BOSTON – A baffling disease that makes lobsters ugly, but not inedible, has crept northward from the Buzzards Bay hotspot where it has afflicted lobsters for several years.

The numbers of infected lobsters are far too tiny to cause panic, but the disease’s progress is being eyed warily by regional researchers and lobstermen. The disease doesn’t affect the meat, but a lobster with a corroded, blackened shell is a tough sell.

“You go and spend $8 for a lobster, you want a good-looking lobster,” said Edward Heaphy, a lobsterman of 50 years from Dover, N.H.

In 1998, diseased lobsters began filling traps in the Buzzards Bay area, off the coast of southeastern Massachusetts. Almost a quarter of all lobsters sampled by the state in the bay that year had the disease, known as shell burn.

In the years since, the diseased lobsters were found in lesser numbers in Cape Cod Bay and Boston Harbor. Last year, according to preliminary numbers, 3 percent of lobsters caught off Salem and Cape Ann had the disease – the first time since sampling began there in 2000 that any infected lobsters were recorded.

“We’ve seen, year by year, a slow, steady progression northward,” said Bob Glenn, a biologist leading the coastal lobster studies at the state Division of Marine Fisheries.

Arthur Sawyer, a second-generation Gloucester lobsterman, said he has spotted a couple diseased lobsters in the last year or two, but added, “You’re still talking about nothing.”

He said the disease is worth watching because of its mobility and unexplained cause.

“To say whether it’s going to get worse or not, nobody knows,” Sawyer said. “Those guys got creamed down there [in Buzzards Bay].”

The state’s lobster catch was worth $56.7 million in 2002, the most recent year for which statistics were available.

The shell disease hasn’t been tied to any mass lobster deaths and the shellfish seem to survive it reasonably well, though perhaps in a weakened state, Glenn said.

The disease is caused by the chitinolytic bacteria that eat chitin, a celluloselike substance in the shells. The disease has been around forever, but the strain that has hit Buzzards Bay lobsters could be new and more virulent, Glenn said.

Other theories tie it to pollution, warmer coastal waters or some weakness in the lobsters. Glenn said that unless researchers find an easily contained cause of the disease – say, a leaking chemical pipe – nature likely will have to fix the problem.

“It’s not like livestock, where you could inoculate them,” he said.

The disease has yet to significantly affect Maine – where fishery officials recorded a minuscule 44 cases of shell disease among 130,000 lobsters sampled in 2003 – or New Hampshire, where the disease turned up in 43 of 14,308 lobsters.

“Right now, I don’t think it’s anything to be concerned about,” Heaphy said. “We’re keeping our fingers crossed.”