Mosquitoes testing positive for West Nile increasing in Phoenix area

This year, there are more than 150 positive mosquito samples for the virus.
Published: Sep. 26, 2023 at 5:22 PM MST
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PHOENIX (3TV/CBS 5) — Despite the hot and dry summer, more mosquitoes are testing positive for West Nile virus across the state. The Maricopa County Environmental Services Department collects and tests mosquitoes for West Nile. In one night, crews collected 89,000 mosquitoes in one trap in a neighborhood in Mesa. “Throughout the week, we set over 900 CO2 traps, these attract mosquitoes, and they bring them into the lab,” said vector lab division manager John Townsend.

In the lab, swarms of mosquitoes are collected, tested, and studied inside the vector lab. “This year started off really fast. May, June, lots of positive mosquitoes,” he said.

But a dry, hot summer made samples a lot smaller this year. “A mosquito that normally lives four weeks to 6 weeks, probably only three weeks it was living,” Townsend said. “Less chance of it to come in contact with a bird with West Nile to have it go through and become infected.”

He says mosquitoes become infected by biting infected birds. This year, there are more than 150 positive mosquito samples for the virus, compared to 62 last year. “Normally, we have a little more activity in the southwest Valley, but this year, we’ve had it northwest, southeast, and kind of spread all over the place,” he said.

Townsend says that the most important part of their lab work is tracking which areas are being impacted. “We can set it up for treatment and try to find out where breeding sites are,” he said.

As the weather begins to cool down, he expects both positive samples and cases to trend down, too. “We had a positive West Nile last week, but that was the first one we’d had in probably close to the month,” Townsend said.

Although there is concern this year, it does not come close to the 2021 West Nile virus outbreak in Maricopa County. The vast majority of cases that year came in September and October.

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