Print this page    •   Back to Web version of article

Important Health Information for People With Immune Disorders
Protect Yourself: Only Eat Fish That's Been Thoroughly Cooked


This article is to inform people with immune disorders of the hazards they face if they eat raw molluscan shellfish: oysters, mussels, clams, and whole scallops. The article also contains useful information about safe handling and consumption of seafood in general.

Fish -- fin fish and shellfish -- have many of the qualities that health-conscious food shoppers look for. They're generally low in saturated fat and are excellent sources of protein, vitamins and minerals, although nutrition values differ depending on the type. Properly handled and thoroughly cooked, fish is tender, easy to digest, and safe to eat.

But sometimes shellfish, especially mollusks -- oysters, clams, mussels, and whole scallops -- are eaten raw, as in oysters-on-the-halfshell. Eating raw or undercooked shellfish can be a serious problem for persons with:

The problem occurs because raw mollusks sometimes carry bacteria called Vibrio vulnificus that may multiply after the shellfish are caught, even with refrigeration. These bacteria are completely killed when the shellfish are thoroughly cooked, removing all danger of the bacteria causing food poisoning.

But if mollusks are eaten raw or partially cooked, the bacteria remain alive and may make you very sick. Symptoms include sudden chills, fever, nausea, vomiting, and stomach pain. Vibrio vulnificus can cause blood poisoning, a condition that could be fatal in up to half the people who get it. Death usually occurs within two days.

Certain viruses known as Norwalk viruses also could contaminate oysters, clams and mussels and cause severe diarrhea in those who eat them. Here again, thorough cooking kills the virus.

How Do Mollusks Get Contaminated

Mollusks usually live where rivers and seas meet. Because many cities are located near those places, the waters are more likely to be polluted than offshore waters. For this reason, shellfish harvesting is prohibited in areas contaminated by sewage. To enforce this, these areas are patrolled by state health and fishery agencies.

Mollusks feed by filtering water through their systems, so they are more likely to pick up and store bacteria or viruses from the water, including those that can cause illness in humans. If you eat mollusks raw, you eat the live viruses and bacteria too.

Another source of contamination of shellfish can come from naturally occuring algae blooms called "Red Tides." Waters are closely monitored for these blooms to prevent shellfish poisoning in humans. The Food and Drug Administration and the coastal states all test for these blooms, and when they appear, the waters are closed to all fishing.

Is Raw Fin Fish Safe To Eat?

Raw fish dishes, such as sushi and sashimi, can be safe for most people to eat if they are made with very fresh fish, commercially frozen (at temperatures lower than in home freezers), and then thawed before they're eaten. This kills any parasites that may be present.

Parasites are also killed by thorough cooking, and, once killed, they are no longer a danger to you. But people with immune disorders should not eat raw fin fish because freezing does not kill bacteria. Persons with immune disorders need to take extra precautions to thoroughly cook all fish.

Seafood Safety Tips




Following these steps for buying, storing and cooking will protect you and still allow you to enjoy seafood.

Keep cold seafood cold: 40 degrees and below.
Keep hot seafood hot: 140 degrees or above. Avoid the Danger Zone -- 40 degrees Fahrenheit to 140 degrees Fahrenheit.

This article was provided by U.S. Food and Drug Administration. You can find this article online by typing this address into your Web browser:

General Disclaimer: is designed for educational purposes only and is not engaged in rendering medical advice or professional services. The information provided through should not be used for diagnosing or treating a health problem or a disease. It is not a substitute for professional care. If you have or suspect you may have a health problem, consult your health care provider.