Public health officials this week confirmed the third case of tularemia contracted on the Vineyard. It's the pneumonic form, and it has convinced officials that the Island is in the midst of yet another outbreak of the rare bacterial disease that last year struck 15 people, one of them fatally.

Simply put, whatever caused last year's outbreak is back or maybe never even left, according to David Dennis, chief medical director of the bacterial zoonoses branch at the Centers for Disease Control and Prevention in Fort Collins, Colo.

"What we're seeing is a continuing transmission of the bacteria," said Dr. Dennis, "at levels that are exposing people to the pneumonic form [of tularemia]."

The latest victim is a 52-year-old landscaper. And while health officials never reveal the personal identities of cases, it was widely known that Tisbury selectman Tristan Israel, a landscaper, was very ill for the past few weeks.

While Mr. Israel had not been officially notified by either his physician or any public health agency, he confirmed this week that he is 52 and conceded that in all likelihood he is the third case of the year.

"I got sick on June 17. When it hit me, it hit me hard and fast," he said in a telephone interview. Mr. Israel spent 10 days in the hospital on intravenous antibiotics. "The one important thing I did was after a day of high fever, I went and sought help."

Mr. Israel said he is still weak, but he is trying to return to work on a gradual basis. He mows lawns across the Island, including in Katama and Chilmark, the two areas where investigators think many of the victims have been exposed to the tularemia - either through tick bite or actually breathing in contamined air particles.

The outbreak has stumped scientists. For the third time in a year, a team of researchers came to the Island last week to collect evidence that would unlock the mystery. They tested more than 100 landscapers, who are at the highest risk for getting tularemia, and they bagged as many ticks and mammals as they could get in a few days.

Already, one dead rabbit found in Chilmark has tested positive for tularemia. Tests on other mammals - about 50 mice and voles - trapped last week have not yet been completed.

Mr. Israel said he sees plenty of rodents, alive and dead, while mowing. The problem is, he added, "In the high grass, you don't see it until you're right on top of it." Mr. Israel said he does not wear a dust mask when he works.

Health officials have urged people mowing lawns or brush-cutting to wear a dust mask as a precaution, even though they are not sure it can actually prevent the bacteria from being inhaled.

Meanwhile, a total of six other cases are under evaluation as potential tularemia. Two of them are landscapers whose initial blood tests came back negative. Four other cases were discovered by CDC officials who examined records at the Martha's Vineyard Hospital looking for symptoms that matched tularemia.

No other place in the country has ever experienced an outbreak of pneumonic tularemia. Last summer marked the Island's second outbreak, the first one taking place back in 1978, when 15 people also came down with the disease.

Public health advisories have warned people to check themselves diligently for dog ticks and to seek medical attention if they suffer a sudden onset of flu-like symptoms. But they have also tempered their warnings by pointing out that the disease is not contagious and is most likely to be contracted by people who work outdoors.