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Common house mosquitoes (Culex pipiens) are the main carriers of West Nile virus.

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Q: How come there hasn’t been any West Nile virus in the area this summer? (Not that anybody wants it.) Isn’t it unusual for East Central Illinois to still be West Nile-free when summer is nearly over?

A: It is unusual, at least for the past decade.

“I can’t really speak to the other counties, but it has been at least 10 years since Champaign County has gotten this far in the mosquito season without a West Nile virus positive mosquito test,” said JEFF BLACKFORD, program coordinator at the Champaign-Urbana Public Health District’s environmental health division.

“That doesn’t mean that the risk is over,” Blackford warned. “Last year, due to an extremely warm September, we were collecting positive West Nile virus mosquitoes in October.”

The risk really won’t end for the year until there’s been a hard freeze that kills vegetation and exposed mosquitoes, so continue to take precautions against mosquito bites, he advised.

By way of comparison, last year, the first batch of West Nile-infected mosquitoes had already turned up in Champaign County in mid-June. In 2017, West Nile was already in Champaign County in July.

This year, West Nile virus has surfaced in 36 counties in Illinois, and nearly all the positive tests have been among mosquito batches, as opposed to birds, horses or other mammals, according to the Illinois Department of Public Health.

Statewide, there have been three human cases of West Nile so far this year (in Kankakee and Cook counties) compared to 176 human cases statewide for the entire season last year.

CHRIS STONE, medical entomologist at the Illinois Natural History Survey, said what’s happened this year is likely a combination of low numbers of Culex mosquitoes (the ones that can transmit West Nile through their bites) and a very low West Nile transmission in the mosquitoes.

This has been a summer of weather extremes — heavy rains, then drought — and this kind of mosquito is particular about the weather conditions it needs for large populations to take off, Stone said.

Too much rain, and the mosquitoes get flushed out of their habitats, he said. What they prefer is a little rain, followed by a dry period.

If the current weather conditions continue, this area may still see a late-season bloom of Culex mosquitoes and some West Nile virus, Stone said. His guess is that the virus level in birds is low enough that West Nile wouldn’t ramp up a lot at this point, but given favorable weather conditions continuing there could still be some transmission, he said.

For at least the next two or three more weeks, Culex mosquitoes can still be breeding, Stone said. Then they’ll begin preparing to over-winter in sheltered areas, such as basements, and lose interest in biting. Some of the survivors over the winter may be infected and they’ll be the ones kicking off West Nile virus season next year, he said.