A woman over 50 who has not been named is the fourth case of electrical and electronic equipment in that state since early August, according to the Massachusetts Department of Public Health.
The other cases this month are those of two men over 60 and another man between 19 and 30 years.
This is the first time since 2013 that Massachusetts reported cases of electrical and electronic equipment.
State epidemiologist Catherine Brown told CNN in a statement that warmer temperatures and above-average rainfall in July accelerated virus replication in mosquitoes, which may explain the increase in EEE activity.
Migratory birds could even have spread a new strain of virus, though lab tests could prove that this is not yet complete, they said.
In Florida and Delaware, however, the virus was detected in sentinel chickens No cases have been reported in humans in any state.
EEE kills one third of those infected
EEE is rare but potentially fatal. According to the Centers for Disease Control and Prevention, 5 to 1
0 human cases are reported each year, but about 30% of them are fatal.
The disease can cause a brain swelling that is preceded by flu-like symptoms such as high fever, chills and nausea. Severe cases could lead to seizures or coma, which can lead to brain damage, according to the CDC.
The CDC recommends residents of affected areas to recharge the repellent and avoid the outdoors after dark to reduce the risk.