Vibrio and Raw Oysters - What you need to know.
Updated: Jul 26
People love their fresh raw oysters, and there are many delicious ways to consume them. For example, you can go with oysters on the half shell with a traditional Mignonette Sauce, maybe even a coconut Thai curry sauce, or as a ‘shooter’, typically served in a shot glass a spicy tomato sauce. But there are risks when consuming raw oysters, and as a food service provider, you need to know what the risks are and your responsibilities if you chose to serve them.
During warm months, a bacterium naturally multiplies more rapidly in the waters that shellfish filter and consequently collect. About 80% of these infections occur between May and October when water temperatures are warmer.* This greater amount of bacteria can cause people to get sick. You cannot see, smell or taste this bacterium. The symptoms are too familiar - diarrhea, abdominal cramps, nausea, vomiting, headache, fever, and chills. This bacterium is called Vibrio. The most common species causing human illness in the United States are Vibrio parahaemolyticus, Vibrio vulnificus, and Vibrio alginolyticus.
This vibrio poisoning is no small matter, and your record-keeping should not be taken lightly. Vibriosis causes an estimated 80,000 illnesses and 100 deaths in the United States every year.* Raw oysters contaminated with Vibrio vulnificus can be life-threatening, even fatal when eaten by someone with liver disease, diabetes, or a weakened immune system.† About 52,000 of these illnesses are estimated to be the result of eating contaminated food as opposed to infection through an open wound.* Some scientists speculate that climate changes and the increased sea temperatures create a more hospitable environment for Vibrio to flourish in water that used to be too cold.
If a customer becomes ill from eating raw or undercooked shellfish at your food service, you need to immediately report it to our department. We also learn about confirmed Vibrio, Norovirus, and even Hepatitis cases from primary care doctors, who then report to our Communicable Disease nurses. We will come to your facility, pick up the shellfish tags, and turn them over to the Oregon Department of Agriculture. These tags provide them with a starting point for tracking down the area where they were harvested. We also review your oyster handling practices. We will collect customer receipts, menus, and other documents to help track the illness outbreak if needed.
And lastly is the question - Are there ways to kill the bacterium and still have a decent ‘raw‘ oyster? Below is an excerpt from a CDC podcast with Dr. Duc Vugia, chief of the Infectious Diseases Branch at the California Department of Public Health.
Three methods are available commercially to make them safer. One approach is called individual quick freezing, by which half-shell oysters are rapidly frozen then stored until consumption. A second method is a heat-cool pasteurization, by which oysters are heated in warm water and then dipped in cold water. And the third method is high hydrostatic pressure processing, in which oysters are subjected to pressure up to 45,000 pounds per square inch which kills the oyster. All three methods can reduce Vibrio vulnificus bacteria to “non-detectable” levels.
Oregon Health Authority Fact Sheet #26 on Raw Molluscan Shellfish