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Vibrio: Shellfish Farmers’ Summertime Foe

The summer heat that ripens vast fields of gorgeous strawberries in Washington’s Skagit Valley also raises anxiety levels among shellfish farmers on the nearby Samish Bay tideflats.

The longer the heat spell, the higher the risk that bacteria called Vibrio or other such microbes will multiply among their millions of prized oysters, says Bill Dewey of Taylor Shellfish, the nation’s largest shellfish company. 

So it was that last week, when Pacific Northwest temperatures finally edged into the 70s and stayed there for a while, oyster-lovers and oyster growers paid the price. By week’s end 18 illnesses had been linked to commercial shellfish operations, and four more to recreational harvesting in Puget Sound and the Washington coast in an outbreak of Vibrio parahaemolyticus.

State health officials warned shellfish lovers to ice or refrigerate oysters, rinse them and cook them thoroughly -145 degrees for 15 seconds – to destroy the vibrio bacteria.

Dewey knew this was coming, because it happens virtually every summer.  But there is nothing they can do about it because, to date, growers have not found a way to monitor for vibrio.  All he and other growers can do is wait for somebody to get sick  — bad news for the seafood lovers who crave their raw oysters, and for entrepreneurs who grow and sell them.

Standing on the mudflats, surrounded by hundreds of acres of shellfish beds, Dewey looks every bit the farmer he is — faded jeans, a work shirt and heavy hip-wader boots sunk a couple of inches in tidal muck.

As he discusses the problem, midday heat creates a steamy mist across the dark tideflats, reminding him of the microbial rogue’s gallery lurking in his crop. Those rogue microbes generally fall into one of three biological groups that are all naturally occurring and frequently confused with each other, Dewey says.

First there is Paralytic Shellfish Poison, sometimes known misleadingly as “red tide,” which is perhaps the best-known and certainly the most frightening.  PSP is not a bacterium itself, but rather a biotoxin that is produced by certain species of microscopic  algae. 

That toxin can be carried by all shellfish with hinged shells – from oysters and scallops to clams and mussels. It can be  carried by crab as well, but does not infect the meat, so is not considered dangerous.

PSP is highly toxic, and frequently fatal.  The toxin is tasteless, and does not cook out of the shellfish.  And there is no known antidote.

Richard Lillie, a shellfish specialist at the Washington Department of Health, warns that consumers are easily confused by the term, “red tide.”  In Atlantic waters, the water may turn red with the biotoxin, but not in the Pacific, he says.

However, PSP is also fairly rare, thanks in large part to scientists who have been studying the stuff and come up with ways to identify harmful blooms before they make people sick.  In Washington’s case, this consists of caged shellfish in strategic spots around Puget Sound and other coastal waters.  Those cages are checked frequently, and if any of those sites tests positive for PSP, the area is immediately closed to harvest.

For most shellfish growers, the more common problem is Vibrio, a family of bacteria that bear a cosmetic similarity to distant cousins such as Salmonella. And, while it may not be as deadly as PSP, Vibrio poses another problem – it is extremely difficult to detect.

Around the Gulf of Mexico, which still dominates the oyster markets, the major concern is Vibrio vulnificus. Every year, Dewey says, it sickens and kills people – but only people with compromised immune systems, generally due to illnesses.  “The same number of people die each year from Vibrio after swimming in the gulf – people who are immuno compromised,” he explains.

But Taylor and other Pacific Northwest shellfish growers don’t have to worry about that one, Dewey adds. It is rare in Northwest waters.

Instead, they worry about Vibrio parahaemolyticus, a close cousin which gets along just fine in the Pacific – as demonstrated by last week’s outbreak in Washington.  This microbe probably won’t kill you, Dewey says.  But it causes a gutache and severe diarrhea that may make you wish you were dead.

“I’ve had it,” Dewey says.  “And I won’t forget it.”

Most frustrating, shellfish growers have no effective means of monitoring for Vibrio.  Basically, all they can do is wait until somebody gets sick, and the illness is linked to consumption of shellfish.

“We don’t know what to monitor for, other than for actual illnesses,” Dewey explains.

Clams and mussels are not a problem, because the bacteria is killed in the cooking process.  But what about all those aficionados of raw oysters?

Health officials have proposed various processing strategies to kill  bacteria – mechanized processing, pasteurization, and irradiation.  But each of those processes kills the oyster, Dewey says. And oyster lovers aren’t going to like that idea.

So, for shellfish growers in the Pacific Northwest, the best bet seems to be cold weather.   As air and water temperatures rise, Vibrio populations tend to take off.  And, while the rest of the nation has been sweltering, the Northwest seems stuck in a perpetual spring – 70 degree days and frequent overcast.

That’s bad news for Vibrio, but great news for oyster growers and oyster lovers.

© Food Safety News
  • Minkpuppy

    The oyster industry is really getting slammed this summer.
    Down here on the Gulf Coast, we still have beds that were fouled in the BP spill last year and the drought is wiping out the oysters along the Texas coast. The Oyster guys are harvesting private leases here in TX right now and have been pulling up empty shells.