Eating Raw Oysters Can Make You Sick Even in Cooler Months

You may have heard the saying that raw oysters are safer to eat in months with an “R” in them—from September to April, when it’s cooler in the Northern Hemisphere.

But this old rule isn't true. Eating raw oysters is risky year-round, and the danger has little to do with temperature or storage. Although modern farming and supply chains have improved food safety, the risk of eating raw oysters goes beyond how they’re harvested or stored.

Oysters are filter feeders, meaning they absorb whatever is in the water around them, including harmful bacteria and viruses. Because many water sources are contaminated with fecal matter, oysters from those waters can also carry harmful pathogens.

Cooking oysters kills many of the harmful microbes they may contain. Eating them raw means ingesting whatever the oysters have absorbed, including bacteria and viruses that can cause foodborne illness.

One of the most common risks is Vibrio bacteria, which thrives in warm water. The risk may be lower in colder months, but it’s never zero. And the warmer ocean temperatures due to climate change meanVibrio can now survive in more places and throughout the year.

Norovirus, one of the leading causes of “stomach flu," can also spread through raw oysters. It’s a tough virus that can withstand harvesting, shipping, and storage conditions.

To reduce your risk, cook oysters until they reach an internal temperature above 145°F. This kills many, though not all, pathogens. Spoiled oysters usually have a strong, unpleasant odor, especially after cooking. If they smell rancid or like ammonia, don’t eat them. However, the smell test isn't always reliable: Some raw oysters can smell fine even if they contain harmful pathogens.