Connoisseurs of clams on the half shell be warned: Citing new evidence, health experts from New York and Texas cautioned yesterday against the consumption of raw or poorly cooked shellfish.

"Potential consumers should be warned that eating poorly cooked shellfish is currently a high-risk venture at best," said Dr. Herbert L. DuPont of the University of Texas Health Science Center in Houston.

He said that "reports appearing with alarming frequency in the United States, Europe and Australia" demonstrate a "clear risk" of gastrointestinal illness and hepatitis-A infection from the consumption of raw or steamed clams and oysters contaminated with viruses or bacteria.

"We say avoid the consumption of raw shellfish," said John J. Guzewich of the New York State Department of Health in Albany. "The average consumer has no way of knowing if their shellfish are contaminated . . . . It's a roll of the dice."

A study reported in the current New England Journal of Medicine found widespread outbreaks of clam and oyster-related gastrointestinal illness in New York that were linked to the consumption of oysters and clams from the Northeast and Canada.

The study, which documented 103 outbreaks in which 1,017 people became ill over an eight-month period in 1982, concluded that the illness usually was associated with raw clams or oysters, but that there was an "unexpectedly high attack rate" among people who ate steamed clams that had not been cooked sufficiently.

Most cases involved gastroenteritis, which is characterized by nausea, vomiting, diarrhea or abdominal cramps within a day or two after eating contaminated shellfish.

While gastroenteritis usually lasts only a few days, rarer cases of hepatitis-A can result in a longer illness, DuPont said in an editorial in the journal. And shellfish-borne infections can be life-threatening for individuals with cancer, diabetes, immune-deficiency diseases or liver disease, he warned.

Local officials said yesterday that Chesapeake Bay shellfish pose no problem.

"We haven't historically had any outbreaks and we don't have any now," said Mary Jo Garreis of the Maryland Department of Health. "I have no reason to advise against eating raw shellfish in Maryland."

Robert Croonenberghs of the Virginia health department concurred, saying that recommendations not to eat raw shellfish go "too far."

But Dr. Martin Levy of the D.C. Commission of Public Health said there has been an "occasional" case of cholera traced to local shellfish. "We say you shouldn't eat raw seafood," he said. "There isn't any question there is a higher risk."

Dr. Michael St. Louis of the Centers for Disease Control said that "there is a clear risk of contracting gastrointestinal disease from raw shellfish" but that, since state reporting systems vary, it is difficult to determine the degree of risk in a particular area.

Guzewich, who coauthored the New York study, said the "potential" for illness exists wherever raw shellfish is consumed. "It's not a localized problem because these things are shipped all over," he said. One outbreak of illness in New York and New Jersey was traced to English clams that also had been shipped to Hawaii, he said.

"We dump our sewage on the water where these animals grow, then we harvest them and eat them alive," he said. "Whatever they ate, they're going to give back to us."

Guzewich urged cooking shellfish thoroughly before eating. He noted that steamed clams often are cooked just until their shells open -- about a minute -- but said it may take four to six minutes to kill viruses. "You've almost got to cook these things until they're chewy, like rubber," he said.