Chicago is now at a high risk level of West Nile virus after reporting its first three confirmed human cases of the virus this year, according to local health officials.
This week, the Chicago Department of Public Health announced that three city residents from the Northwest and South sides between the ages 40 and 80 tested positive for the virus and that their symptoms began in late July or early August.
The state saw its first human case of the year on June 24, which was the earliest case of West Nile in Illinois since 2016.
“We are seeing a higher proportion of mosquitoes testing positive for West Nile virus than usual this year,” CDPH Commissioner Olusimbo Ige said in a news release.
In 2024, the Illinois Department of Public Health reported a total of 69 cases, leading to 13 deaths of West Nile virus, with nearly half of the cases coming out of Cook County. To date, in addition to those reported by CDPH this week, the state agency has reported five other human cases this year in DuPage, DeKalb, Lake
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, Madison and Wayne counties.West Nile virus is primarily spread to people through mosquito bites and is not transmitted by human-to-human contact, according to the U.S. Centers for Disease Control and Prevention. Most people infected with West Nile do not feel sick, the CDC says, though about 1 in 5 people develop a fever and other flu-like symptoms and 1 in about 150 people develop a serious, sometimes fatal, illness.
Most human cases of West Nile occur in late summer and fall, when mosquitoes are most active. With no vaccines or medicines to prevent or treat the virus, the most effective method to prevent West Nile infection is to avoid mosquito bites, according to CDPH.
To track the risk level of West Nile in Chicago, CDPH from June through October collects mosquitoes from traps placed across the city and tests them for West Nile. Last week, CDPH tested 121 pools of collected mosquitoes and detected West Nile in 59, or 48.8%, of those pools, according to city surveillance data. The week before, the department tested 130 pools of mosquitoes and 81, or 62.3%, tested positive for the virus.
In an email to the Tribune, CDPH spokesperson Jacob Martin stated that this year “has been an unusual year for West Nile virus because we have seen mosquitoes testing positive for (West Nile virus) earlier in the season … than we typically do.” And while the city has seen a roughly “average number” of total mosquitoes, there’s been a higher proportion of pools testing positive for West Nile, Martin said.
That means Chicago has reached a “high risk” level of West Nile and, because mosquitoes are testing positive earlier, could see that heightened risk last longer this year. That also means the CDPH has sprayed more insecticide to reduce mosquito numbers than it has in recent years, Martin said.
Over the past few weeks, CDPH has been spraying insecticides to kill mosquitoes in various areas across the city, including O’Hare International Airport, Logan Square and Avondale, with the department spraying seven more communities — Forest Glen, Norwood Park, Mount Greenwood, Beverly, East Garfield Park, Near West Side and North Lawndale — this week.
To curb infection, CDPH is encouraging Chicagoans to: use Environmental Protection Agency-registered insect repellant; eliminate standing water; keep grass and weeds short to eliminate hiding places for mosquitoes; and make sure all screens, windows and doors are shut tight. CDPH also has advised the people who are immunocompromised to consider avoiding outside activity during peak mosquito hours from dusk to dawn.
The public is encouraged to track CDPH West Nile surveillance data.