A Norfolk County woman in her 80s has died of Eastern equine encephalitis. She was the first Massachusetts resident known to be infected this season with the often lethal mosquito-borne illness, state public health officials said Tuesday.
The woman, who has not been identified, was hospitalized earlier this month and died a few days later.
Health officials said they are tracing the woman’s activity in recent weeks to determine where she may have been bitten by an infected mosquito, and they will determine whether to increase warning levels and urge cancellation of night-time activities based on their findings.
“We are right in the middle of peak transmission for EEE,” said Dr. Catherine Brown, state public health veterinarian. “Given the activity we have seen over the last decade for EEE, I am afraid this is the type of thing we are going to see happen.”
Eastern equine encephalitis was once considered rare in Massachusetts, with isolated cases occurring years apart. But infections have increased in recent years, and last season, with seven people infected, was unprecedented. Three of those infected last year died.
In the past week, several batches of mosquitoes were detected carrying Eastern equine in three communities in Bristol County — Easton, Dighton, and Mansfield — where the virus has historically been prevalent because of the region’s sprawling Hockomock Swamp, a prime breeding ground.
Brown said that Eastern equine cases in Norfolk County are unusual, but they are uncertain whether that is where the woman who died was infected.
“EEE has been in Massachusetts since the 1930s, but we are still learning things about it,” Brown said.
Primarily seen in Bristol and Plymouth counties in the past, the disease is now more widespread. The type of mosquitoes that carry it are seen across the state: Two horses infected with EEE have been reported in Belchertown this year. The risk of infection to residents there is critical, the state says, and it is also high next door in Amherst.
Brown said climate change and development have changed habitats for birds and mosquitoes that spread Eastern equine, and scientists are trying to discern what those changes mean for future Eastern equine infections.
The risk of infection from mosquitoes continues until the first hard frost, and health officials urge residents to take precautions against being bitten by wearing insect repellent, and curtailing outdoor activites between dusk and dawn when mosquitoes are most active.
Mosquitoes also carry West Nile virus, which is less lethal but more common in Massachusetts. The risk level for West Nile infections is moderate so far this year in parts of Southeastern Massachusetts, Greater Boston, northern Essex County, and in the Northampton area.Kay Lazar can be reached at firstname.lastname@example.org. Follow her on Twitter @GlobeKayLazar.