Heads up, raw oyster lovers. New research from China shows the bivalves not only transmit human norovirus, they also serve as a reservoir for the harmful and highly contagious virus.
In an expansive study published recently in Applied and Environmental Microbiology, a journal of the American Society for Microbiology, scientists discovered that more than 80 percent of the known noroviruses matched those found in oysters.
Also known as “cruise ship flu” or “stomach flu,” norovirus causes varying degrees of gastrointestinal distress, including nausea, vomiting, diarrhea, and stomach pain. It can be deadly for infants, the elderly, and people with weakened immune systems.
“More than 80 percent of human norovirus genotypes were detected in oyster samples or oyster-related outbreaks,” study lead author Yongjie Wang, a professor in the college of food science and technology at Shanghai Ocean University, said in a release. “The results highlight oysters’ important role in the persistence of norovirus in the environment, and its transmission to humans, and they demonstrate the need for surveillance of human norovirus in oyster samples.”
In the study, researchers analyzed the genetic sequences of 1,077 samples of noroviruses found in oysters. Some sequences had been stockpiled in genetic databases since 1983.
Most of the genotype matches came in oysters from coastal waters, which are more likely to be contaminated by human waste.
Every year, between 19 million and 21 million norovirus cases occur in the U.S. Those illnesses caused up to 71,000 hospitalizations and 800 deaths, according to the Centers for Disease Control and Prevention. Norovirus can stay on objects and surfaces, and infect people for days or weeks.
Food-safety experts have cautioned against eating raw oysters because of the health risks they pose — and the recently published research offers another reason to perhaps pick another shellfish on which to feast. Even common recommendations for reducing the risks of illness, such as avoiding them from warm-water sources, aren’t a guarantee you won’t get sick, food safety expert Ben Chapman told LifeZette.
“Considering where they’re from, and not eating them at a certain time of year, may reduce the risk, though risk is always there,” said Chapman, a researcher at North Carolina State University.
You should avoid oysters if your immunity is compromised or you are on medications, like chemotherapy, or antibiotics for surgery or medications for chronic arthritis.
Chapman also notes that cooking oysters may lower risk, but that steaming — a popular way to prepare them — isn’t likely to get them above 140 degrees, which isn’t high enough to kill noroviruses. Frying is a safer option.
Food-safety experts and health care providers also advise that certain groups of people avoid raw oysters altogether.
“Anybody whose immunity is compromised or on medications, like chemotherapy, or antibiotics for surgery or medications for chronic arthritis, and certain people with inflammatory bowel disease, should stay away from them,” Dr. Matilda Hagan, a leading gastroenterologist at Mercy Medical Center in Baltimore, told LifeZette. “People have to be mindful that in even in a healthy person, it can result in a lot of discomfort.”
“I’m a pretty fit and active guy, and I could not stand up during this entire experience.”
Discomfort is an understatement for what happened 11 years ago to Eric Bray, CIO of an Atlanta-based financial software firm, during a business trip to Seattle. The last night of the trip, Bray indulged in some raw oysters, as he and colleagues had been doing throughout the trip. A few hours later, in the middle of the night, he was so violently ill he missed his flight the next day and was barely able to move.
“I had to crawl back to the bed, and crawl back to the bathroom,” said Bray, who stayed away from raw oysters for several years after that. He eats them occasionally, but cautiously. “It completely drained my energy. I’m a pretty fit and active guy, and I could not stand up during this entire experience.”
What should you do if you should you find yourself, like Bray, hugging the toilet after slurping back a dozen raw oysters? Be sure to stay hydrated, Hagan said, and if you can’t keep anything down, including liquids, it’s time to head to the hospital for treatment.