Doctors have issued a warning to sushi lovers: Beware of a parasitic worm disease that has become much more common as the popularity of the trendy food has increased.

Doctors from the University of California and Food and Drug Administration said at least 50 cases of the disease anisakiasis have been reported in the United States, with a majority of the cases occurring since 1980."This is probably only the tip of the iceberg, because the infection is difficult to diagnose," said Dr. James H. McKerrow, a pathologist at the University of California in San Francisco.

In a letter in The New England Journal of Medicine, McKerrow and two colleagues warned physicians to be on the lookout for the often-misdiagnosed disease and urged sushi lovers to be careful what they eat.

"Much of this increase can be attributed to changes in our dietary habits, for example the introduction of raw-fish dishes such as sushi and sashimi and the current trend of cooking seafood for shorter periods."

The doctors speculated the increased incidence of the disease also may be due to a rise in the population of dolphins, sea lions, seals and other marine mammals since the Marine Mammal Protection Act of 1972. These mammals play a key role in the life cycle of the parasite, they said.

The disease is caused by tiny worms known as a anisakis simplex and pseudoterranova decipiens. These worms initially infest small shrimplike ocean crustaceans known as krill, which are eaten by many ocean creatures, including squid and some fish, which are then consumed by larger fish that serve as food for marine mammals.

In many cases, a human who unknowingly consumes the worm by eating raw or undercooked fish infected with the worm will not develop any serious complications, though the experience can be unnerving.

In more serious cases, the worms penetrate the stomach or intestine, causing abdominal pain, fever, nausea, vomiting or diarrhea, symptoms often confused with appendicitis, the doctors said.

The worms, which are white, average about one-half inch long and about a tenth of an inch in diameter, McKerrow said. "They look like little white wiggling threads."