Dry, hot weather keeping cases of West Nile virus low in Maricopa County

PHOENIX (AZFamily) — This year, at least five states have confirmed human cases of Eastern equine encephalitis (EEE).

It’s rare but can infect mosquitoes, humans and some animals. It’s already been deadly in at least one case in the U.S.

Arizona has seen EEE before. In 2023, a horse in Cochise County tested positive for the virus. However, the larger problem nationwide and here in Arizona is the other mosquito virus, West Nile.

This year, there have been 18 reported cases of West Nile Virus in Arizona, 15 of which were in Maricopa County. That’s rather low compared to this time last year when we had 70 cases.

“Your mosquito population can go from no mosquitoes biting you during the day time to hundreds of thousands of them in a week’s time,” said John Townsend, division manager of the Maricopa County Environmental Services Department’s Vector Control Division.

He says this year’s heat is impacting the life span of mosquitoes.

“Believe it or not, mosquitoes don’t like that,” he said.

While the overall number of bugs may be down, there are still cases of infected mosquitoes.

“We’ve had quite a bit of positive West Nile Virus mosquito samples come in,” Townsend said. “In a year where it’s really active, and we have a lot of monsoon rains, we might have 10 times that amount of positive mosquitoes or more.”

Every week, crews set about 860 mosquito traps in Maricopa County.

“The attractant is carbon dioxide and we put dry ice in the bucket that’s got holes in it, so it releases the CO2,” Townsend explained. “That makes the mosquitoes think there is an animal present. They come towards it and get caught in a fan.”

The mosquitoes biting you at night are often the ones you should worry about. West Nile has been prevalent in the state since about 2003, but according to Townsend, they also test for other viruses.

“We’ve seen co-infections of Saint Louis Encephalitis since 2016, and a couple of weeks ago was our very first Saint Louis Encephalitis-positive mosquito for this year,” he said. “So we’ve kept that streak alive of those two viruses circulating in the nighttime mosquitoes.”

Townsend says the best way to protect against mosquitoes is to cover yourself with long sleeves and pants when out at night.

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