UPDATE: West Nile risk highest it’s been since 2017, says local abatement district

(Editor’s Note: This story was updated with data from a Sept. 1 report from the North Shore Mosquito Abatement District and followup questions to the district.)

The danger of the West Nile virus is currently amid its annual peak, and it’s important to take precaution, said Dave Zazra, of the North Shore Mosquito Abatement District. 

The district, which covers portions of 14 northern suburbs, and the Cook County Department of Public Health, which represents all of suburban Cook County, has labeled the current risk of human infection “high”, as more than half of mosquito traps collected in the county over the past two weeks have tested positive for the West Nile virus.

The designation from the abatement district came after the district’s vector index — a formula that measures West-Nile-infected mosquitos across the district — more than doubled in a week, from 1.47 to 3.85.

The district says the metric is directly related to human risk for the virus; however, as of Sept. 1, only one human case of West Nile virus has been reported this summer in the district.

The virus follows a predictable path most summers in the area, as the abatement district keeps data based on samples collected in traps throughout its coverage area.

This summer’s numbers were trending above the 10-year average, but by a slim margin until the Sept. 1 report, which measures mosquitos collected in district traps between Aug. 21-27.

The 3.85 vector index is the highest since early August of 2017, when it was also 3.85, said Zazra in an email.

Across the district, of the 132 mosquito samples tested from Aug. 21-27, 87 percent of them (or 115 samples) tested positive for West Nile virus. That total includes eight samples from Northfield, four from Wilmette, four from Winnetka, three from Glencoe and one from Kenilworth. Twenty-two of 26 samples out of Skokie and 34 of 40 from Evanston tested positive as well.

Zazra and the district urged residents to clear their property of any and all standing water, which breeds mosquitos.

“One of the things (we) have noticed is that anything that can hold water will hold water. We’re not kidding,” he said. “We’re seeing it on site inspections and home inspections. If not the house itself, it’s another house nearby. We treat it, dump out the water. There’s basically been a lot of that this year.”

A culprit could be “a wheelbarrow, a saucer under a planter, clogged gutters” or even a tarp that is not secured, he said.

The best defense for humans, Zazra said, is the application of an EPA-certified repellant, which are “very effective,” on any exposed skin.

If residents notice an abundance of mosquitoes near their home, they can call the abatement district to review the area. 

Zazra said it is important to control mosquitos in the early stages, especially because adult mosquitoes have built up a resistance to common treatments.

“Mosquitoes have become resistant to the adult mosquito control pesticides commonly used in the United States, making them less effective when we need to use them should an outbreak occur,” Zazra wrote to The Record. “This places additional importance on larval control and source reduction in order to reduce mosquito populations and minimize potential health risks form mosquito-borne illnesses.”

Higher temperatures also cause mosquitoes to mature quicker, making it more difficult to treat the problem, he said.

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