Massachusetts shut down oyster beds in the state after illnesses linked to bacterial contamination were confirmed.
The culprit is the Vibrio parahaemolyticus bacterium, which has occurred in state waters since the 1960s, according to the Associated Press.
Last month, Connecticut closed oyster beds and issued a voluntary oyster recall after its first Vibrio outbreak, which sickened at least 14 people. Also last month, officials in Seattle warned oyster fans that Vibrio had sickened twice as many people as normal.
The bacterium causes gastrointestinal problems, including vomiting and cramping, but the illness is generally severe only in people with weakened immune systems. The Centers for Disease Control and Prevention estimates the U.S. has about 4,500 cases of Vibrio infection annually.
Since May, Massachusetts has had 50 laboratory-confirmed cases of Vibrio, compared with 27 during the same period last year.
Other states have also seen increases in Vibrio-related illnesses.
Experts are unsure what has prompted the recent upticks in bacteria-related illness.
"Honestly, I'm confused by the whole thing," Don Merry, an oyster grower from Duxbury, where oyster beds have been closed, told the AP.
Average monthly daytime water temperatures in the region rarely approach the 81 degrees believed to be the threshold that triggers dangerous Vibrio growth. Rising average water temperatures locally, while not reaching that threshold, could be causing environmental changes that cause strains of Vibrio to thrive, Suzanne Condon, associate commissioner of the Department of Health, told the AP.
In addition, virulent Vibrio strains that aren't as temperature-sensitive may have been carried from overseas in ships' ballast water in the past decade, said the state's chief shellfish biologist, Michael Hickey.