Raw Shellfish Food Safety

Fresh raw shellfish (e.g. oysters, clams, mussels) pose an increased food illness risk  – as they are filter feeders and become contaminated when their waters are polluted with raw sewage and bacteria.
Shellfish can be contaminated with a variety of foodborne pathogens such as E. coli, norovirus, hepatitis A, and Vibrio vulnificus – which can put anyone at risk for infections.

Norovirus

Norovirus is a highly contagious virus that can cause viral gastroenteritis, often called “food poisoning” or the “stomach flu.” Eating raw or partially cooked shellfish can cause norovirus infection.
Norovirus makes its way into the marine environment through untreated human sewage (poop) and vomit. This may come from leaky septic systems, faulty waste water treatment plants, boaters, or beach-goers. Shellfish are filter feeders, which means they filter seawater through their bodies to get food floating in the water. When norovirus particles are in the water, shellfish can accumulate the virus in their bodies.

Vibrio and Vibriosis

About a dozen Vibrio species can cause human illness, known as vibriosis. The most common species causing human illness in the U.S. are Vibrio parahaemolyticus, Vibrio vulnificus, and Vibrio alginolyticus.
The CDC estimates 80,000 people become sick with vibriosis, and 100 people die from their infection, in the United States every year. Most of these illnesses happen from May through October when water temperatures are warmer. However, you can get sick from eating raw or undercooked oysters during any month of the year, and raw oysters from typically colder waters also can cause vibriosis.
Remember, you can’t tell if raw shellfish contains Vibrio germs from the way it looks, smells, or tastes. Always make sure oysters are fully cooked to stay safe from food poisoning.

Most Vibrio infections from oysters, such as Vibrio parahaemolyticus infection, result in only diarrhea and vomiting.
However, people with a Vibrio vulnificus infection can get very sick. Some people can even get infected through an open wound when swimming or wading in brackish or salt water.
This is because the infection can result in bloodstream infections, severe blistering skin lesions, and limb amputations. If you develop symptoms of vibriosis, tell your medical provider if you recently ate or handled raw shellfish.
Anyone can get sick from vibriosis, but you may be more likely to get an infection or severe complications if you:

• Have liver disease, alcoholism, cancer, diabetes, HIV, or thalassemia (blood disorder).
• Receive immune-suppressing therapy for the treatment of disease, such as for cancer.
• Have an iron overload disease, such as hemochromatosis.
• Take medicine to lower stomach acid levels, such as Nexium and Pepcid.
• Have had recent stomach surgery.

How can I reduce shellfish infection?

Follow these tips to reduce your chances of getting an infection when eating or handling shellfish and other seafood:

• Don’t eat raw or undercooked oysters or other shellfish. Fully cook them before eating, and only order fully cooked oysters at restaurants. Hot sauce and lemon juice don’t kill Vibrio bacteria and neither does alcohol.
  • Some oysters are treated for safety after they are harvested. This treatment can reduce levels of Vibrios in the oyster but it does not remove all harmful germs. People who are more likely to get vibriosis should not eat any raw oysters.
• Separate cooked seafood from raw seafood and its juices to avoid cross contamination.
• Wash your hands with soap and water after handing raw seafood.
• Cover any wounds if they could come into contact with raw seafood or raw seafood juices or with brackish or salt water.
• Wash open wounds and cuts thoroughly with soap and water if they have been exposed to seawater or raw seafood or its juices.

Other Shellfish Illness from Natural Toxins

Natural toxic compounds can cause a variety of adverse health effects and pose a serious health threat to humans. Adverse health effects can be acute poisoning – ranging from allergic reactions to severe stomachache and diarrhea, and even death. Long-term health consequences include effects on the immune, reproductive or nervous systems, and also cancer.
Shellfish (bivalve and molluscan) can become contaminated by toxin-producing algae because of red tide (algal bloom) events. During red tide, shellfish harvested from the affected areas are not safe to eat. Toxic shellfish will taste and appear no different than nontoxic shellfish, and cooking does not destroy the red tide toxin. Testing is the only way to determine if shellfish contain unsafe levels of toxin. Algal blooms are most common in the spring and summer months when sunlight, temperature, and precipitation favor algal growth.

