As the map of communities at risk for eastern equine encephalitis continues to expand, health authorities across Massachusetts are testing mosquitoes for the potentially deadly virus.
As of Monday evening, 334 of them have tested positive for the mosquito-borne virus this year. That's "quite a lot, even compared to other active EEE years," state epidemiologist Catherine Brown told NBC10 Boston in an email.
Those tests are one way the state has been assessing the risk that communities face, as officials order aerial spraying and changes to outdoor events to limit people's exposure to EEE.
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Four people have tested positive in the outbreak this year — the first in Massachusetts since 2010-2012, when 22 people were diagnosed with it. Twenty-four communities are under critical risk for EEE, another 24 are under high risk, and planes have been spraying parts of the state to control mosquito populations for weeks.
Methuen was placed under critical risk Monday, the only community in Essex County facing that level of risk. Mayor James Jajuga said in a statement that that level of risk is uncommon for communities in the county.
The state uses the number of mosquitoes that test positive for EEE to help assess risk, but there's more to it than that, Brown said.
Local mosquito control agencies, each of which represents up to a few dozen communities, send mosquito traps to a state laboratory for analysis. Researchers sort through the thousands of mosquitoes in each to find the four to five species of mosquito important for virus transmission, and up to 50 of those are tested, according to Brown.
"Even the mosquitoes that are not tested provide important information," she said. "The total number of a particular species of mosquito in a particular trap helps us understand the relative abundance of the different mosquito populations."
Those population numbers are factored into the state's human risk assessment, she added.
A city may also have its risk level rise when an animal tests positive for EEE. That happened to Methuen Monday. Four animals have tested positive for the virus so far this summer.
"We're being extremely cautious with this," Methuen Mayor James Jajuga said Monday. "We don't want anybody to get sick, and obviously, most importantly, we don't want anybody to lose their life."
The virus is deadly to about a third of the people who contract it, according to the U.S. Centers for Disease Control and Prevention, though Massachusetts has reported no deaths from EEE this year.
But the family of a woman who died in Fairhaven Sunday said she had been diagnosed with EEE. If confirmed by health officials, she would be the first EEE-related death this year.
Health officials have been warning people to cover up and use bug spray to avoid getting bitten by mosquitoes, while having planes spray to control the insects' population.
Outdoor events in high-risk areas are being canceled during the early morning and at dusk, when mosquitoes are most active.