This year’s infestation of mosquitoes infected with Eastern equine encephalitis is the most intense that Massachusetts has experienced in three decades, a top health official said Tuesday as the state announced a second round of aerial spraying.
Barely two weeks after blanketing 21 Southeastern communities with pesticide, officials said that planes will again take to the skies over six of the cities and towns to prevent human-biting mosquitoes from spreading the often-fatal virus to residents.
No human cases have been reported yet, but health specialists are concerned about the threat to the public.
“It’s extremely important that residents in these communities take immediate steps to protect themselves and their families from mosquito bites,” said John Auerbach, the state’s public health commissioner.
Typically, the first batches of infected mosquitoes are seen in late July or early August, but this season, disease trackers found them weeks earlier.
“We have seen mammal-biting EEE-positive mosquitoes earlier this year than in anyone’s memory,” Auerbach said in an interview. He added that surveillance traps are catching an unusually high concentration of the infected insects.
A weekend of spraying July 20-22 reduced the mosquito populations by 60 percent in the areas covered, but large numbers of the insects are being seen in the region.
The communities to be sprayed again are Bridgewater, Easton, Norton, Raynham, Taunton, and West Bridgewater. Auerbach said two-thirds of the mosquito samples with Eastern equine encephalitis found since the initial round of spraying have been found in these six towns, with 21 insect pools found in Easton alone.
“It isn’t surprising that we have found a concentration in those communities,” Auerbach said, noting that they surround the Hockomock Swamp, a vast wetland that is a prime breeding ground for the type of mosquitoes that carry the virus.
Health officials raised the risk level from high to critical in West Bridgewater, Easton, Raynham, and Taunton and from moderate to high in Norton and Bridgewater. Already in the high-risk zone were Canton, Lakeville, Middleborough, and Rehoboth, and health officials urged residents in those 10 communities to avoid outdoor activities at dusk and at night, when mosquitoes are most active.
But as dusk fell in Easton Tuesday, 33-year-old Melissa MacKinnon pushed her 2-year-old son in a stroller toward the Center School playground.
She said there has been talk around town about mosquitoes, because many people are shocked that the state has found high concentrations of EEE virus in Easton.
MacKinnon is generally not out after 7 p.m. because her son goes to bed early, but she said she applies bug spray to him several times a day.
“We have a can of bug spray at our [front] door; we have a can of bug spray at our back door,” MacKinnon said. “I think that everyone does.”
She said the threat of mosquitoes has not changed her way of living.
“It’s not stopping me from doing anything that I would normally do, just way more cautious,” MacKinnon said.
Eastern equine encephalitis has not been found in human-biting mosquitos outside Southeastern Massachusetts and Cape Cod this year, but mosquitoes infected with the less serious West Nile virus have been found across the state.
Insect specialists say it is not clear what is driving the spike in EEE-carrying mosquitoes this year and the earlier detection of infected insects. Some point to the unusually mild winter that may have protected many more mosquito larvae that normally would have died.
“You would expect that the warm temperatures would speed the development of those mosquitoes that overwintered as larvae,” said Ellen Bidlack, staff entomologist at Plymouth County Mosquito Control.
Reginald Zimmerman, spokesman for the state Office of Energy and Environmental Affairs, said his agency requested $2 million from lawmakers for aerial spraying this season and is awaiting approval. He said the agency has not tabulated costs for the July spraying.
Not everyone was happy at the prospect of more aerial spraying. Beth Connors, 47, a lifelong Easton resident, was walking her cockapoo along with other walkers around the gravel track at Frothingham Park as the sun set.
She said she is more concerned about the pesticide than the mosquitoes, explaining that she thinks most of the infected mosquitoes were found in Hockomock Swamp, where there is stagnant water but not many people. Connors said she has seen more dead birds and small animals around town this year than in the past.
“You’re poisoning all the stuff around here, and you’re killing all our other birds, and I’m not happy about that,” she said.
Connors said she pulls anything she eats from her yard, such as fresh basil, into her house because she does not want it outside when chemicals are sprayed. She said people can avoid mosquitoes easily.
“It’s dusk, you know there’s bugs outside, you put bug spray on your kids, or you stay inside,” she said.
Aerial spraying often sparks concerns about health risks to residents and potential environmental damage to crops and waterways. State officials say they have taken those concerns seriously in selecting sumithrin, a pesticide that is combined with piperonyl butoxide, a compound that activates sumithrin. The pesticide is also known by the brand name Anvil 10+10. They say the combination breaks down very rapidly when exposed to sunlight and has very low toxicity to humans and animals.
Behind the West Bridgewater Middle-Senior High School, Kenny and Joehnna Barros of nearby Brockton said they had not heard about the EEE threat. They said they routinely work out at the playing fields behind the school, surrounded on two sides by trees.
As night fell and Joehnna, 30, swatted a mosquito near her face, Kenny, 29, said the couple would guard more closely against mosquitoes. “We’ll come a little bit earlier,” he said, adding that they will also use bug spray now that they know about the presence of EEE.
Two human cases of EEE were reported last year in the state, including a Raynham man who died.
Officials said spraying will begin as soon as possible, following appropriate public notification and outreach. They said ground-spraying cannot be done in the six communities because trucks cannot easily reach the areas around the swamp.
Residents are encouraged to check local news outlets and the state website at www.mass.gov/dph for further details.
Clarification: An earlier version of this story did not make clear that human-biting mosquitoes infected with the potentially fatal virus have not been found this year outside Southeastern Massachusetts and Cape Cod.