Gulf Coast oyster reefs may be home to emerging infection threat

oystersgeneric.jpgView full sizeScientists are concerned that dinophysis acuminata, an algae normally seen on reefs in the North Sea, has made its way to the Gulf Coast.
When Alabama officials closed state waters to oystering last week, it marked just the second time one of the nation's oyster reefs had been shut down because of a one-celled organism that causes diarrhetic shellfish poisoning.

Scientists fear members of the Dinophysis family of algae represent an emerging threat on the Gulf's shellfish beds, one that could force closures during the height of the winter harvest season.

Dinophysis acuminata, which was found last week on Alabama oyster reefs,  is typically seen in colder oceans such as the North Sea. It is a common problem in Scandinavian countries, where its presence can force the closure of mussel beds for weeks or months at a time.

People eating infected oysters may experience abdominal cramping, nausea, vomiting and diarrhea -- sometimes as quickly as 30 minutes after consumption. Unlike the viruses and bacteria that can sicken people who eat raw oysters, the acid that causes diarrhetic shellfish poisoning does not disappear when oysters are cooked, meaning even cooked oysters from infected beds can cause illness.

Scientists found 3,800 cells of the algae per liter of water in Grand Bay during routine sampling around the oyster reefs there on March 30. At concentrations over 1,000 cells per liter, shellfish, including oysters, accumulate high levels of the algae in their tissue as they filter water to feed.

"We consulted with the FDA. They said we should close the reefs, so we did," said Jeff McCool with the seafood branch of the Mobile County Health Department. "The FDA went back a few days later and tested in Grand Bay. All their samples came back negative."

The Alabama reefs reopened Friday morning.

Texas oyster reefs were closed for a month because of a 2008 bloom of another member of the Dinophysis family, the first time a U.S. shellfish bed was closed for Dinophysis contamination. Scientists believe it reached American waters via the ballast water of ships from Europe.

The FDA attributed a recent rash of oyster-related illnesses in Louisiana and Mississippi to the more-common norovirus. But some biologists have speculated they are actually cases of diarrhetic shellfish poisoning linked to algae blooms, such as the one last week in the Mississippi Sound. FDA officials did not return calls seeking comment.

The two illnesses have identical symptoms lasting one to two days. The primary difference is the speed with which a person becomes sick -- norovirus typically takes 24 to 48 hours to incubate, while diarrhetic shellfish poisoning usually takes less than 24 hours.

Norovirus is commonly known as the stomach flu. It can be passed from person to person and is the culprit behind the mass sickness episodes seen on cruise ships.

Three of Louisiana's seven oyster harvesting areas were shut down in the last few weeks, with norovirus blamed. It is unclear if Louisiana officials looked for D. acuminata. Neither Louisiana nor FDA officials responded to questions about sampling on those reefs.

Ed Cake, an oyster biologist registered with the state of Louisiana, said he believed the disparate locations of the affected Louisiana reefs suggest diarrhetic shellfish poisoning.

"We've had an extremely cold winter, plus a lot of rain that put a lot of nutrients in the water. That's a recipe for these algae blooms, not norovirus. The only way to get norovirus on all these reefs is for an infected person to spread it to each reef," Cake said. "The problems are too widespread to be caused by people vomiting or going to the bathroom overboard on the reefs."

Reefs must be infected by humans with norovirus for there to be an outbreak, Cake said, while algae blooms can occur without any help from people. He also noted that there have been a number of dinoflagellate blooms along the Atlantic seaboard this year, most attributable to excess nutrients washed into the sea by unusually heavy rainfall.

In e-mail exchanges with oyster biologists and Alabama officials, FDA scientists said that, based on the time it took for people to get sick in the several episodes this year, they believed people were afflicted with norovirus.

In those e-mails, an FDA scientist acknowledged that the agency had little experience with Dinophysis because it is so seldom seen in the United States, and he held out the possibility that the incubation time for diarrhetic shellfish poisoning may not be fully understood.

"This thing is here now so we are adding it to our list of things we look for in our routine samples," said McCool, with the Mobile health department. "It's just a good idea."

Scott Gordon, head of the Mississippi Shellfish Bureau in the Department of Marine Resources, said his agency's sampling is stepped up when there are algal blooms in either Louisiana or Alabama.

"We are seeing Dinophysis in some of our samples, but not at the levels associated with a bloom. It's something we find from time to time," Gordon said. "We're watching for it, but so far it is not an issue on the Mississippi reefs."