Amnesic Shellfish Poisoning (ASP)
Amnesic Shellfish Poisoning (ASP) is an illness caused by eating bivalve and other molluscan shellfish that have been contaminated with domoic acid, a naturally occurring acid which is produced by certain species of marine algae.
Symptoms usually occur 30 minutes to 6 hours after consumption and can include:

• nausea, vomiting, and diarrhea,
• muscle weakness,
• disorientation, and
• memory loss.

If poisoning is not severe, symptoms usually disappear within a few days. If a severe case is suspected, or should problems persist, immediately seek medical attention, because death can occur.
Diarrhetic Shellfish Poisoning (DSP)
Diarrhetic Shellfish Poisoning (DSP) is an illness caused by toxins that are produced by certain microscopic plants. Generally, DSP is often short-lived and non life-threatening – except for vulnerable groups (young children, pregnant women, and people with weakened immune systems). The toxins are not destroyed by cooking.
Symptoms usually occur 30 minutes to 6 hours after consumption and can include:

• nausea, vomiting, and diarrhea,
• headache,
• abdominal cramps, and
• chills.

Symptoms usually disappear within a few days. If a severe case is suspected, or should problems persist, immediately seek medical attention.
Paralytic Shellfish Poisoning (PSP)
Paralytic Shellfish Poisoning (PSP) is caused by eating bivalve and other molluscan shellfish that have been contaminated by toxins produced by certain species of marine algae.
Symptoms of PSP could begin within a few minutes through up to 10 hours after consumption and can include:

• a tingling sensation or numbness around the lips that gradually spreads to the face and neck,
• a prickly sensation in the fingertips and toes,
• drowsiness,
• headache and dizziness, and
• difficulty swallowing.

Respiratory difficulty, salivation, temporary blindness, nausea, and vomiting may also occur. In extreme cases, paralysis of respiratory muscles may lead to respiratory arrest and death within two to twelve hours after consumption. Seriously affected people must be hospitalized and placed under respiratory care. There is no known cure for PSP. If you suspect you have PSP you should immediately seek medical attention.

Safety Tips for Cooking Oysters

The USDA recommends all seafood (fish, shellfish – shrimp, lobster, crab, scallops, clams, oysters, mussels, etc.) be cooked to a minimum internal temperature of 145°F (62.8°C) – as measured with a food thermometer.
For oysters in the shell, either:

• Before cooking, throw out any shellfish with open shells.
• Boil until the shells open and continue boiling 3–5 more minutes, or
• Steam until the shells open and continue steaming for 4–9 more minutes.
• Only eat shellfish that open during cooking. Throw out shellfish that do not open fully after cooking.

For shucked oysters, either:

• Boil for at least 3 minutes or until edges curl;
• Fry for at least 3 minutes at 375°F;
• Broil 3 inches from heat for 3 minutes; or
• Bake at 450° F for 10 minutes.

Summary

To ensure you’ll have a shuckin’ good time – don’t gamble on your chances of getting sick with raw contaminated shellfish. Always cook shellfish – i.e., by baking, grilling, or broiling – to a minimum internal temperature of 145°F (62.8°C) – as measured with a food thermometer.

Additional information

• Take a look at CDC’s questions and answers about Vibrio and vibriosis
• Listen to CDC podcast on the dangers of eating raw oysters
• Be food safe – learn how you can protect yourself from food poisoning
• Read more guidance on FoodSafety.gov for preventing Vibrio infections
• Learn more about aquaculture: http://www.fisheries.noaa.gov/aquaculture/index.htm
• Learn more about Pacific and Eastern oysters from FishWatch: http://www.fishwatch.gov/profiles/search/oyster