An insidious disease that afflicts oysters but is not harmful to humans is widespread in Edgartown Great Pond. While there are not yet any reports of die-offs, there is concern that at least a portion of the oysters in the pond will die.

Edgartown shellfish constable Paul Bagnall said he learned this week that 70 per cent of the oysters in the pond may have the disease. The disease is called Dermo and it is no threat to humans, even if you eat the infected oysters. The disease is responsible for wiping out the oyster fisheries in some southern waters. But because of the Vineyard's colder water temperature, the disease is not as harmful here.

The appearance of Dermo is disappointing news to Edgartown commercial fishermen, who have been looking forward to the full recovery of the oyster fishery in that pond. Throughout the summer, shellfishermen have reported seeing a significant return of oysters to that pond after years of effort. The news comes a year after the town of Edgartown spent $13 million to rebuild its wastewater plant and it comes after the formation of a community action group to oversee the activities of the pond and explore ways to help the shellfish recover.

Because affected oysters are harmless and their taste is not affected, they may be commercially harvested and sold.

Roxanna Smolowitz, a senior research investigator for the University of Pennsylvania lab at the Marine Biological Laboratory in Woods Hole, has monitored the disease for more than a year. She told the Gazette yesterday that the disease normally affects 10 per cent of the population of oysters in New England waters. Detection came after water samplings were done late in August as part of a widespread test of waters on Cape Cod. If there is any good news, it is that the scientists believe the disease hasn't killed any oysters yet. The bad news is that it still might. Research into the way the disease destroys shellfish in New England waters is still going on. The scientists believe it won't be so harmful here as it is farther south.

The study of Edgartown Great Pond and several other sites is part of a $100,000, two-year study of the disease. The Marine Biological Laboratory scientists are also looking at ponds in Cotuit, Wellfleet, Chatham, Sandwich, Connecticut and New York. The water quality for all the ponds is similar, the scientist said.

The researchers are interested in what environmental conditions supporting the disease are present in all locations. "We want to know what the water temperature is and the salinity levels are to bring on a major die-off," she said. "We want to know how much we need to worry about our animals, what are the factors, what kind of environmental factors affect the growth of the oyster and the disease."

The disease has no impact on humans. It only affects oysters and scientists don't yet know how severe the disease must be to kill an oyster. The scientists believe that with cold winters on the Vineyard, shellfish do appear to be able to shake off the disease and survive. "Dermo is a protozoan disease. It is a single cell organism, more formally called perkinsus marinus, and affects the eastern oyster. It occurs all up and down the Atlantic Coast of the United States and has been responsible for great mortality in oysters, especially in the mid to South Atlantic Coast. It was first seen in the Upper Eastern end of the east coast in the early 1970s," she said.

The University of Pennsylvania and Rutgers University are working together on ways to understand the disease and offer communities ways to cope with it. It is an interesting disease in that none of it has been found in either Oyster Pond or Tisbury Great Pond, where oysters are plentiful. The disease has nothing to do with water quality.

Dr. Smolowitz said: "We are finding it is present in most cultured oysters, in most locations. But it does not cause the severe mortality that we've seen around the Delaware [River]. It appears this organism is greatly influenced by temperature and salinity."

"What is so baffling is that it is not detectable in Oyster Pond," said Mr. Bagnall. "If you get Dermo in one oyster, it infects others." The shellfish constable said Dermo was not responsible for the extensive die-off of oysters that occurred in 1993. In that year, the whole oyster fishery collapsed in the Edgartown Great Pond. Scientists suspect it was an algae bloom of some kind. Edgartown Great Pond has shown remarkable signs of recovery since the die-off. Mr. Bagnall said water clarity in the pond has improved and there is an increase in finfish, including perch, herring and minnows. "We are looking at an oyster season this fall. Historically we go oystering after the scallop season opens."

Scientists and the fishing community are interested in watching this disease over time. There is no cure, but the scientists feel that learning how to live with the disease and stifle its growth in shellfish will save the oyster fishery. They just have to find out how. The disease is expected to decline as the water temperature gets colder this winter, and observers will closely monitor the number of oysters that survive over the winter.