A second round of blood tests has confirmed that two Island men who fell ill earlier this month with symptoms of pneumonia actually had the pneumonic form of tularemia, the rare bacterial disease that killed a Chilmark man almost two years ago.

This marks the third summer in a row that the disease has infected people here. The year's first new cases bring the total tularemia count to 21, proving that the disease has not loosened its grip on the Island.

In 2000, there were 15 cases. Last year, that number fell to four. All but five cases have been the rarer pneumonic form of the disease.

That fact is what's alarmed the medical community. More commonly, humans contract tularemia after being bitten by a dog tick infected with the bacteria.

Public health officials are convinced that landscapers are at the highest risk for contracting the pneumonic form of the disease. Of the 21 total cases since 2000, 17 of the victims were either landscapers or people who worked outdoors.

Scientists from the federal Centers for Disease Control (CDC), who have come to the Island three times to investigate the tularemia outbreak, believe that most of the tularemia victims must have inhaled the bacteria while mowing a lawn or cutting brush.

The theory is that the bacteria can persist in the environment, possibly for weeks at a time, in the feces or urine or carcasses of animals susceptible to the disease such as rabbits, rats or skunks.

After questioning victims, the CDC found that most had been mowing a lawn within the two weeks prior to becoming ill with the flu-like symptoms.

This year, the two confirmed cases were two males, aged 22 and 35. One is a full-time landscaper, and the other frequently does landscaping work. They were treated with intravenous antibiotics and released after two days in the Martha's Vineyard Hospital earlier this month.

The first blood test was not conclusive. But the second tests, taken a couple weeks later, "were significantly elevated by comparison, showing a four-fold rise in antibodies," said Dr. Bela Matyas, epidemiologist for the state Department of Public Health (DPH).

Three weeks ago, the DPH released new advisories, one aimed specifically at landscapers, warning them to wear dust masks or respirators while they work and to check an area for animal carcasses before mowing or brush-cutting.

The advisory urged landscapers to dispose of animal carcasses in double plastic bags. The general advisory repeated the same warnings and recommended that people seek immediate medical attention if they "develop fever or respiratory symptoms within seven days of potential exposure to aerosols of dust, soil or grasses, or after direct contact with a wild animal."

Skin sores or swollen lymph glands after a tick bite are considered telling symptoms of the more common form of tularemia.

In a typical year, tularemia infects only one or two people statewide, sometimes none. On a national level, the Vineyard occupies a unique status when it comes to tularemia.

No other place in the country has experienced an outbreak of pneumonic tularemia. And the Island has seen two such outbreaks, the last in 1978 when 15 people were infected.

"There are environmental circumstances that are carrying a risk for pneumonic tularemia," said Dr. Matyas. "Those circumstances have stretched over three years."

It's not known right now whether the latest confirmed cases of tularemia will prompt another visit to the Island by the CDC.

Last week, Harvard parasitologist Sam Telford and his team of three scientists were on the Island collecting ticks and deer flies and trapping skunks, hoping to find some evidence of the tularemia bacteria.

They focused most of their efforts around the Katama and Squibnocket areas, where CDC epidemiologists believe many of the victims were exposed to the bacteria.

Dr. Matyas said there is no point in calling the CDC to the Island to do the same work that the team from the Harvard School of Public Health is doing.

But the CDC may want to return to the Island with high-tech machinery designed to detect the bacteria in the aerosol form.

From the beginning of the Island outbreak, the CDC has made it clear that tularemia's classification as a bio-terrorist agent has intensified its interest in what's happening on the Vineyard.

Meanwhile, health officials on the Island are continuing to investigate the possibility of more new cases. Dr. Dennis Hoak, an infectious disease specialist in Edgartown, told the Gazette yesterday that the Martha's Vineyard Hospital has sent blood samples from six people for tularemia testing. Dr. Hoak described these cases as "low suspicion" for the disease.

For more public health information on tularemia, people may call the DPH at 617-983-6800.