Each year, consumers eat millions of oysters worldwide, with a significant number of the mollusks grown in the Gulf of Mexico. While illness from eating oysters is rare, it can be fatal. Here, Razieh Farzad, a UF/IFAS and Florida Sea Grant seafood safety expert and Keith Schneider, a UF/IFAS food safety expert answer commonly asked questions about eating oysters and minimizing the risk of illness.
Q: What are oysters?
A: Oysters are bivalved (two-shelled) molluscan shellfish filter feeders living in marine and brackish waters. Filter feeders take in water and digest nutrients from the water they “filter” through their body. An adult oyster can filter up to 50 gallons of water in just one day. They provide ecosystem services such as water filtration and habitats for other species. Oysters are also considered a seafood delicacy.
Q: Are there health benefits to eating oysters?
A: Yes. Oysters are a moderate source of highly digestible proteins and an excellent source of vitamins and minerals such as vitamin D and B12, zinc, selenium and copper. They are extremely low in fat and calories. In a serving of six medium-sized oysters, you will consume less than 50 calories, while consuming 12 grams of protein, equivalent to 2 ounces of meat.
Q: Are there risks? What are they?
A: As with eating any raw or undercooked meat product, eating raw or undercooked oysters comes with some risk of exposure to foodborne pathogens. Coastal waters and estuaries can contain Vibrio bacteria. Although many Vibrio species are harmless, several can cause severe human and animal diseases. As filter feeders, oysters are the most common vehicle for Vibrio vulnificus and Vibrio parahaemolyticus. In rare cases, these bacteria can lead to foodborne illness and wound-related illnesses. In addition to biological contamination, oysters can bioaccumulate chemical contaminants, such as harmful algal blooms toxins (red tide), especially in the spring through fall. There are regulations in place to help minimize these risks.
Q: Who is most at risk?
Although anyone can get sick by eating raw or undercooked oysters, the risk of illness increases for young children, older adults, pregnant women and individuals who are immunocompromised or have other conditions such as chronic liver disease, kidney disease or a weakened immune system. These individuals should be more cautious and avoid eating raw oysters.
Q: What are the signs of illness? Can early detection make a difference?
Vibriosis can cause watery diarrhea, often accompanied by abdominal cramping, nausea, vomiting, fever and chills. Usually, these symptoms occur within 24 hours of ingestion and last about three days. In deadly cases, particularly when infected with V. vulnificus, the infection can lead to bloodstream infections known as septicemia.
Starting antibiotics within 24 hours will increase the chance of survival, so early detection is critical. After 72 hours, treatment is less successful.
Q: Can a consumer tell if an oyster is not fresh or could make them sick? Any tell-tale signs?
No, consumers cannot tell if the oyster is contaminated or could make them sick just by its smell, taste or look. What consumers perceive as freshness in oysters does not always equal safety. When looking to buy oysters, avoid purchasing them if the shell is cracked, has holes or is open and will not close to the touch. If the oyster shows any of these signs, avoid it even if it smells and looks fine.
Q: If vibrio is present, is that the fault of the oyster farmer or fishery?
A: No, Vibrio bacteria are naturally occurring organisms present in salt and brackish water.
Q: Is oyster farming regulated?
Yes, oyster farming and the seafood industry are highly regulated in the United States. Farming is only allowed in designated water bodies. Farmers and harvesters follow safety requirements to manage safety risks by ensuring that harvested oysters are chilled and kept cold within specific time windows to prevent the Vibrios from multiplying and increasing the food safety risk.
Additionally, regulatory agencies monitor red tide blooms and do not allow shellfish harvesting in affected areas during these events.
Q: Are there regulations for serving raw seafood? What does the raw food warning on menus mean?
A: Food service establishments are allowed to serve raw or undercooked foods at a customer’s request as long as the customer is informed about the risks associated with consuming undercooked food. Based on the FDA’s Food Code, restaurants serving raw oysters, or other certain raw or undercooked products, must display a public health warning from the medical health officer of their health authority. This warning can be placed on a display card at the table or included in the menu. Restaurants must follow food code and other food safety regulations to ensure the risk of pathogens contamination is minimized when serving raw fish and shellfish.
Q: How can I tell what is raw on a menu and what is not?
A: If you are unsure which foods on a menu are raw, you should ask. Pay attention to the health advisories and do not hesitate to ask questions.
Q: Are there questions to ask the restaurant to ensure we’re eating safer oysters?
A: Consider ordering properly cooked oysters, especially if you are immunocompromised. Check the source of oysters and ask for the supplier’s tag to ensure the oysters were legally harvested from an approved body of water. Based on the National Shellfish Sanitation Program regulation, the suppliers’ tag that includes the time and place of harvest should be kept for 90 days. Also, ask about their cooking methods to ensure that the oysters are not undercooked.
Q: If you avoid eating raw oysters for months without the letter “R,” are you safe?
A: During these months, the water temperature is higher, which is favorable for Vibrio growth. These warmer temperatures might increase the chance of a Vibrio infection if the oysters are not stored at a proper temperature after harvest and if they are served raw or not cooked properly. However, pathogens exist all year round, and we recommend that you always follow the food safety recommendations to keep yourself safe.
Q: How can you minimize the risk of eating raw oysters?
A: At restaurants, consider ordering properly cooked oysters, especially if you are immunocompromised. For at-home consumption, avoid purchasing the oyster if the shells are open. Always ask for a certified shipper’s tag. Properly wash your hands before and after handling the raw oysters and avoid handling the raw shellfish if you have an open wound on your hand.
Follow the FDA recommendation for safe handling and cooking of oysters to minimize the risk.
Q: Are shellfish particularly susceptible to bacteria more than other meat products?
A: Any meat or seafood product, when eaten raw, comes with risks. Filter feeders such as oysters get their food by filtering large quantities of water through their shells. In doing so, they can accumulate pathogens, thus making raw mollusks less safe to eat compared to cooked shellfish.