This information has been reviewed and adapted for use in South Carolina by D.C. Smith, Seafood Industry Specialist; P.H. Schmutz, HGIC Food Safety Specialist; and E.H. Hoyle, Extension Food Safety Specialist, Clemson University. (New 12/99.)
Foodborne illness is not limited to seafood but is a common concern of all food industries. The recent media attention to seafood has led to an increase in public awareness and a number of misconceptions about the safety of eating seafood.
The incidence of illness attributed to seafood can be reduced if the public is better informed, understands the risks, and most important, learns to prevent seafood-borne illness. When handled properly, finfish and shellfish are as safe to eat as any other source of protein. For healthy individuals, the nutritional benefits of seafood far outweigh the safety concerns. Persons with compromised immune systems, such as those with liver disease, can also benefit from eating seafood but should follow a few precautionary measures when preparing seafood.
About 20 million Americans eat raw oysters. However, for some people, eating raw oysters can cause serious illness or even death. What causes this? How do you know if you are at risk? What can you do about it?
The Cause: Vibrio vulnificus is a bacterium that occurs naturally in warm marine waters. V. vulnificus infections are transmitted to humans either through open wounds in contact with seawater or by eating certain improperly cooked or raw shellfish. V. vulnificus is most likely to be present during warm months. In South Carolina, shellfish harvesting (both commercial and recreational) is generally not permitted between April and October. The harvest season will vary depending on environmental conditions.
While not a threat to most healthy people, V. vulnificus can cause sudden chills, fever, nausea, vomiting, blood poisoning and death within two days in people with certain medical conditions. Forty percent of V. vulnificus infections from raw oyster consumption are fatal. The bacteria are not a result of pollution; so, although oysters should always be obtained from reputable sources, eating oysters from "clean" waters or in reputable restaurants with high turnover does not provide protection. Eating raw oysters with hot sauce or while drinking alcohol does not kill the bacteria, either.
High-Risk Factors: All individuals who eat foods contaminated with this organism are susceptible to gastroenteritis, which usually develops within 16 hours of eating the contaminated food. However, certain health conditions put you at risk for serious illness or death from V. vulnificus infection. Some of these conditions have no signs or symptoms so you may not know you are at risk. If you are an older adult, you also may be at increased risk because older people more often have these risk conditions than younger people. Check with your doctor if you are unsure of your risk. These high-risk conditions include:
IF YOU ARE OR THINK YOU MAY BE IN ANY OF THESE RISK CATEGORIES, YOU SHOULD NOT EAT RAW OYSTERS.
How can "high risk" individuals avoid Vibrio vulnificus?: Avoid exposure of recent or healing wounds, cuts, punctures, or burns, to warm seawater. When swimming or wading, temporarily cover wounds with watertight wrap. The V. vulnificus lives naturally in warm seawater, can enter a person’s wound and, in some cases extend to the bloodstream and cause a potentially fatal illness. The highly invasive nature of this bacterium is cause for special concern.
People in high-risk categories should avoid consumption of raw shellfish, particularly oysters. Oysters are filter-feeding animals that can concentrate Vibrio bacteria from the water into their system. This concern exists for any raw oysters regardless of harvest from approved or questionable waters.
When eating shellfish, particularly oysters, be sure they are properly and thoroughly cooked. Fully cooking oysters completely kills the bacteria, so you can continue to enjoy oysters in many cooked preparations. However, steaming oysters as is done at an oyster roast may not be sufficient to destroy the bacteria.
Avoid cross-contamination of previously cooked shellfish with raw shellfish. A common cause of cross-contamination is storing cooked shellfish in the original container used for raw shellfish, or storing raw and cooked shellfish in the same area.
Drinking Alcoholic Beverages Regularly & Liver Disease: If you drink alcoholic beverages regularly, you may be at risk for liver disease, and, as a result, at risk for serious illness or death from raw oysters. Even drinking two to three drinks each day can cause liver disease, which may have no symptoms. Liver disease will put you at increased risk for V. vulnificus infection from raw oysters. The risk of death is almost 200 times greater in those with liver disease than those without liver disease.
What are the chances for an infection?: Rare! No major outbreaks of illness have been attributed to this organism. Sporadic cases have occurred in South Carolina, becoming more prevalent during the warmer months. To date no fatalities have been related to eating oysters harvested in S.C. waters. Extensive federal and state regulatory programs monitor the production and marketing of raw shellfish to assure product safety. Most healthy individuals are not troubled by V. vulnificus infections from water or food. Thus, the V. vulnificus problem is primarily restricted to individuals in the risk categories. These individuals are advised not to eat raw shellfish.
At Restaurants: Order oysters fully cooked. Some states display notices for those at risk. Use them as reminders of how to avoid illness.
Cooking at Home:
In the Shell: Cook live oysters in boiling water for three to five minutes after shells open. Use small pots to boil or steam oysters. Do not cook too many oysters in the same pot, because the ones in the middle may not get fully cooked. Discard any oysters that do not open during cooking. Steam live oysters four to nine minutes in a steamer that’s already steaming.
Shucked: Boil or simmer for at least three minutes or until edges curl. Fry in oil for at least three minutes at 375 °F. Broil 3 inches from heat for three minutes. Bake (as in Oysters Rockefeller) for 10 minutes at 450 °F.
Many consumers enjoy raw or lightly marinated seafood products such as sashimi, sushi, ceviche, gravlax, cold-smoked fish and raw shellfish. Eating raw seafood (and raw meat, poultry or dairy products) has a greater food safety risk than eating properly cooked products. Follow these tips to reduce the risk of illness:
Ingestion of raw or insufficiently steamed clams and oysters also poses a high risk for infection from the Norwalk virus, a cause of viral gastroenteritus or the stomach "bug." (Foods other than shellfish are contaminated by the Norwalk virus by ill food handlers.)
It is estimated that Norwalk viruses are responsible for about one-third of the cases of viral gastroenteritis not involving the 6- to 24-month age group. Approximately 181,000 cases occur annually, with no known associated deaths. Norwalk and Norwalk-like viruses have been associated with outbreaks on cruise ships, in communities, camps, schools, institutions and families. Foods such as raw oysters, cake frosting and salads, as well as drinking water, have been implicated as a common source of viral infection in several outbreaks.
A mild and brief illness usually develops 24 to 48 hours after contaminated food or water is consumed and lasts for 24 to 60 hours. Norwalk virus infection symptoms include nausea, vomiting, diarrhea and abdominal cramps. Symptoms experienced less often include headache, fever, chills and muscle pain. Fluid replacement is the common therapy. For further information on Norwalk virus request HGIC 3720, Foodborne Illnesses: Viruses.
This information is supplied with the understanding that no discrimination is intended and no endorsement by the Clemson University Cooperative Extension Service is implied. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.