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:
- liver disease, including cirrhosis, hemochromatosis, and chronic alcohol use
- diabetes mellitus
- immune disorders, including AIDS, cancer, and reduced immunity due to steroid or immunosuppressant therapy
- gastrointestinal disorders, including previous gastric surgery, and low gastric acid (for example, from antacid use or achlorhydria).
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
- Fresh seafood should not smell unpleasantly "fishy."
- Fresh fish steaks and fillets should be moist, with no drying or browning around the edges. The eyes of fresh whole fish should be bright and clear, not cloudy or sunken. Scales should not be "slimy" and should cling tightly to the skin. Gills should be bright pink or red. Frozen fish should not be freezer burned or have damaged packaging.
- Mollusks in the shell should always be alive when you buy them. When a clam, oyster, mussel, or scallop is alive, the shells will be tightly closed or will close when tapped lightly or iced. A test for freshness is to hold the shell between your thumb and forefinger and press as though sliding the two parts of the shell across one another. If the shells move, the shellfish is not fresh. Throw away any that do not close tightly.
- Buy seafood only from reputable dealers. You can't know what you're buying from the back of a pickup truck. It could have been caught by someone not subject to FDA or state inspection.
- Ask to see the shipper's tag for molluscan shellfish.
- Cook fish no later than two days after purchase.
- Keep fresh fish cold-- in the coldest part of your refrigerator, usually under the freezer or in the meat drawer, until it's ready to cook and serve.
- Store fresh fish in your refrigerator in the same wrapper it had in the store.
- Store live mollusks in your refrigerator in containers covered loosely with a clean, damp cloth. Do not store live shellfish in airtight containers or in water.
- Canned fish should be refrigerated after opening.
- Smoked fish, pickled fish, vacuum-packed fish, and modified-atmosphere-packed fish products should always be refrigerated.
- Keep cooked and raw seafood separate. It's not safe to put cooked seafood back in the original container used for raw seafood, or to store raw and cooked seafood together.
- The safest way to thaw frozen seafood is in the refrigerator in its own container. Allow about one day for defrosting.
- For fin fish (baked, broiled, poached, fried, or stewed): allow 10 minutes cooking time for each inch of thickness. Turn the fish over halfway through the cooking time unless it is less than a half inch thick. Add 5 minutes to the total cooking time if the fish is wrapped in foil or cooked in a sauce. Properly cooked fish will flake easily with a fork and should be opaque and firm. It should not be translucent.
- For molluscan shellfish: Boiled-- shells will open during boiling. After shells open, boiling should continue 3 to 5 more minutes. Steamed-- cook 4 to 9 minutes from the start of steaming.
- Use small pots to boil or steam shellfish. If too many shells are cooking in the same pot, it's possible that the ones in the middle won't get thoroughly cooked. Discard any clams, mussels or oysters that do not open during cooking. Closed shells may mean they have not received adequate heat.
- Shucked oysters: Boil or simmer for at least 3 minutes. Fry in oil for at least 10 minutes at 375 degrees Fahrenheit. Bake for at least 10 minutes at 450 degrees Fahrenheit.
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.