Katama Bay oyster farms will remain closed for another week following additional confirmed cases of Vibrio illness tied to the area, a state Department of Marine Fisheries official said Tuesday afternoon.

The 12 shellfishermen harvesting oysters out of Katama Bay were scheduled to resume operation on Thursday morning after a one-week precautionary closure announced last Wednesday by state health and fisheries officials. The closure was prompted because of three reported cases of Vibrio parahaemolyticus caused by consumption of raw oysters from Katama Bay.

Now the closure will be extended for another seven days due to three more confirmed cases of Vibrio from mid-August, state Vibrio program coordinator Chris Schillaci told the Edgartown shellfish committee at a meeting Tuesday. The meeting was attended by a small group of Katama Bay oyster farmers.

Chris Schillaci, state vibrio control coordinator, told Edgartown oyster farmers: "It's not the handling that's the problem." — Mark Lovewell

Under state guidelines, two to four illnesses within a 30-day period would result in a seven-day closure, while four or more illnesses require a 14-day closure. More than 10 illnesses would require a 21-day closure and a recall.

There have been six Vibrio cases reported from Katama Bay in the past 45 days, Edgartown shellfish constable Paul Bagnall said, and three in the last 30 days.

Based on the weather and the number of illnesses, plus a lag time in reporting illnesses, “it made more sense not to put more product on the market just to close again for one more illness,” Mr. Schillaci said. If one more illness was reported, he added, an additional 14-day closure would have been mandated.

“I know it’s not what you guys want to hear,” he said.

Vibrio, a naturally-occurring bacteria, is more abundant in warm waters, and with the Vineyard experiencing a prolonged heat wave in recent weeks, bay waters are warm. The water temperature in Edgartown harbor last week was 76 degrees.

Consuming raw oyster with the bacteria can cause gastrointestinal illness with abdominal cramping, nausea, vomiting, fever and chills. Severe illness is rare and most likely to occur in people with weakened immune systems.

The Katama Bay closure is the first Vibrio-related shellfish closure in Massachusetts this year.

Katama and other oyster-producing areas around New England have seen more reported cases of Vibrio in recent years. This marks the third year in a row that Katama Bay oyster farms have been closed for some period of time in the late summer because of Vibrio.

Ryan Smith, a member of the shellfish committee and a Katama oyster farmer, said while demand remains high for farmed oysters, the repeated closures are stressful and there is a loss of business.

“It seems like nobody really knows what’s going on,” he said, adding that the closures seemed to start “out of nowhere” three years ago.

Oyster farmers peppered Mr. Schillaci with questions about how the illnesses were reported, accounted for and traced to the growing area, and discussed whether more stringent measures might be put in place in future years. In 2013 the state adopted strict protocols for handling oysters, and Mr. Schillaci said he’s observed Katama oyster farmers following those guidelines.

“It’s not the handling that’s the problem here and I truly believe that,” he said.