Eating Raw Oysters Can Prove Deadly To Some

When the Walrus and the Carpenter beseeched some young oysters to join them for a walk and dinner in Lewis Carroll’s famous 1925 poem, not much was known about microbial contamination and compromised immune systems.

But today infants, the elderly and people with diabetes, liver and kidney diseases, cancer, AIDS, alcoholism or other chronic disorders must avoid raw oysters or any raw mollusks. Those succulent bivalves could wreak deadly revenge for the nasty trick played on their ancestors by Carroll’s duplicitous duo.

Raw oysters kill about 15 people a year and cause dozens of others to be hospitalized, mostly because of a common seawater bacterium, Vibrio vulnificus. Oysters eat by filtering microorganisms from seawater, so they are prime receptacles for V. vulnificus as well as any other pathogens that happen to be swimming in the oyster beds.

V. vulnificus isn’t the result of pollution or human contamination. It thrives in the warmer waters of the Gulf of Mexico, especially during summer, says Robert Grodner, professor emeritus of food science at Louisiana State University in Baton Rouge. It does exist in cooler waters but in much smaller amounts, he says.

It almost never causes symptoms in people with fully functioning immune systems. “Any normally healthy person can eat all the raw oysters he or she wants with no problem with it,” Grodner says.

But V. vulnificus is a serious risk for those whose bodies are less strong.

“The bacteria release a poison that kills tissue, causing black skin lesions resembling those produced by flesh-eating bacteria,” says Dr. Herbert Dupont, vice chairman of Baylor College of Medicine in Houston. “Initial symptoms develop 24 to 48 hours after eating and include fever, muscle aches, skin redness, a drop in blood pressure and lightheadedness. Infection can be fatal in 24 hours, so medical help should be sought immediately.”

Even if you are not in the at-risk category, you should see your doctor if you have those symptoms after eating oysters, he says: “Be sure to tell your doctor you’ve eaten raw oysters within the last few days, since treatment requires a special antibiotic.”

Uncooked oysters (as well as any uncooked seafood or other meats) also can be contaminated by hepatitis A and Norwalk viruses and other pathogenic bacteria, he says, which can come from infected food handlers as well as polluted waters. Most of these infections, however, are not lethal, though they can send victims to the hospital.

Commercial oyster beds and shucking houses are inspected for coliform bacteria, which often come from human waste, but not for viral contaminants, DuPont says. One of the largest outbreaks of hepatitis A in Houston came from oysters harvested from a certified bed.

Most restaurants buy from reputable processors, who must tag every bag of oysters traveling interstate so any contamination can be traced to its source, Grodner says. Restaurants have to hold the tags for 90 days after the oysters are sold.

Meanwhile, Grodner and his associates at LSU have patented a pasteurization process to kill V. vulnificus bacteria by subjecting unshelled raw oysters to mild heat followed immediately by rapid chilling in an ice bath. The system is being used commercially by AmeriPure Oysters, a New Orleans company that markets the mollusks across the country.

Though some object that such pasteurization degrades the flavor, Grodner says only an oyster gourmet would notice a difference. The process also prolongs the oyster’s shelf life because it destroys spoilage bacteria. It will not, however, protect against hepatitis A or any contamination by handling after the treatment.

Another process, called depuration, submerges the live oysters in a bath of specially treated water so they purify themselves by purging out any contaminants. But connoissuers say deputation destroys the oysters’ flavor.

One way to eliminate the threat of contamination without affecting quality is gamma-ray irradiation, which also would eliminate any threat of hepatitis. Irradiation, which is used on some fruits and chicken, has yet to be approved for seafood by the Food and Drug Administration.