Oysters harvested from Katama Bay in Edgartown earlier this summer have been linked to two confirmed cases of Vibrio parahaemolyticus infection, according to a letter received by the town last week. Health inspectors found no evidence that the oysters had been mishandled anywhere along the supply chain.

In the letter, the state Division of Marine Fisheries reported that oysters tied to the first vibrio illness were harvested on June 30 and consumed raw on July 2. The second incident occurred on July 15.

A bacterial pathogen commonly found in warmer waters, vibrio causes gastrointestinal illness. Severe disease is rare and most often affects those with weakened immune systems.

Late last summer, the state closed Katama Bay to oyster farming after two confirmed illnesses were linked directly to the bay. This year no closures have been ordered and state and local health officals said strict controls now in place are being followed to the letter of the law.

The state Department of Public Health and the Division of Marine Fisheries perform joint regulation of the state shellfish industry. They visited the Island following each incident this summer to conduct an audit.

“After a review of harvester practices, no significant noncompliance with the vibrio control plan was identified,” the letter said. Along the supply chain, no evidence of mishandling was found.

The control plan mandates that growers chill oysters immediately following harvest and provide shade for them while in transport.

They must also record time of harvest, time of icing, the quantity of oysters harvested and the primary buyer, among other data.

Edgartown shellfish constable Paul Bagnall said within a two-week period the Katama oyster farmers harvest 100,000 oysters, so to expect that no one will get sick may not be realistic.

“There is a reason why there is a yellow cautionary sign on the raw bar,” he said, also expressing concern for those who had suffered the illness.

If four illnesses are found to have occurred within a 30-day period, the state will close the harvest area, according to the Division of Marine Fisheries.

Statewide, there have been four confirmed infections this summer, including two traced back to Dennis, a town on Cape Cod.

But tracing the illness back to the source isn’t an exact science, Mr. Bagnall said. Often oysters grown in one bay are served alongside oysters from another geographical area, so it’s hard to tell which was the culprit.

“You might end up with an illness with multiple areas implicated,” Mr. Bagnall said.

Last summer’s closure was disruptive for local oyster farmers and businesses and precipitated the set of more restrictive regulations for the handling of raw oysters. Mr. Bagnall said last year was the first time confirmed cases of vibrio had appeared on the Island.

This summer, consumers are benefitting from the stricter rules and relatively colder bay waters, he said.

“I think this cool summer is helping,” Mr. Bagnall said.