Oysters are officially to blame for a norovirus outbreak that originated in Tofino last month.

“We do know of at least 120 people who became ill with norovirus and it was because of exposure to raw oysters,” Island Health Officer Dr. Paul Hasselback told Andrew Bailey of Westerly News on Monday.

The Department of Fisheries and Oceans Canada shut down all shellfish harvesting in a portion of Lemmens Inlet last week and Hasselback suggested further closures could be coming.

“The investigation isn’t quite complete. There are some loose ends and there may be further actions,” he said. “We can’t put every oyster back exactly where it came from but, believe it or not, we can actually track lots of oysters as to where they were processed, harvested and transported and that’s all been part of this investigation.”

Oysters were the primary suspect in Island Health’s investigation from the onset as roughly 30 reports of norovirus cases came in in the immediate aftermath of Tofino’s Clayoquot Oyster Festival.

Hasselback said the number of reported cases ballooned from 30 to 120 after anyone who became sick after attending the festival was encouraged to report in.

“We certainly did get individuals who had consumed the product in Tofino that had gone to other provinces, or even south of the border, who were notifying us of illness so it’s good to know that the communication channels worked well,” he said.

He said the oysters were likely contaminated before arriving at the Oyster Festival’s tables.

“The investigation strongly suggests that the oysters were already contaminated with norovirus before they came to any of those locations so there was nothing that the festival people or other locations would have had any control over or would have known about,” he said.

“Unfortunately we don’t have easy lab testing for things like viruses that would make it simple to screen the product before it gets out and then we end up finding out afterwards that potentially was contaminated.”

He said he has spoken with festival organizers to hash out strategies for next year.

He said the recent Tofino outbreak is the largest norovirus cluster he’s seen in the past five years but noted it was not unprecedented.

“We have seen it before,” he said. “We know this can occur.”