It's peak season for West Nile virus, and the Chicago area is a hotspot: What's being done

In the more than 20 years since West Nile virus established itself in North America, local, county and state organizations have built robust systems to monitor and abate mosquitoes that carry it.

The systems are especially vital in the Chicago area, which experts call a West Nile hot spot. While drought and mild winters have also been associated with disease outbreaks, climate change further increases the risk of human exposure.

The peak of West Nile virus season begins in mid-July and lasts through September. Municipalities, county health departments and abatement districts test mosquito pools regularly during this time to detect the presence of the virus.

The first three batches of mosquitoes to test positive for West Nile virus in Illinois in 2023 were both in Cook County: Positive batches of mosquitoes were collected in Evanston May 30 and in Park Ridge May 31.

Mosquitoes carrying the virus were found in Medinah, Wheaton and Wood Dale July 5, the first in DuPage County to test positive this year.

A Batavia sample tested positive for the virus July 24, Kane County health officials said, while the Lake County Health Department detected the virus in mosquito pools in Bannockburn, Lake Villa and Vernon Hills sampled July 27.

“If we start to see high numbers in a certain area, we'll work with that municipality and township to make sure that they're doing preventative stuff like larviciding early in the season,” said Alana Bartolai, the ecological services program coordinator at the Lake County Health Department, referring to applying insecticide to kill larvae.

The department maintains 23 traps throughout the county that are tested weekly. Bartolai regularly relays the data to municipal officials, especially during this time of year.

Officials encourage people to practice the “Four D's of Defense”:

• 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 and 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.

Bartolai said draining is especially important because a lot of people don't realize they could be inadvertently helping mosquitoes breed by leaving standing water out in their kiddie pools, buckets or gutters.

That's because mosquitoes lay their eggs in standing water. Some mosquitoes can lay their eggs in as little as a quarter-inch of water, laying hundreds of eggs at a time. The eggs then hatch into larvae that live in the water.

Though scientists haven't reached a definitive conclusion as to why the Chicago region is so affected by West Nile virus, there are several factors.

“Climate is just one of many important factors that influence the transmission, distribution and incidence of West Nile virus disease,” according to the U.S. Environmental Protection Agency. “Human exposure to infected mosquitoes is also influenced by multiple factors, including changes in the proximity of human populations to mosquitoes and host bird species, increased awareness of West Nile virus, and modified behaviors, such as spending less time outdoors during peak mosquito-biting times and taking precautions to avoid being bitten.”

Birds are the primary hosts of West Nile virus. It can infect more than 250 species of birds, with larger species such as crows and blue jays especially prone to infection. Mosquitoes become infected when they feed on infected birds.

While the virus is the leading cause of mosquito-borne disease in the continental United States, most people infected with the disease do not feel sick: About 1 in 5 people who are infected develop a fever and other symptoms, according to the Centers for Disease Control and Prevention.

Still, about 1 out of 150 infected people develop “a serious, sometimes fatal, illness.”

“Even though we don't see generally a ton of cases, this is a serious, serious disease,” said Patrick Irwin, the assistant director of the Northwest Mosquito Abatement District. “With Chicago being a hot spot, we try to be very diligent in trying to manage mosquito populations, specifically the ones that can spread West Nile virus.”

One of four in Cook County, the abatement district was formed in 1956 and is funded by property taxes. Its main purpose is to exterminate mosquitoes, flies and other insects and to abate stagnant pools of water and other breeding places.

Irwin, who has been studying mosquitoes as an entomologist for nearly two decades, tests the district's 40 mosquito traps daily for West Nile virus. Because the virus has been in Illinois for a relatively short time — it was first detected in the state in September 2001 — Irwin hopes outbreaks will become more predictable as data builds.

“It may turn out that 50 years from now, we may be able to start to predict some of these outbreaks. I work with a lot of professors and we do a lot of research trying to figure out why Chicago is such a hot spot. How can we start to predict when we're going to see human cases?” he said. “We're trying to figure out exactly what is going on in our area. As you can imagine, it's incredibly complicated, but it doesn't mean we stop.”

The Illinois Department of Public Health monitors animals and insects that may carry the virus, including mosquitoes along with crows, robins, blue jays and horses. IDPH updates surveillance data weekly on their website, at tinyurl.com/IllinoisWNV.

Data includes human cases, positive mosquito batches, and counties with positive humans, birds, mosquitoes or horses.

Meanwhile, Illinois is considered to be at low risk, according to the IDPH, of widespread, yearlong transmission of the Zika virus, another virus that may bring only mild symptoms such as fatigue and joint and muscle pains, with rare exceptions. The Aedes aegypti species mosquito, which is the primary vector for Zika virus transmission, is rarely found in Illinois.

Local transmission of Zika in the United States has occurred only in southeastern Florida and Texas, and currently, the only Zika virus cases in Illinois are a result of travel.

• Jenny Whidden is a climate change and environment writer working with the Daily Herald through a partnership with Report For America supported by The Nature Conservancy. To help support her work with a tax-deductible donation, see dailyherald.com/rfa.