Two Batches of 2024 Mosquitoes Test Positive for West Nile Virus: Hoffman Estates and Jacksonville, Illinois

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Inland Floodwater (Aedes vexans) or Asian Tiger Mosquito (Aedes albopictus) on Broccoli
Inland Floodwater (Aedes vexans) or Asian Tiger Mosquito (Aedes albopictus) on Broccoli.

Two batches of mosquitoes tested positive for West Niles Virus in Illinois for the first time in 2024, according to the Illinois Department of Health.

The Northwest Mosquito Abatement District collected the first batch of mosquitoes in Hoffman Estates, Cook County, on Tuesday. A second batch was found in Jacksonville, Morgan County, on Thursday.

The Illinois Department of Public Health announced Friday, May 17, 2024 that the positive tests in the batches follow a mild winter and spring.

“The report of the first two batches to test positive for West Nile virus serves as a timely reminder for Illinoisans to begin protecting themselves from vector-borne diseases.”

— IDPH Director Dr. Sameer Vohra

The batch that tested positive in Hoffman Estates was collected by the Northwest Mosquito Abatement District on May 14, 2024.

The second batch that tested positive was collected May 16, 2024 in Jacksonville in Morgan County. The first bird to test positive for West Nile Virus was found April 2, 2024 in Douglas County, according to IDPH.

In 2023 mosquito “pools” (batches of mosquitoes) sampled on June 27, 2023 in Bannockburn, Lake Villa, and Vernon Hills tested positive for West Nile virus. The mosquito pools were the first confirmed indicators of West Nile virus presence in Lake County in 2023.

By June 15, 2023 statewide in Illinois, a total of ten batches of mosquitos tested positive for West Nile virus in Cook County, LaSalle County, Morgan County and St. Clair County.

Worst-Case Illnesses

“Less than 1% will develop severe West Nile Neuroinvasive Disease (WNND).”

— US Department of Health and Human Services

In 2023, a total of 67 Illinois counties reported finding a West Nile virus-positive mosquito batch, bird case, a horse case, and/or a human case, compared to 44 counties in 2022. In 2023 (provisional data), there were 119 human cases of West Nile virus, and six deaths reported in Illinois, compared to 33 human cases and seven deaths in 2022.

Where Is West Nile Virus (WNV) More Likely?

“According to the US Department of Health and Human Services, The Great Plains and western states are more likely to have high incidence of WNV (defined as more than 1.10 cases per 100,000 people). Six counties with both high incidence and large populations reported 23% of all WNND cases (West Nile Neuroinvasive Disease) during 2009– 2018 (Cook County, IL; Dallas County, TX; Harris County, TX; Los Angeles County, CA; Maricopa County, AZ; and Orange County, CA). Although certain areas of the country are more likely to have higher WNV incidence, the number of cases reported in a given county varies greatly each year. This makes accurately predicting the number of WNV disease cases that will occur each year and in each county challenging.

While we do not fully understand how climate change impacts WNV transmission across the United States, seasonal weather patterns can have an effect. This was observed in 2021 in Maricopa County, AZ, when the county experienced the largest-ever WNV outbreak, resulting in a reported 1,487 WNV cases, 1,014 hospitalizations, and 101 deaths.”

— US Health and Human Services

The first human cases for West Nile Virus usually test positive late summer or fall in Illinois, but cases can occur earlier. Most people infected with the virus show little or no symptoms, but unlucky victims, people over 50 years-old, or victims with weakened immune symptoms, can develop symptoms such as high fever and nausea, tremors, unsteady gait, or life-threatening symptoms, such as respiratory arrest. West Nile Virus infections can also put people in the Intensive Care Unit with treatment that may even require endotracheal intubation with the assistance of a ventilator, and sedation to tolerate the discomfort of being treated with a ventilator. West Nile Virus can also cause brain damage, and damage to the rest of the nervous system. Brain damage occurs when victims suffer meningitis, encephalitis or both meningitis and encephalitis together.

Less than 1% will develop severe West Nile Neuroinvasive Disease (WNND), according to the US Department of Health and Human Services.

“It is important for everyone – and especially older people and those with weakened immune systems – to safeguard themselves and their families from mosquitoes and the viruses they carry by wearing insect repellent and eliminating standing water around their home. You can ‘Fight the Bite’ by practicing the three R’s – reduce, repel, and report.”

— IDPH Director Dr. Sameer Vohra

Illinois monitoring procedures for West Nile virus include laboratory tests for mosquito batches, laboratory tests of dead birds, testing of sick horses, and testing of humans with West Nile virus-like symptoms. The lab results that confirm a human is sick with West Nile Virus take a few days for the results.

The Lake County Health Department recommended people follow the “4 Ds of Defense” to protect yourself and your family from mosquitoes:

Drain: Drain standing water from items around your home, yard, and business.

Defend: When outdoors, use an insect repellent containing DEET, picaridin, oil of lemon eucalyptus, 2-undecanone, or IR3535 and reapply according to label directions.

Dawn to Dusk: Protect yourself all day and night, and wear repellent outdoors during these prime times for mosquito activity.

Dress: Wear long sleeves, pants, and closed toe shoes when outdoors to cover your skin.

The Illinois Department of Health also has a mnemonic “REDUCE, REPEL, REPORT.

Find more prevention tips and information on West Nile virus at Residents can also call the Health Department’s West Nile hotline to report areas of stagnant water, report locations of dead birds, and obtain more information on the signs and symptoms of West Nile virus. The West Nile hotline number is (847) 377-8300.



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The first cases of infected humans often begin in late June. Monitoring for West Nile virus in Illinois includes laboratory tests for mosquito batches, dead crows, blue jays, robins and other perching birds, as well as testing humans with West Nile virus-like symptoms. Human testing for West Nile Virus takes about 4-5 days for a confirmation.

People who observe a sick or dying crow, blue jay, robin or other perching bird should contact their local health department, which will determine if the bird will be picked up for testing.

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