Why it’s possible you will never eat oysters raw again

The most common foodborne illness is caused by bivalve shellfish that are contaminated with human waste. Recent research in the United Kingdom has found that nearly 70% of raw oysters in the U.K. are infected with norovirus. However, it is not known if the entire virus is infectious.

Norovirus outbreaks associated with oysters are often caused by contamination in the growing waters. Oyster beds can be contaminated by sewage from land or oyster harvesters.

How can you tell if an oyster is bad if it’s raw?

The oysters that contain harmful bacteria or viruses do not smell, taste, or look any different than other oysters.

It is safest to cook oysters until their internal temperature reaches 90@C. 

Raw oysters are not only a source of norovirus but also a host of bacteria and viruses.

Vibrio is the most important genus of bacteria. Vibrio parahaemolyticus is one, and Vibrio vulnificus is the other. The second bacterium is more serious but more common in Canada than in the U.S.

Salmonella was also found in raw oysters. Hepatitis A, which can cause illness, is also found in raw oysters. Norovirus, however, has been the most common virus to cause problems.

How are raw oysters contaminated?

The most likely cause of norovirus contamination in oysters is human sewage.

Vibrio bacteria, however, are thought to be an inherent part of the marine ecosystem where oysters reside. These bacteria are only a problem when the water temperature rises.

Some people believe that Vibrio species outbreaks are linked to global warming.

Both bacteria and viruses can concentrate in oyster tissues before consumption.

Who is at the greatest risk?

In North America, acute gastrointestinal diseases such as vibriosis and norovirus are very common. They can affect people of all ages.

Pregnant women, those with compromised immune systems, and children under the age of five are more susceptible to serious complications.

Fishermen rake clams in the North River near Charlottetown. They scoop up their catch using a hinged rake and discard seaweed, rocks, and muck. 

People with health conditions such as liver disease are more likely to develop more serious complications or even die when they have a particular type of Vibrio bacterium.

What to look out for when buying oysters

Shop from reputable sources, and select fresh oysters that have intact shells without an abnormal smell. The body should be tightly closed when tapped. Any oysters that have shots open should be thrown away.

Check the expiration date on pre-packaged shucked oysters. Store them at 4@C for chilled products or -18@C for frozen ones.

When transporting oysters from the store, it is best to put them in a thermos bag that has been chilled. The oysters will spend less time at room temperature, which can encourage the growth of bacteria such as Vibrio.

Consume chilled oysters within one or two days. To prevent cross-contamination in the fridge, oysters in their shells should be stored in containers that have a lid.

How to cook oysters in the home

Do not eat raw or undercooked oysters. It is best to heat your oysters until the internal temperature reaches 90@C.

Boil the oysters in the shell for another 3 to 5 minutes after the shells have opened. Do not cook more than one oyster in a pot, as the middle ones may not be fully cooked. Lastly, throw away any oysters which do not open when cooking.

In a steamer, add oysters to water that is already steaming. Cook the oysters live for 4 to 9 minutes.

The shucked product should be boiled for at least 3 minutes, then fried for 3 minutes at 375degF or baked for 10 minutes at 232degC.

After handling raw oysters, wash your hands thoroughly with soap and hot water. Do not cross-contaminate any other foods that are ready to eat in your kitchen.

Some oyster myths

1. Only during the warmest months of the year can oysters make you sick.

Although the majority of vibriosis indeed occurs during the summer months, when water temperatures are higher, and the Vibrio bacteria have a greater chance of growing faster, there have been cases reported throughout the year.

Studies have shown that commercially harvested oysters contain higher norovirus levels during winter. According to some studies, there is a correlation between cold weather and the amount of norovirus gastroenteritis cases.

2. You will be safe if you do not eat raw oysters that come from contaminated waters.

False. Vibrio bacteria do not result from pollution, as they are a natural part of the marine environment. Oysters can be purchased from reliable sources, but eating them in “clean” waters or at a reputable restaurant with high turnover does not guarantee you won’t get sick.