West Nile virus specimens confirmed in 17 Illinois counties

West Nile virus specimens have been confirmed in 17 counties across the state, including Bureau, Grundy, DeKalb, DuPage, McHenry and Winnebago counties.

No human cases of West Nile virus have been reported in Illinois so far this year. In 2024, there were 69 confirmed human cases of the virus, including 13 deaths, according to a news release from the Illinois Department of Public Health.

That was the highest number since 17 West Nile virus-related human deaths were recorded in 2018, according to the release.

A county is considered positive for West Nile virus if a mosquito, horse or human from that county tests positive for the virus. The counties to report West Nile virus so far this year included Bureau, Christian, Cook, DeKalb, DuPage, Fulton, Grundy, Hancock, Henry, Marshall, McDonough, McHenry, Peoria, Scott, Stark, Tazewell and Winnebago.

“With mosquito season underway in Illinois, it is now time for Illinoisans, especially seniors and those with weakened immune systems, to start taking precautions to prevent mosquito bites,” IDPH Director Dr. Sameer Vohra said in the release.

“A bite from a mosquito infected with West Nile virus can cause serious illness,” Vohra continued. “The best way to ‘Fight the Bite’ is to practice the three Rs – reduce the opportunity for mosquitos to enter your home, repel mosquitos by wearing insect repellent outdoors and report standing water in your community where mosquitoes can breed.”

West Nile virus is transmitted through the bite of a infected Culex mosquito, commonly called a house mosquito, that has picked up the virus by feeding on an infected bird. Common symptoms include fever, nausea, headache and muscle aches.

Symptoms may last from a few days to a few weeks. Most people infected with West Nile virus will not show any symptoms; however, in rare cases it can lead to severe illness including paralysis, meningitis or encephalitis (i.e. brain and nerve infections) or even death, according to the release.

People older than 60 and those with weakened immune systems are at highest risk for severe illness.

Monitoring for West Nile virus in Illinois includes tests for mosquito batches as well as testing sick horses and humans with West Nile virus-like symptoms.

Last year, the virus was confirmed in 72 counties in Illinois.

The first WNV-positive mosquito batch of this year was found May 9 in Rockford, according to the release.