Information for Health Care Professionals on Preventing Vibrio vulnificus Infections
Vibrio vulnificus infections can lead to septicemia and death in high-risk patients. Infections can occur through: 1) exposure of wounds to seawater and/or 2) consumption of raw or undercooked bivalve (two shells) molluscan shellfish (oysters, clams, and mussels). Healthy people are usually not at risk for serious infection from raw shellfish consumption, but can develop cellulitis from wound infections.
Need for Educating Patients
Awareness of the risks of V. vulnificus infection is low among most raw oyster consumers and high-risk patients. Results from a 2001-02 survey of 1,963 raw oyster consumers in Florida, Louisiana, Texas, and California reveal:
- 50% know that people with liver disease can become extremely ill from eating raw oysters,only 34% know that raw oysters are risky for people with weakened immunity, and
- only 19% know that diabetics are at risk for illness (2).
In addition, the survey found that high-risk patients who eat raw oysters do so more frequently (ten times per year) than other consumers (six times per year).
The 50 – 60% mortality rate from V. vulnificus septicemia in high-risk populations (1) makes prevention a safer and more effective strategy than post-infection treatment. Because of low awareness of V. vulnificus infection, educating high-risk patients about infection risks is an important prevention component.
Doctors and other health care professionals play a vital role in patient education. According to the same survey, raw oyster consumers who are informed by health care professionals about the risk of V. vulnificus infection are more concerned about raw oyster consumption and health than those who receive information from other sources (2). Education is a crucial first step in affecting healthy behavior changes. We ask health care professionals treating or counseling high-risk patients to:
- advise abstaining from eating raw or undercooked molluscan shellfish (oysters, clams, and mussels) and to avoid exposing open wounds and sores to seawater, especially in the Gulf of Mexico and southern U.S. states,
- distribute “The Risk of Eating Raw Molluscan Containing Vibrio vulnificus” (English, Spanish)* brochures to all high-risk patients, and
- encourage high-risk patients to eat thoroughly cooked oysters and clams.
V. vulnificus educational brochures, available in English or Spanish, can be obtained free from the Interstate Shellfish Sanitation Conference (ISSC) by phoning 1-800-416-4772, accessing their website (www.issc.org), or by downloading and faxing their order form*.
Preventing Wound Infections
In the United States, 60% of all V. vulnificus cases are caused by wound infection. Exposing pre-existing wounds or open sores to seawater or acquiring a wound while engaging in a marine-related activity, especially in the Gulf of Mexico during warm weather months, can produce V. vulnificus infected wounds in healthy and high-risk people and potential septicemia in high-risk patients.
Coastal recreational activities that may produce wound infections include swimming, wading, boating, fishing, and shellfish harvesting. Occupational, marine-related activities that may cause wound infection include harvesting/handling Gulf of Mexico seafood, shucking Gulf oysters, and picking crab meat from Gulf coast crabs. In addition, cases of wound infections have been reported from marine-related activities in the mid and south Atlantic coastal regions of the U.S.
Health care professionals (especially those who live in a Gulf coast or southern state) should inform all high-risk patients who frequently engage in marine activities about the risk of wound infection, the symptoms of V. vulnificus infection, and the urgency of obtaining medical treatment.
Preventing Infections from Shellfish Consumption
Molluscan shellfish obtain food by filtering out nutrients and microscopic organisms from the water in which they grow, can accumulate Vibrio bacteria from seawater, and may cause infection in humans. The easiest way to prevent infection is to eat cooked oysters, clams, and mussels and to avoid eating them raw or undercooked.
Raw Molluscan Shellfish
High-risk patients should not eat raw oysters, regardless of their market form (live in shell, on the half-shell, or shucked).
Several fallacies about raw oysters include:
- Raw oysters are safe to eat if you avoid them during months that don't have an "R" (May, June, July, August).
- Eating raw oysters with hot sauce and/or alcoholic beverages will kill harmful bacteria.
- Unsafe oysters can be detected by smell and/or appearance.
- Raw oysters purchased from licensed and reputable seafood dealers or restaurants do not contain bacteria or viruses.
None of these misconceptions provide protection against V. vulnificus infection. Thorough cooking and the avoidance of all raw oysters, clams, and mussels is the only protection from consumption-related infection.
In April 2003, California enacted a state-wide ban on the sale of raw oysters harvested from the Gulf of Mexico from April through October each year, unless they are processed to reduce levels of V. vulnificus to non-detectable (post-harvest processed oysters). In addition, some U.S. states (Alabama, Alaska, California, Connecticut, Florida, Georgia, Louisiana, Maine, Massachusetts, Mississippi, New Hampshire, North Carolina, Pennsylvania, Rhode Island, Texas, Washington) require restaurants and/or seafood markets that sell raw molluscan shellfish to advise consumers about the risk associated with eating these products uncooked. The method of disclosure and terminology used to advise or remind consumers of the risk varies from state to state.
Cooked Molluscan Shellfish
Canned oysters, clams, or mussels are safe to eat for all consumers including high-risk patients, because cooking during the canning process destroys pathogenic bacteria and viruses. These canned products are shelf-stable and need no refrigeration, until the cans are opened.
Live Oysters and Clams in the Shell
- Use small pots so shellfish in the middle receive adequate cooking
- Boil for 3 to 5 minutes after the shells open or
- Steam for 4 to 9 minutes in a pre-steaming pot
Shucked Oysters and Clams
- Boil or simmer for 3 minutes or until the edges curl,
- Fry in preheated 375°F oil for at least 3 minutes,
- Broil 3 inches from heat for 3 minutes, or
- Bake in a pre-heated 450°F oven for 10 minutes
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. These post-harvest processes maintain freshness and quality while killing spoilage bacteria and reducing Vibrio spp. bacteria to non-detectable levels4. 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 patients 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.
- Kizer, K.W. 1994. Vibrio vulnificus hazard in patients with liver disease. Western Journal of Medicine. 161:65
- Flattery, J. and M. Bashin. 2003. A baseline survey of raw oyster consumers in four states. Interstate Shellfish Sanitation Conference*.
- Interstate Shellfish Sanitation Conference. 2003. The risk of eating raw molluscan shellfish containing Vibrio vulnificus (consumer brochure - English)*.
- Mississippi Department of Marine Resources. 2004. Post-harvest oyster processing technologies fact sheet.