Introduction to Vibrio vulnificus
What Consumers Should Know About Eating Raw Oysters and Clams
What is Vibrio vulnificus?
Vibrio vulnificus is a rod-shaped bacterium that can tolerate salt, unlike most bacteria. It is a natural inhabitant of marine environments, especially areas with low salinity (0.5 to 2.0% sodium chloride) i.e., near shorelines and in estuaries where freshwater from rivers mixes with ocean saltwater. The presence of the bacteria is neither a result of chemical or biological pollution.
V. vulnificus bacteria prefer a warm environment and reproduce rapidly when water temperatures are 86°F to 95°F and salinity levels are approximately 0.5% sodium chloride. In the United States, the highest seawater concentrations of the bacteria are found in the Gulf of Mexico from April to October. The bacteria are present, but at much lower levels, in other U.S. coastal waters and have also been detected in seawater and shellfish in other countries.
Can Vibrio vulnificus cause illness in humans?
The Centers for Disease Control and Prevention (CDC) report approximately 90 cases of V. vulnificus infection each year in the U.S. The bacteria primarily causes serious illness only in people who have weak immune systems or certain other health or medical conditions (see list below) that enable the bacteria to infect and rapidly spread throughout their bodies (septicemia). The overall mortality rate for V. vulnificus infection is 40%.
The bacteria may cause mild gastroenteritis (nausea, vomiting, stomach pain, and/or diarrhea) or cellulitis (spreading skin infection) in healthy people, but this is rare and usually not serious.
The following conditions make people susceptible to serious infection:
- Liver disease (from hepatitis, cirrhosis, alcoholism, or cancer)
- Cancer (including lymphoma, leukemia, Hodgkin's disease)
- Iron overload disease (hemochromatosis)
- Stomach disorders (including surgery, prescribed antacids)
- Other illnesses or treatments that weaken the immune system, such as chemotherapy
How does a V. vulnificus infection occur?
Infection can occur in two ways: through eating raw or undercooked oysters, clams, or mussels which contain the bacteria (40% of U.S. cases) or by exposing an open wound or sore to seawater which contains the bacteria (60% of U.S. cases).
What are the symptoms of an infection?
Symptoms of infection from eating raw oysters, clams, or mussels: fever/chills; nausea; vomiting; diarrhea; sharp drop in blood pressure; painful skin lesions that are initially red, develop into blisters and are sometimes filled with blood, then become necrotic ulcers; septicemia; shock; death
Symptoms of wound infection: swelling, redness, and pain around wound; fluid-filled blisters that become necrotic ulcers; septicemia; death
Is medical treatment necessary?
Yes! Because infection can spread rapidly and cause death in as few as one or two days in high-risk people, immediate medical treatment is imperative for these people. In addition, because other people may have undiagnosed health conditions, it is prudent for anyone who has symptoms of V. vulnificus infection or suspects exposure to Vibrio bacteria to quickly consult a physician.
Can V. vulnificus be destroyed in raw shellfish?
Yes, but only after shellfish are harvested, since V. vulnificus is a natural inhabitant of unpolluted marine waters.
Several food processing technologies are being researched and adapted by the U.S. Gulf coast shellfish industry to provide safer and better quality raw oysters. Three technologies - freezing, heat-cool pasteurization, and high hydrostatic pressure - are already being used commercially on oysters after they have been harvested. In August 2005, the Food and Drug Administration approved irradiation as another means to kill Vibrio bacteria in raw oysters. These post-harvest processes maintain freshness and quality while killing spoilage bacteria and reducing Vibrio bacteria to non-detectable levels. The absence of spoilage bacteria increases product shelf life.
However, because these processes may not kill all bacteria and viruses, it is not recommended that high-risk consumers eat raw, post-harvest processed (PHP) oysters. Raw PHP oysters provide a safer product for high-risk patients to prepare and eat cooked oysters and are a safer alternative for healthy consumers who are not at risk for serious V. vulnificus infection and like to eat raw oysters.
Does cooking kill V. vulnificus bacteria?
Yes, thorough cooking kills harmful bacteria and viruses. However, be sure to protect cooked oysters, clams, or mussels from coming into contact with raw seafood, meat, poultry, or their juices.
How can consumers avoid infection?
Consumers who are at risk for serious infection should only eat thoroughly cooked oysters, clams or mussels. (Refer to "Safely Cooking Oysters and Other Molluscan Shellfish" webpage for cooking guidelines.)
If you are not at risk for V. vulnificus infection and like to eat raw oysters, buying PHP oysters, especially during warm weather months, offers additional protection from Vibrio infection. (Refer to "PHP Oysters" webpage for more information.)