Norovirus, the culprit behind a nasty stomach bug, is rising again in Canada

Highly contagious norovirus, known for causing a nasty, days-long stomach illness, is on the rise in Canada after a pandemic lull, federal health officials confirmed to CBC News.

Since early January, reported cases of norovirus have been "increasing both at the national level and within several provinces," including British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, New Brunswick and Newfoundland and Labrador, the Public Health Agency of Canada (PHAC) said in an email response to questions about spikes in cases in the U.S. and U.K.

Infections identified through PHAC's surveillance program since the start of 2023 are "generally comparable" with those reported during the same time period in the last few seasons prior to the pandemic, said spokesperson Anna Maddison. PHAC did not provide hard data, due to the "preliminary nature" of the figures being reported by the provinces.

Given the unpleasant, and in some cases deadly, symptoms — including stomach pain, vomiting, diarrhea, weakness, and dehydration — the virus's rise in Canada offers another rude reminder of how many pathogens are circulating again this winter.

"What we're seeing are the numbers of infections returning to what was the normal baseline before the pandemic," said Lawrence Goodridge, a professor of microbiology at the University of Guelph and an expert in food safety and norovirus surveillance.

For much of the last few years, COVID-19 restrictions and infection prevention measures like handwashing and mask-wearing likely helped keep norovirus at bay, all while people were circulating less than usual, he said.

"But as things have been relaxed, and things have opened up, and things are getting back to normal, we're beginning to see [more cases]."

South of the border, the U.S. Centers for Disease Control and Prevention (CDC) also recently reported a rise in cases nearly reaching pre-pandemic levels. 

From August to early January, 225 norovirus outbreaks were reported by states participating in a federal surveillance program, CDC data shows. That's compared to 172 norovirus outbreaks reported by the same states during the same period last season.

And in the U.K., norovirus is also back with a vengeance, with national surveillance data showing lab reports of the virus are 66 per cent higher than the average for this time of year.

Care homes are experiencing a rise in outbreaks, and the biggest increase in cases is among people aged 65 and up, the UK Health Security Agency said in a release. 

"While high numbers of cases in this age group is expected at this time of year," the agency continued, "these levels haven't been seen in over a decade."

Norovirus typically spreads through contaminated foods, such as raw shellfish or imported fruit, and is also highly transmissible in closed settings, including households, care homes, cruise ships and daycare facilities. 

In 2022, B.C. oysters were linked to hundreds of cases in several provinces and in parts of the U.S., largely among people who'd eaten contaminated raw oysters between January and March that year.