You may have heard about a norovirus outbreak involving oysters in British Columbia about a month ago. Things became quiet for a while in the news, but did you know that the outbreak may have been going on since last December, and is in fact still happening?
Norovirus outbreaks associated with shellfish occur from time to time, and tend to be of fairly small and happen over a matter of days. The mechanism for this is that oysters and other molluscan shellfish filter seawater to feed, and if that water is contaminated with human sewage, viruses (like norovirus and hepatitis A) in the sewage can become concentrated inside the animals’ digestive tracts. These viruses then hang out until we come along and eat the oyster, particularly if we don’t cook it first. A usual recourse when this happens is to find out where the shellfish were harvested and temporarily close those waters, while also removing any implicated shellfish from the market, to prevent more people from getting sick.
What is unusual in this B.C. case is how long the outbreak has been going on, and its spread. As time has passed and the effects of this event are being realized, we wanted to share a broad picture of what’s been going on.
A few days ago, the Public Health Agency of Canada reported 321 cases of gastrointestinal illness to date across three Canadian provinces, thought to be part of the same outbreak. Most of these people were in British Columbia (223), with some in Alberta (42) and Ontario (56), will illness occurring from December 2016 through March 2017, and in each instance, the person remembered eating oysters.
Figure: Reported number of clusters of norovirus or acute gastrointestinal illness associated with the consumption of oysters, by week, for 321 cases. Image from the Public Health Agency of Canada.
As would be expected, epidemiologists have been working to identify and contain the cause of this outbreak for the last couple of months. The Public Health Agency of Canada is coordinating the national response to this event and regularly informing the public and the provinces. In Canada, the Canadian Food Inspection Agency investigates potential sources of foodborne outbreaks and monitors shellfish harvesting areas and processing facilities. Fisheries and Oceans Canada is a separate organization that has the legal authority to open and close shellfish harvesting areas.
The source has indeed been identified as oysters harvested from British Columbia, and seven shellfish farms in B.C. have been temporarily closed (meaning they cannot harvest shellfish), while other sites are being investigated. Several other local suppliers have voluntarily stopped selling product out of caution. How the oysters became contaminated with norovirus is currently unknown, but assumed to be the result of sewage contamination. Since people are continuing to get sick, it is believed the product is still on the market, perhaps at restaurants, seafood markets, or grocery stores.
As for the public, the Agency is urging consumers to fully cook oysters to a temperature of 90°C (194°F) for at least 90 seconds before eating them, and practicing good food safety in the kitchen, such as separating raw and cooked food on separate surfaces with separate tools, and washing hands before handling food.
Not surprisingly, this outbreak has been particularly hard on the shellfish industry in British Columbia, and it is unknown how long the closures will last. For example, one shellfish company in Vancouver reported they had not shipped product since January 19th, while they and other companies try to wait out the event with minimal layoffs. To meet the ongoing global demand for oysters, for which British Columbia was a major supplier, there has conversely been an increased demand for oysters harvested from the east coast.
The U.S. has also been experiencing its own, smaller oyster-associated norovirus outbreak. The Seattle-King County Public Health Department has identified 39 cases of norovirus-like illness since January that have been linked to eating raw oysters harvested along the Washington coast and served at multiple restaurants, mainly in Seattle. An implicated harvesting area in Samish Bay was closed in mid-March. Consumers there have been urged to check any bags of oysters they purchase, discarding those harvested from Samish bay in the last couple of weeks.
We will update this story as more information becomes available.