Pneumonic tularemia is back. Confirming this year's first case of the pneumonic form of the disease, public health officials said yesterday they are prepared to call in the Centers for Disease Control and Prevention (CDC) to investigate.

Five other cases of pneumonia contracted by people who work outdoors as either landscapers or contractors are still being evaluated to see if they test positive for pneumonic tularemia, signaling a repeat of last year's outbreak, which infected 15 people, killing one.

But just the confirmation of one case alone - a 78-year-old Island man who lives on a farm - has already amazed the state's top epidemiologist. "It's very weird, very, very unusual to have another case this year that indicates a persistence of the disease," said Bela Matyas, medical director of epidemiology at the state department of public health (DPH). "Obviously there's some type of exposure that continues to pose a risk to people. What it is, we don't know with certainty."

Two weeks ago, state health officials were not nearly so alarmed when they confirmed the year's first case of tularemia resulting from a tick bite - a four-year-old boy vacationing in Chilmark. Tularemia in any form is rare, limited to one or two cases a year statewide, and sometimes no cases at all.

Rabbits are the most common carriers, but other rodents such as rats and skunks can also become infected. Typically, a dog tick carries the bacteria from rodents to humans, but last year, 11 of the 15 people who got sick with tularemia contracted it by breathing in contaminated air particles, most likely while mowing lawns or cutting brush.

Nationally, Martha's Vineyard is the only place ever to experience an outbreak of pneumonic tularemia. It's happened not once but twice, and now possibly three times. An outbreak in 1978 also infected a total of 15 people. But despite the experience, scientists still don't know the source. Is it rabbit carcasses or rodent feces or soil soaked with a rodent's urine? Is there an overpopulation of rabbits or possibly rats that are to blame?

The good news, said Dr. Matyas, is that investigators can get a jump-start this year, collecting data while the disease is still active. "We need to do a lot more investigating to figure this out," he said. "But now we know about it a lot sooner, giving us a better chance of finding the source while it's still there."

Dr. Matyas said investigators need to return to the Vineyard in July and try again to crack this mystery with fresh evidence. He said that since last year's CDC field studies were done in mid-September, the bacteria might not have been as prevalent as in mid-summer, when most of the people were getting sick.

Results from the CDC study turned up very little hard evidence. After weeks spent on the Island and months back at the division of vector-borne infectious diseases in Fort Collins, Colo., epidemiologists could only say that lawn mowing and brush cutting were the top risk factors for acquiring pneumonic tularemia.

Of 40 mammals trapped and the dozens of air, soil and grass samples collected, the only signs of tularemia bacteria were found in a Chilmark skunk and a Katama rat, both of which tested positive for the disease.

"We need to decide what kind of studies would make sense now," said Dr. Matyas. Ideally, he added, a scientist from the academic world would also get involved. Earlier this year, some scientists from the CDC assigned to the tularemia investigation were pulled off the case and sent to England to help public health officials deal with the foot and mouth outbreak.

Meanwhile, public health officials have sent out new advisories to primary health care providers, alerting them to the first confirmed case of pneumonic tularemia. Physicians are treating many of their pneumonia patients as if they had tularemia even before blood tests come back. In the case of the 78-year-old Islander with pneumonic tularemia, initial tests came back negative because it was too early in the disease for antibodies to have developed.

A DPH spokesman said yesterday that there is no need to cause a panic in the ranks of tourists coming to the Island. "We need to balance this information," said press spokesperson Roseanne Pawelec. "We're not seeing any instances of potential tularemia outside of people who work extensively outdoors. There's no great risk to people just walking around the Vineyard."