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Eating Smart

Raw Shellfish Is Risky


Eating raw shellfish is potentially dangerous regardless of when or where it was caught. Primarily, this is because of widespread bacterial contamination of fresh water and salt water from untreated sewage. Even areas certified as "clean" can contain viruses and bacteria that are not detectable in the water, most notably the Norwalk and hepatitis A viruses.

Usually these better-known bugs will produce a mild to severe gastrointestinal infection that causes diarrhea, nausea, abdominal cramps, vomiting and, in the case of hepatitis A, jaundice. However, older people and those with poor health or who have compromised immune systems can become seriously ill.

Although all shellfish can be dangerous, clams and oysters, which live by filtering 15 to 20 gallons of water a day, become concentrated containers of the bacteria in the water. Eating them raw is just asking for trouble, particularly if you have liver disease or any immune deficiency.

The main culprit is a bacterium, Vibrio vulnificus , which has been found in almost every geographic location in the United States, from the Gulf of Mexico to Cape Cod and the Great Salt Lake in Utah.

V. vulnificus lives only in saltwater, and it doesn't need contaminated water in which to grow. In fact, it thrives in cleaner water. It is most likely to be a problem in the summer months, but the range of reported infections is March to November.

It was once thought that using hot sauce or drinking alcohol with the oysters would kill this nasty bacterium, but these reports have not been borne out by scientific study.

The symptoms of infection include fever, chills, changes in mental status, low blood pressure, nausea, vomiting, abdominal pain, diarrhea and severe skin lesions. It is such a serious infection that treatment is usually given in the intensive-care unit of a hospital. Skin lesions may become so bad, they require limb amputation.

The good news is that this infection is rare in people who do not have one of the known risk factors; the bad news is that a lot of people have these risk factors. They include: liver disease or other diseases with possible liver involvement (including cirrhosis, alcoholism, malignancy, hemochromatosis or thalassemia major); low stomach acid concentrations, which either occur naturally or through heavy antacid use; compromised immune systems; cancer; diabetes; and kidney disease. The disease is far more common in men than women and in people older than age 40. It comes on within 24 hours of eating raw shellfish or even of just being in contact with seawater when there is an open wound on the body.

It also has occurred in otherwise healthy people who have been cleaning shellfish and develop a wound. It can be treated with aggressive antibiotic administration, but if this is not done promptly, the disease can be fatal.

Although it is probably impossible to keep from being exposed to the V. vulnificus organism if you ever go anywhere near the sea or come in contact with shellfish, you can take precautions to protect yourself against infection: Immediately cook or freeze fresh shellfish, wear rubber gloves when cleaning the shellfish and avoid going into the water if you have any type of skin lesion.


Dr. Sheldon Margen is a professor of public health at UC Berkeley; Dale A. Ogar is managing editor of the UC Berkeley Wellness Letter. Send questions to Dale Ogar, School of Public Health, UC Berkeley, Berkeley, CA 94720-7360, or e-mail to

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