Category Archives: Microbiological Risk Assessment

Many people enjoy eating raw oysters, and raw oyster bars are growing in popularity. But eating raw or undercooked oysters and other shellfish can put you at risk for foodborne illness.

Learn about vibriosis, an illness caused by infection with certain kinds of Vibrio bacteria, and steps you can take to protect your health when it comes to oysters and other shellfish.

Vibrio bacteria naturally inhabit coastal waters where oysters live. Because oysters feed by filtering water, Vibrio and other harmful bacteria and viruses can concentrate in their tissues. When someone eats raw or undercooked oysters, germs that might be in the oyster can cause illness.

CDC estimates that about 80,000 people get vibriosis—and 100 people die from it—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.

An oyster that contains harmful bacteria doesn’t look, smell, or even taste different from any other oyster. You can kill Vibrio in oysters and certain other shellfish, such as mussels and clams, by cooking them properly.

What are the symptoms of vibriosis?

Most Vibrio infections from oysters, such as Vibrio parahaemolyticus infection, result in mild illness, including diarrhea and vomiting. However, people with a Vibrio vulnificus infection can get very sick. As many as 1 in 5 people with a Vibrio vulnificus infection die. This is because Vibrio vulnificus infection can lead to bloodstream infections, severe blistering skin lesions, and limb amputations.

If you develop symptoms of vibriosis, tell your medical provider if you recently ate or touched raw oysters or other raw shellfish or came into contact with salt water or brackish water. Brackish water is a mixture of salt water and fresh water. It is often found where rivers meet the sea.

Who is more likely to get vibriosis?

Anyone can get vibriosis, but you may be more likely to get an infection or severe illness if you:

• Have liver disease, cancer, diabetes, HIV, or the blood disorder thalassemia
• Receive immune-suppressing therapy for the treatment of disease
• Take medicine to lower stomach acid levels
• Have had recent stomach surgery
• Are 65 years or older

How do people get vibriosis?

Most people become infected by eating raw or undercooked shellfish, particularly oysters. Other people become infected by:

• Getting brackish or salt water in a wound, such as when they’re swimming, wading, or fishing.
• Cutting themselves on an item, such as a rock or pier, that has come into contact with brackish or salt water.
• Getting raw seafood juices or drippings in a wound.

How can I stay safe?

Follow these tips to reduce your chances of getting an infection when eating or touching 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 vibriosis in the oyster, but it does not remove all harmful germs. People who are more likely to get vibriosis should not eat any raw or undercooked oysters.
• Separate cooked seafood from raw seafood and its juices to avoid cross contaminationexternal icon.
• Wash your hands with soap and water after handling raw seafood.
• Stay out of salt water or brackish water if you have a wound (including from a recent surgery, piercing, or tattoo).
  • Cover any wounds if they could touch raw seafood or raw seafood juices, or if you might come into contact with brackish or salt water.
• Wash open wounds and cuts thoroughly with soap and water if they contacted salt water or brackish water or raw seafood or raw seafood juices or drippings.

What are tips for cooking shellfish?

Before cooking, discard any shellfish with open shells.

For shellfish in the shell, either:

• Boil until the shells open and continue boiling another 3-5 minutes, or
• Add to a steamer when water is already steaming and cook for another 4–9 minutes.

Only eat shellfish that open during cooking. Throw out any shellfish that do not open fully after cooking.

For shucked oysters, either:

• Boil for at least 3 minutes,
• Fry in oil for at least 3 minutes at 375°F,
• Broil 3 inches from heat for 3 minutes, or
• Bake at 450°F for 10 minutes.