- Eating raw food always comes with a risk of bacterial or viral contamination.
- Oysters are especially likely to contain pathogens absorbed from ocean water.
- Warming waters and human population growth has increased the risk of getting sick from raw shellfish.
While many people enjoy slurping raw oysters from the shell, the seafood delicacy may not always be safe to eat for much longer — at least not uncooked.
Cases of foodborne illness linked to raw shellfish have been on the rise over the past decade, food safety attorney Bill Marler told Insider. Though he used to enjoy eating oysters straight off the beach during his childhood, he said uncooked seafood is "not worth the risk" nowadays.
Hundreds of people in the US and Canada got sick with norovirus, a common culprit in food poisoning cases, in spring 2022 after eating raw oysters harvested in British Columbia. The illness causes a sudden onset of diarrhea, vomiting, and stomach pain that typically lasts one to three days.
In addition, a type of bacteria called vibrio sickens about 80,000 people in the US each year, according to the Centers for Disease Control and Prevention. Oysters are known to absorb the bacteria in the coastal waters where they feed, especially during the summer months.
Most cases of vibriosis cause mild stomach upset, but as many as 100 deaths from the infection are reported to the CDC annually. Infections with one species, Vibrio vulnificus, can lead to potentially fatal bloodstream infections, severe skin blistering, and limb amputations.
As ocean waters get warmer and humans move closer to aquatic habitats, the risk of illness associated with eating raw shellfish will only continue to increase, Marler said.
Oysters may absorb harmful viruses and bacteria
Oysters are particularly dangerous to eat raw because they feed by filtering ocean water, Marler explained. As ocean waters warm, more viruses and bacteria can thrive, leading to a greater risk of oyster contamination.
"They're absorbing viruses and pathogens into their meat," Marler said. "It's a process that helps clean the water, but doesn't help the consumer of raw shellfish."
Some bacteria, like vibrio, occur naturally in and around shellfish beds where the water is warm.
Other germs, like norovirus and hepatitis A virus, may end up in water along with sewage materials. Both viruses are known to spread via the fecal-oral route, meaning that untreated sewage poses a contamination risk.
Plastic pollution may also enable certain viruses to survive for longer in bodies of water, Insider previously reported. Studies have found that viruses similar to norovirus can stay infectious for up to three days on a tiny piece of plastic, although the strongest evidence has come from experiments in freshwater.
A growing human population comes with greater amounts of human waste that must be disinfected before it is released into the ocean or other waterways. Some areas of British Columbia have only recently implemented modern sewage treatment plants, so Marler said it is not surprising that recent norovirus outbreaks led back to those communities.
Cooking shellfish will kill off most pathogens
Oysters contaminated with vibrio bacteria or norovirus typically don't look, smell, or taste different from other shellfish, according to the CDC.
Marler said cooking shellfish to an internal temperature of 194° Fahrenheit (90° Celsius) for a minimum of 90 seconds will kill any viruses and bacteria inside.
The CDC recommends boiling shellfish until the shells open and for an additional three to five minutes. Throw out any shellfish that does not open during cooking.
If a shell is open before you cook it, the meat inside is not safe to eat. Healthy shellfish should have a mild odor, but you can discard any shellfish that has an especially pungent or offensive smell. Bad smells and broken shells may indicate the oyster has gone bad.
In the future, shellfish lovers may opt to only indulge in raw oysters imported from northern areas where the waters are cooler and there are less people around, Marler said.
"As water warms and the population grows, it becomes a bigger and bigger challenge for people to simply go, 'Oh, I love raw oysters, so I'm going to eat them.'"