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Massachusetts public health authorities are urging South Coast residents to take precautions against mosquitoes. It could be a significant year for eastern equine encephalitis, which has appeared early in local mosquitoes this summer. 

One animal EEE case — in a horse — was reported in Plymouth on Aug. 6. A human-biting mosquito carrying the EEE virus was identified in a state trap in Wareham on July 30. 

The rare yet potentially deadly virus was found in both human-biting and bird-biting mosquitoes in Plymouth County over the past month, concerning local mosquito control authorities. State officials announced the first EEE virus-positive mosquitoes of the season on July 3, detected in Carver. 

The early appearances tell them that the EEE virus may be established enough in local mosquito and bird populations to amplify and sustain viral activity, which boosts the risk of human infection. 

“You find it in September, you know there’s not a lot of time for the season to progress,” said Priscilla Matton, Bristol County Mosquito Control Project’s superintendent. “You find it in July, you have many more months to go.”

Local mosquito control authorities had expected to see the EEE virus in mosquitoes in 2024, because of recent wet weather and recovering populations of black-tailed mosquitoes — an important virus-spreading species.

No human cases of EEE have been reported thus far to the Massachusetts Department of Public Health. DPH has detected EEE virus in 29 mosquitoes in Plymouth County, including in 14 human-biting mosquitoes. 

Early-season EEE activity in Plymouth County already seems to be amplifying transmission in the environment, posing a risk to residents, said Plymouth County Mosquito Control Project superintendent Ross Rossetti. 

“I think it’s definitely taking a step up,” Rossetti said. “Concentrate on being safe.” 

Bristol County has not seen any EEE-virus carrying mosquitoes yet. Still, “mosquitoes don’t know boundaries very well,” Matton said, and the county has seen EEE before. So the virus may show up, given its appearance in neighboring Plymouth County. 

“I think that it does lead to a little bit more urgency that people do take precautions,” Matton said. 

What is EEE?

Eastern equine encephalitis — or EEE — is a rare and serious viral disease found in Massachusetts and other East Coast states. The virus is transmitted to humans through mosquito bites. It mostly circulates between birds and black-tailed mosquitoes around freshwater hardwood swamps. 

The first symptoms of EEE are fever, stiff neck, headache, and loss of energy. These show up within three to 10 days after a bite from an infected mosquito. 

In severe cases, EEE may also cause inflammation of the spinal cord membrane and brain. Symptoms can progress to seizures, vomiting, and coma. Approximately one in every three people with severe EEE dies. Many of those who survive have significant brain damage or a disability. There is no commercial human vaccine nor specific antiviral treatment available. 

Massachusetts is a nationwide hotspot for EEE. The state’s abundant red maple and white cedar swamps serve as important larval habitat for black-tailed mosquitoes, as well as seasonal habitat for migratory birds which can carry the disease. The sizes and concentrations of these ecosystems in southeastern Massachusetts are among the largest on the East Coast.

Still, contracting the disease is relatively uncommon, and requires a complex chain of events. 

Black-tailed mosquitoes — the major amplifier of EEE in the environment — rarely bite humans. They mostly take blood meals from birds. The mosquitoes pick up the EEE virus from infected birds and spread it around the local bird population. 

For a person to get EEE, a more opportunistic mosquito (called a “bridge vector”) must bite an infected bird, and then bite a human. These more opportunistic bloodsuckers include species like floodwater mosquitoes and cattail mosquitoes.

Roughly 5% of people infected with EEE develop a severe case of the disease, according to the U.S. Centers for Disease Control and Prevention.

Since EEE was first identified in Massachusetts residents in 1938, roughly 115 human cases have been reported in the state. Between 2000 and 2023, 43 reported human cases occurred. 

Getting ready for EEE

To prepare for a potentially serious EEE season, Plymouth and Bristol County Mosquito Control Projects cleared thousands of feet of ditches, culverts, streams, and areas of standing water across 48 towns during the fall and winter. 

They have also been larviciding, as they do every year, throughout area swamps, ponds, and other wetlands since the spring. This will continue through the summer. 

Plymouth County Mosquito Control Project is stepping up ultra-low volume adulticide spraying with trucks around the Carver, Plymouth and Middleborough areas, Rossetti said. It’s also spraying a wider range of towns, and conducting weekly on-demand truck spraying. 

Matton said Bristol County Mosquito Control Project started weekly on-demand truck spraying in June, and has been conducting normal catch basin treatments and surveillance activities. 

Still, these efforts cannot mitigate all risks of contracting EEE. And with West Nile virus also starting to appear in the region’s mosquitoes, authorities say it’s important that South Coast residents take action to protect themselves. DPH announced the state’s first case of West Nile Virus this year — in the Springfield area — on Aug. 6. 

“We can’t kill every mosquito, and we don’t want to kill every mosquito,” Matton said. “It’s important for people to just make sure that they’re doing the best that they can to avoid mosquito bites on themselves and their family.”

People should clear gutters, buckets, and areas of standing water on their properties, so as not to accidentally breed mosquitoes, Rossetti said.

Residents should avoid going outside during hours of peak mosquito activity like dusk and dawn, wear long sleeves and pants, and use plenty of repellent with an Environmental Protection Agency-registered ingredient, Matton said. 

Email environmental reporter Adam Goldstein at agoldstein@newbedfordlight.org.