BOSTON – A 5-year-old girl from Halifax and an 83-year-old man from Kingston have died after being diagnosed with the mosquito-borne eastern equine encephalitis, bringing the number of human EEE cases in Massachusetts this year to three.

The girl became ill Aug. 26, and died Sept. 4, public health officials said. Lab tests later identified the disease, although the results of a final test are pending. The man became ill on Aug. 21 and died five days later. Tests confirmed the disease a week after he died.

It wasn’t clear how or when the two victims contracted the virus, said Dr. Al DeMaria, Department of Public Health director of Communicable Disease Control. He released few other details and declined to identify them by name, saying the department needed to protect their privacy.

“Obviously this is a horrendous situation for these families to have something like this happen,” he said.

Last week, the health department announced that a 63-year-old Duxbury woman had contracted the state’s first human case of the disease this summer. The dangerous virus also turned up in four New Hampshire residents in recent weeks; none has died.

The Duxbury woman was the first case diagnosed and reported, but she became ill on Aug. 26 after the man from Kingston had already developed symptoms. She continues to be hospitalized in serious condition.

Last year, there were four human cases of EEE confirmed in Massachusetts, two of them fatal.

Public health officials said residents can take steps to protect themselves from the disease by trying to avoid mosquito bites until the first frost.

As of last week, the state had tested around 120,000 mosquitoes for EEE, and only 15 of those insects tested positive for the virus.

The Massachusetts Department of Public Health has also confirmed reports of EEE in two horses, one in Haverhill and one in Wrentham. This season both the EEE virus and West Nile virus have been detected in mosquitoes and birds in a number of communities across the state.

EEE can be transmitted to humans through mosquito bites, and August and September are typically the months when it emerges in humans. Symptoms range from mild flulike illness to inflammation of the brain, coma and death.

The disease tends to appear in cycles, DeMaria said. The last outbreak was in 1990; with the cases last year and the three cases this year, public health officials believe that they are in a “high part of the cycle,” DeMaria said.

“We were concerned that there might be more cases this year than last year. There was a lot of discussion, there was a lot of planning, and mosquito control efforts that went into place … hopefully we won’t have any more,” he said.

The disease kills around 35 percent of the people who contract it, according to the Centers for Disease Control, making it one of the more serious mosquito-borne diseases in the United States. Some 35 percent of those who survive it have mild to severe neurological problems.

There have only been about 200 confirmed cases in the U.S. since 1964, according to the CDC. Massachusetts is among the states that has the largest number of cases; others are Florida, Georgia, and New Jersey. There is no licensed vaccine for humans.