Oysters and illness: Research suggests potential for norovirus early warning system

Could an early warning system based on ocean conditions predict oyster-related norovirus outbreaks before they happen?

U.S. Gulf Coast study suggests it's possible but B.C. scientist warns conditions much different on West Coast

Two oyster farms on Vancouver Island have been closed following a norovirus outbreak. (David Horemans/CBC)

Could an early warning system based on ocean conditions predict oyster-related norovirus outbreaks before they happen?

A recent study from the Gulf Coast of the U.S. suggests it's possible. The researchers studied environmental conditions and norovirus outbreaks over an 18-year period, and found that just three factors could explain more than three quarters of the outbreaks.

"The environmental indicators, along with the predictive model can be utilized to develop an early warning system," authors Shima Shamkhali Chenar and Zhiqiang Deng wrote in the journal Marine Environmental Research.

"The early warning system can be used for informing oyster farmers and seafood safety monitoring programs of where and when the environmental conditions are likely to lead to an oyster-related outbreak."

They suggest that by monitoring the three major factors — water temperature, sunlight and water depth — health officials could close harvesting areas when the conditions suggest an outbreak is likely or use DNA testing to detect the presence of the virus.

2 recent outbreaks in B.C.

It's an intriguing idea in B.C., where health officials announced this week that about 40 people had become ill after eating raw oysters. It's the second year in a row that the B.C. Centre for Disease Control has warned people about a norovirus outbreak linked to the shellfish.

Marine microbiologist Curtis Suttle, a professor at the University of B.C., said the idea of an early warning system is intriguing, but what works in the southern states may not work on the Pacific Coast.

"I think it's certainly an approach that should be looked at, but things are quite different here than they would be in, say, southern Louisiana," Suttle told CBC News.

Health officials announced this week that about 40 people had become ill after eating raw oysters (Getty Images)

The U.S. study, published last August, was based on environmental data from harvesting areas in the Gulf of Mexico dating from 1996 to 2014, as well as records of norovirus outbreaks linked to oysters.

The researchers found that in the month before an outbreak, there were common threads in the weather and ocean conditions: low water temperatures, lack of sunlight, low salinity, heavy rainfall, strong winds and low water levels above the shellfish.

Just three of those conditions could explain close to 78 per cent of all norovirus outbreaks: cold water, lack of sunlight and low water levels. On its own, extremely low water temperature was linked to 37 per cent of the outbreaks.

Winter weather helps virus proliferate

The link between water conditions and the proliferation of norovirus was already well established.

After more than 400 people fell ill after eating West Coast oysters last winter, scientists at the B.C. Centre for Disease Control said that the unseasonably cold, wet and dark weather likely helped the virus multiply.

But as Suttle points out, cold, wet and dark have much different meanings in B.C. than they do in the southern U.S. The water here stays cold for most of the year, and rain and darkness are much more common throughout the entire winter season.

"Some of the factors that are at play, even though they may be the same factors, the range of those variables would be quite different," he said.

The conditions that lead to norovirus outbreaks are more common in the winter. (Rafferty Baker/CBC)

He also pointed out that there may also be differences in where the virus is coming from.

Human sewage is suspected as the source of B.C.'s recent outbreaks, but the exact source of that sewage is unknown.

Suttle said more research would be necessary to determine if and how an early warning system would work in B.C.

That's a sentiment shared by Darlene Winterburn, executive director of the B.C. Shellfish Growers Association. She said that members of her group have been meeting with government officials and researchers since last winter's outbreak to discuss where the norovirus came from.

"There are no easy answers for the detection of norovirus in any environment,  but particularly in the marine environment. At the core, we need to be more cognizant of what we are putting into our water," Winterburn wrote in an email.

About the Author

Bethany Lindsay

Journalist

Reach me at bethany.lindsay@cbc.ca or on Twitter through @bethanylindsay.