Suspicions that tularemia has made a comeback on the Vineyard for the third summer in a row have prompted a series of new health advisories aimed at the group of people at highest risk for the disease - landscapers.

Public health officials are still waiting on blood tests that would confirm whether two landscapers - ages 22 and 35 - who ended up in the hospital early this week had the pneumonic form of tularemia. They were treated with intravenous antibiotics and released after two days in the Martha's Vineyard Hospital.

But even the likelihood of tularemia's presence has set off an alarm. Both the state Department of Public Health (DPH) and the Martha's Vineyard Hospital have issued warnings that urge people to take one simple precaution: Wear dust masks when mowing lawns or cutting brush.

"We had two cases in the hospital, and I'm highly, highly suspicious about them," said Dr. Dennis Hoak, an infectious disease specialist in Edgartown. "The main point is to have people start to wear the masks, especially the landscapers."

Dust masks could be the best barrier against pneumonic tularemia, which is contracted simply by breathing in contaminated air particles. While the disease is rare in any form - infecting only one or two people a year statewide - the pneumonic form is even rarer and more virulent than typical cases of tularemia, which are transmitted by a bite from a dog tick.

What has puzzled physicians and scientists is that the Vineyard seems to breed airborne tularemia bacteria like no place else in the country. In the last two years, 14 people have contracted pneumonic tularemia on the Island, most of them landscapers. In August 2000, the disease killed David Kurth of Chilmark, who had been mowing a lawn near Squibnocket about a week before he developed flu-like symptoms and then failed to seek medical attention soon enough.

Another five cases in that time period were the more common - ulceroglandular - form of the disease.

But while scientists from the federal Centers for Disease Control were able to pinpoint landscapers as the group at highest risk for infection, they have never been able to crack the code for why Martha's Vineyard holds a geographic monopoly on outbreaks of pneumonic tularemia.

Back in 1978, another outbreak infected 15 people with pneumonic tularemia, seven of whom were exposed to the bacteria in one cottage in Chilmark.

If the most recent suspected cases are confirmed, the CDC would very likely return to the Island, making it their fourth trip here in two years.

They were here last June, dragging for dog ticks, trapping rodents and drawing blood samples from landscapers. Of the more than 100 landscapers tested, nine per cent tested positive for tularemia antibodies, according to Dr. Katherine Feldman, an epidemic intelligence officer from the CDC's division of vector borne infectious diseases in Fort Collins, Colo.

"Nine per cent positive, that's a large percentage of people in that particular occupation," said Dr. Hoak.

The real question is whether tularemia has remained active for another year. And even more interesting to the CDC is that tularemia appears to infect people on the Vineyard in an aerosol form.

"The clustering that occurred in the last two years there was unusual," said Dr. David Dennis, chief of the CDC's branch in Colorado. "We really want to understand what's going on in the environment that creates this episodic risk."

Dr. Dennis acknowledged that tularemia's classification as a bio-terrorist agent has intensified his agency's interest in the Island outbreaks, especially in the aftermath of the Sept. 11 terrorist attacks.

"Tularemia is a disease that was put on the back shelf because of the kind of resources needed to examine how it behaved in the environment. Now, of course, thinking about bio-terrorism, we are concerned about aerosolized exposure and what that would mean and to quantify the risk," said Dr. Dennis. "We're looking for field opportunities to answer those questions. Certainly, Martha's Vineyard is one of the best places we can think of to do that."

Indeed, the Island has become something of a working laboratory for studying tularemia. Sam Telford, a parasitologist from Harvard's School of Public Health who has studied Lyme disease both here and on Nantucket, became so fascinated with the tularemia outbreak that his team of researchers joined the CDC investigation last year.

Both Mr. Telford and the epidemic intelligence officers from the CDC believe the answers lie somewhere in the Island ecology, possibly emanating from some peculiar mix of climate and mammal population. Both have trapped dozens of animals and dragged fields for hundreds of ticks in pursuit of clues. They have concentrated their efforts around the Katama section of Edgartown and the Squibnocket region of Chilmark, two areas where experts believe many of the victims were infected.

Tularemia, also known as rabbit fever, is most commonly associated with that animal, which is plentiful on the Island. While Mr. Telford's team reported that 12 rabbits trapped in Chilmark tested positive for tularemia, he also harbors strong suspicions that the Island's abundant rat and skunk populations could play a role in the tularemia cases on the Vineyard.

Still, after all the grass clippings, ticks and rodents culled from the Island, scientists still have no hard answers to the mystery. So far, prevention has been the best tool. With the health advisories urging use of dust masks, tularemia cases between 2000 and 2001 fell from 15 to four.

That could mean that the bacteria was less present, said Dr. Hoak, or it could mean that getting the word out to landscapers effectively reduced exposure to the airborne bacteria.

Prompt medical treatment is the other key. Symptoms resemble the flu and can include swollen glands, sore joints, chest discomfort, vomiting and sore throat. For more information about tularemia and the health advisory, people should call the hospital at 508-693-0410, extension 804.