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High number of mosquitoes test positive for West Nile virus, health officials say

The areas with the greatest amount of mosquito activity tend to be Boston and the Metro West area, eastern Middlesex County, and the southern portion of Essex County, she said.
The areas with the greatest amount of mosquito activity tend to be Boston and the Metro West area, eastern Middlesex County, and the southern portion of Essex County, she said.(YE AUNG THU/AFP/Getty Images/File 2016)

An unusually high number of mosquitoes in Massachusetts have tested positive for West Nile virus this year, but only two people have been diagnosed with the mosquito-borne disease, state public health officials said.

The high number of mosquitoes infected with West Nile virus, up to 289 samples this year from 189 samples in 2016, caused the Department of Public Health to raise the risk level for the virus from low to moderate in several counties across the state, state public health veterinarian Catherine Brown said.

The areas with the greatest amount of mosquito activity tend to be Boston and the Metro West area, eastern Middlesex County, and the southern portion of Essex County, she said.

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“The area around Worcester also tends to be one of our hotter spots,” Brown said.

These areas are currently at moderate risk.

Every county in southeastern Massachusetts except Dukes County and Nantucket are also at moderate, as well as many counties in western Massachusetts.

“We had unusual amounts of West Nile virus in mosquitoes this year, but there was no correspondence in humans,” Brown said. “We’re not the only state that’s seeing this.”

Other states in New England, as well as New York City, experienced a similar phenomenon, she said. Typically, five to seven human cases of West Nile virus are reported annually in the state, although there have been years with no cases and years with as many as 33.

While public health officials are still unsure as to why the number of mosquitoes and humans with the disease do not correspond, Brown said the primary theory has to do with the fact that the weather has been relatively cool this summer during the day.

“When that happens, that actually changes the behavior of the mosquitoes, and it shifts the time of day at which they do most of their feeding,” she said.

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Mosquitoes are typically most active at dawn, when people are out for runs, and dusk, a popular time for outdoor parties during the summer, but cooler weather means they can feed during the day when most people tend to be inside at work, Brown said.

While the mosquitoes season is winding down, Brown said it’s important to remain vigilant and heed the moderate risk warning.

Most people bitten by an infected mosquito won’t get sick, she said, but about 20 percent of people bitten by an infected insect will come down with a “flu-like illness” and experience fevers, headaches, muscle aches, and chills.

“It’s not very serious, but you certainly won’t feel very well,” she said.

However, for the small number of people whose immune system cannot fight off the virus and it ends up in their central nervous system, the disease can be deadly.

“During the entire West Nile virus season, we recommend that people should be vigilant in using repellents and protecting themselves from mosquito bites,” Brown said.

The Department of Public Health also recommends wearing clothing that covers the skin as much as possible and insect-proofing homes by draining standing water and installing screens in all windows and doors.

“As the cooler weather comes in, people tend to forget about those precautions, but particularly on warm days, which we will continue to have for the rest [of the] month at least, there are days when that mosquito activity is still going to be particularly significant.”

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Alyssa Meyers can be reached at alyssa.meyers@globe.com. Follow her on Twitter @ameyers.