Despite it being well known that eating raw oysters is a risk, many people choose to enjoy them uncooked and on ice anyway. Photo by SARYMSAKOV ANDREY/Shutterstock

SHANGHAI, Aug. 28 (UPI) -- Researchers found that many of the world's strains of human norovirus can be found in oysters, leading them to suggest people avoid eating the shellfish in their most popular form -- raw.

There are about 20 million cases of norovirus reported each year in the United States. The resulting infection from exposure causes stomach pain, diarrhea, nausea and vomiting.


"More than 80 percent of human norovirus genotypes were detected in oyster samples or oyster-related outbreaks," said Dr. Yongjie Wang, a professor in the college of food science and technology at Shanghai Ocean University, in a press 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."

Researchers reviewed 1,077 oyster-related norovirus sequences from the GenBank and Noronet databases that were collected between 1983 and 2014. The sequences were studied for genetic diversity and geographic distribution over time.

In a previous study, researchers found that 90 percent of human norovirus samples had come from along the Chinese coastline.

The new study similarly found that, worldwide, about 80 percent of the samples could be found in oysters in coastal waters. Wang suggested that some of the explanation could be human waste expelled along coastlines, which can result in oysters picking up norovirus, where it persists for weeks in their tissues.

In addition to suggesting that people not eat oysters raw because of the risk, the researchers suggest a better method of detecting norovirus in the shellfish in order to prevent its frequent human infection.

The study is published in Applied and Environmental Biology.