northern Illinois and said West Nile activity had been detected in 50 counties through positive mosquitoes, horses or humans.

Cases have since been reported around the Chicago suburbs. Cook County health officials announced the first four human cases in suburban Cook County on Aug. 28, all involving adults. At the time, the risk of human infection in suburban Cook County was considered high.

RELATED: 4 Human West Nile Virus Cases Reported In Suburban Cook County

DuPage County reported its first human case of the year Sept. 1 involving an adult resident.

RELATED: 1st Human Case Of West Nile Virus Reported In DuPage County For 2026

Mosquitoes carrying West Nile virus have also been detected across northern Illinois this summer.

The North Shore Mosquito Abatement District reported its first West Nile-positive mosquitoes in Northbrook in June after a batch collected June 8 tested positive. Positive mosquito samples have also been reported in DuPage and McHenry counties.

RELATED: West Nile Virus Found In Northbrook Mosquitoes

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RELATED: Mosquitoes In McHenry County Test Positive For West Nile Virus

West Nile virus is rarely fatal, but it can cause meningitis or encephalitis, a severe inflammation of the brain, in a small fraction of cases.

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

Health officials are also tracking Eastern equine encephalitis, or EEE, a rare but more deadly mosquito-borne virus that affects humans and horses.

Human cases of EEE are rare. The CDC says an average of about 11 human cases are reported annually in the United States, with transmission most common around freshwater hardwood swamps in the Atlantic and Gulf Coast states and Great Lakes region.

Human cases have already been reported this summer in Massachusetts and Michigan.

A horse in Michigan was euthanized after testing positive for EEE, and another equine case was confirmed in Georgia.

Illinois health officials say EEE has been found in birds and horses in the state on rare occasions. IDPH does not list EEE among the mosquito-borne diseases that regularly occur in people in Illinois.

West Nile virus is transmitted primarily by Culex mosquitoes. Eastern equine encephalitis virus circulates between birds and Culiseta melanura mosquitoes in swamps, spreading to humans and horses through other mosquito species.

A Big Year For Mosquitoes

2026 has been a big year for mosquitoes and the diseases they carry. By the end of June, the United States had recorded its earliest start to the West Nile virus season and the most cases by that point since 2004.

Weather conditions have been ideal for mosquito populations.

June was unusually wet across parts of the South, Midwest and Great Lakes, with Kansas, Illinois, Indiana and Kentucky recording one of their 10 wettest Junes, according to NOAA.

Rain fills storm drains, ditches, birdbaths and other places where mosquitoes lay eggs, and some mosquitoes can develop from egg to adult in seven to 10 days, according to the CDC.

A "heat dome" made conditions even more ideal for mosquitoes. July was the warmest month recorded in the contiguous United States since recordkeeping began in 1895, with every state warmer than its 20th-century average, NOAA reported.

Warm weather can speed mosquito development, increase biting and shorten the time West Nile virus needs to become transmissible inside a mosquito.

Warm weather can also extend the season. A recent Stanford University analysis of 239 U.S. cities found that 95 percent now average more days warm enough for Culex mosquitoes to transmit West Nile than they did in the early 1970s.

The mosquito season does not wind down until the first hard frost.

In Illinois, the timing varies considerably by location. The Old Farmer's Almanac lists an average first fall frost of Oct. 12 in Rockford, Oct. 17 in Springfield and Nov. 1 at Chicago's Northerly Island climate station.

How To Control Mosquitoes

Neither West Nile nor EEE has a human vaccine or antiviral treatment, making prevention the primary defense.

The cheapest and most environmentally friendly mosquito control is to make the backyard a lousy nursery.

Weekly, empty and scrub birdbaths, flowerpot saucers, toys, buckets and pet bowls; clear gutters; turn over kiddie pools; and cover rain barrels. Mosquito eggs cling to containers, so dumping the water alone may not work.

Mosquitoes rest in cool, damp vegetation. Mow overgrown areas and thin shrubs, and use a fan on the patio; mosquitoes are weak fliers. Screens provide another chemical-free barrier.

Treat water that cannot be drained or covered with a larvicide. Products containing Bacillus thuringiensis israelensis, or Bti, kill larvae while posing little risk to people, pets and most wildlife when used as directed. The EPA recommends targeting mosquitoes at the larval stage, before they take flight.

If mosquitoes remain thick, the CDC advises using an EPA-registered outdoor insecticide where mosquitoes rest, such as beneath decks, shrubs and patio furniture. Follow the label, avoid wind and rain, and keep chemicals away from people, pets, food and water.

Broad spraying can kill bees, butterflies and other beneficial insects, and some insecticides are toxic to fish. Automatic misting systems have not been studied sufficiently to prove they control mosquitoes or prevent disease, the EPA says.

Bug zappers kill harmless insects but few mosquitoes. Citronella candles and repellent plants offer limited protection.

For the best results, remove water, treat unavoidable breeding sites and use insecticides sparingly.