The Center for Disease Control and Prevention (CDC) has confirmed the first human case of West Nile virus (WNV) in Illinois for 2025. The individual, from Southern Illinois, was hospitalized due to complications from the infection. This disease is transmitted by mosquitoes, and approximately one in 150 infected individuals may develop severe illness affecting the brain, spinal cord, or nerves.
This case represents the earliest onset of WNV-related illness in Illinois since 2016. It highlights the ongoing risk posed by mosquito bites, particularly to individuals over 65 or those with compromised immune systems.
In 2025, WNV has been identified in 19 counties. The initial positive mosquito batch was discovered on May 9 in Rockford. In comparison, there were 69 confirmed human cases and 13 deaths related to WNV in Illinois last year—the highest number since 17 fatalities recorded in 2018.
Dr. Sameer Vohra, Director of IDPH, emphasized the importance of taking precautions against mosquito bites: “The fact that we are seeing the first human case of West Nile virus so early in the season serves as a timely reminder – especially for seniors and those with weakened immune systems – to protect yourself from illnesses caused by mosquito bites,” he said.
To combat this threat, IDPH recommends following three strategies: reduce exposure by ensuring doors and windows have tight-fitting screens; repel mosquitoes using EPA-registered insect repellents; and report stagnant water sources that could serve as breeding grounds for mosquitoes.
A county is considered positive for WNV if a mosquito, horse, or human tests positive for the virus. This year’s affected counties include Bureau, Christian, Clinton, Cook, DeKalb, DuPage among others. Last year saw confirmations across 72 counties statewide.
IDPH supports mosquito control efforts with $2.8 million allocated to local health departments across Illinois for surveillance and control activities. These efforts involve applying larvicide and collaborating with municipal governments on prevention initiatives.
Monitoring for WNV involves testing mosquito batches along with symptomatic horses and humans. Transmission occurs through bites from infected mosquitoes that have fed on birds carrying the virus. While most people show no symptoms after infection—fever nausea headache muscle aches being common—severe cases can result in paralysis meningitis encephalitis or even death especially among older adults or immunocompromised individuals.
For further information about West Nile virus visit IDPH’s website.
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