'Extremely high' number of mosquitoes and West Nile Virus found locally

No human cases yet, but risk bigger than other years

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By Maria Maxham

An increased number of mosquitoes and higher levels of West Nile Virus (WNV) among mosquitoes tested in Forest Park, Oak Park and River Forest have led to spraying in these towns, an action by the Desplaines Valley Mosquito Abatement District (DVMAD) to control the adult mosquito population.

In contrast, no spraying was done locally in 2019, according to DVMAD. This year, however, dry conditions and high temperatures have caused ideal conditions for mosquitoes to thrive and reproduce heavily, and enough mosquitoes have tested positive for WNV to make transmission to humans likely.

Mark Tomek, a biologist at DVMAD, said in an email that the problem is bigger this year than in past summers.

"West Nile Virus and the mosquitoes that carry the virus are some of the highest we've seen in a long time in the Chicago suburbs, especially in Oak Park," said Tomek.

He explained that high temperatures coupled with dry conditions have promoted both WNV transmitting mosquitoes and the transmission cycle between host birds and infected mosquitoes.

"The number of mosquitoes we have collected this summer has been extremely high, even if not noticeable by many residents," Tomek said. "These mosquitoes are generally considered 'quiet biters,' active at or after dusk and mostly biting unnoticed (though the itchy welts will still appear after some time!)."

WNV, said Tomek, was "essentially nonexistent" in 2019. In 2018, DVMAD saw some of the highest numbers since 2012. This year, DVMAD is using more sensitive testing processes, which can find the virus at lower levels; this might partly explain an increase in 2020. But not completely.

"Either way, this does not make up for the astounding number of positive mosquito pools we have been finding, and these findings match other local district and health department results," said Tomek.

As a result, the risk of human transmission of WNV over the past three weeks "has been significantly increased and high this past week," Tomek said.

No human cases have yet been reported, "though based on the rate of infection in mosquitoes and the high number of vector mosquitoes, this is unlikely. Many cases of WNV are mild and mostly go undiagnosed (headaches, fever, possibly being falsely labeled as COVID)," said Tomek. In a very small percent of cases, less than one percent, severe encephalitis can occur, which can cause brain damage and even death, said Tomek.

And there's more. The invasive Asian Tiger Mosquitoes, capable of transmitting multiple viruses including Zika and Dengue, have been detected in fairly high numbers this year, both in the city and extending into Oak Park, said Tomek.

In the past few years, the numbers of these mosquitoes have been low, but the increase now suggests "they may be here to stay and likely grow in prevalence." They're active all day long, unlike the WNV-carrying mosquitoes, which only come out at dusk. They're aggressive and persistent.

But, Tomek said, any virus these mosquitoes can spread would need to be introduced into the population first.

So for now, Zika and Dengue don't need to be added to anyone's list of things to worry about.

According to DVMAD, residents should take the following steps to eliminate mosquito populations:

  • Throw away all trash that can hold water, such as cans, jars, bottles, etc.
  • Clean rain gutters/downspouts to prevent water from standing in gutters or on flat roofs.
  • Change water in bird baths, wading pools, etc. at least once a week.
  • Maintain swimming pools properly.
  • Remove or turn over containers, buckets, wheelbarrows, etc. which may accumulate water.
  • Screen rain barrels to prevent adult mosquitoes from depositing eggs on the water's surface.
  • Aerate ornamental ponds and stock with goldfish or other surface-feeding fish to control mosquito production.
  • Dispose of any used tires without rims to prevent water accumulation.

To avoid WNV-transmitting mosquitoes, don't go out after dusk. If you do, wear long-sleeved shirts, long pants, sock and shoes. Loose fitting, light colored clothing is best. Appropriate use of insect repellents per manufacturer's instructions is recommended.

The good news is that with cooler weather coming soon, mosquito populations should fall.

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