People hit hard by a severe form of shellfish poisoning suffered chronic memory loss that persisted for at least several months, according to a study of an outbreak that struck parts of Canada in 1987.

A research team led by Jeanne S. Teitelbaum of the Montreal Neurological Institute at McGill University found that four to six months after becoming sick, 12 of the 14 severely ill patients could no longer easily remember objects presented to them a short time before.

Some severely afflicted people suffered from amnesia and could not remember anything that had happened in recent years.

The outbreak of the illness occurred in late 1987 and was associated by the researchers with the consumption of mussels from Prince Edward Island. At least 107 people became violently ill within 48 hours after eating mussels contaminated with domoic acid, which destroys nerve cells.

The chemical came from a form of algae known as Nitzschia pungens, which experienced a short-lived population explosion in river estuaries on the eastern coast of Prince Edward Island. Nineteen of the 107 people affected were hospitalized, and three died. The outbreak has not recurred.

In addition to the vomiting, cramps, diarrhea and other hallmarks of shellfish poisoning, this new form of poisoning also produced a short-term memory loss in one out of every four victims and long-term memory and nerve problems among the people hit hardest.

The more traditional form of shellfish poisoning, known as paralytic shellfish poisoning, is caused by the algae responsible for "red tides." Another form, called diarrhetic shellfish poisoning, causes gastrointestinal problems.

Earlier this year, an oceanographer at the University of Rhode Island warned that pollution spilling into the seas, along with the dumping of algae-laden seawater from ships' bilges into untainted waters, is dramatically increasing the number of algae blooms, which can increase the risk of shellfish poisonings, even in areas that have never had them.