BOSTON – A 53-year-old Lakeville woman has been diagnosed as the first human case of mosquito-borne eastern equine encephalitis in Massachusetts this year, public health officials said Wednesday.

The woman, whose name was not released, was in serious condition at an undisclosed Boston hospital, said Dr. Alfred DeMaria, the chief medical officer at the state Department of Public Health.

Tests at the State Laboratory, the only lab in the Commonwealth that can make a positive diagnosis, confirmed the presence of EEE on Wednesday, he said.

The woman’s symptoms started about Aug. 10 and she was hospitalized Saturday. Symptoms usually begin about six days after infection, DeMaria said, so the woman likely contracted the virus around the first of the month, before the state conducted aerial insecticide spraying in Lakeville last Tuesday.

And although he said the spraying reduced mosquito populations by at least 60 percent, “It’s a misapprehension to think that spraying eliminated the risk. It only reduced the risk from ‘really bad’ to ‘bad,”‘ he said.

Almost 160,000 acres in Plymouth and Bristol Counties were sprayed with insecticide for the first time in 16 years. Officials said they expected a larger-than-usual population of mosquitoes after this year’s record rainfall.

The EEE virus, like West Nile, is transmitted to humans through mosquito bites. Symptoms range from mild flu-like illness to encephalitis – inflammation of the brain – coma and death. There is no cure for EEE. According to state health officials, about three of every 10 people who get the virus could be expected to die from it.

Mosquitoes and birds in the state also have tested positive for West Nile virus this year, state officials said.