Cases of norovirus, a highly contagious stomach bug, are climbing across the United States, specifically in northeastern areas, according to the latest data from the U.S. Centers for Disease Control and Prevention (CDC).
And north of the border in Canada, the virus is also spreading, with numbers surpassing those of previous years.
The Public Health Agency of Canada (PHAC) told Global News on Thursday that preliminary data from 2024 shows at the national level, norovirus cases are being reported at a “higher frequency” compared to the 2019-2023 average.
“We’ve only been through three months of this year, and the rate seems to be comparable to 2023, but higher than the five-year average in the preceding years,” Dr. Theresa Tam, chief public health officer at PHAC, told Global News in an interview Thursday.
“Norovirus is quite a common virus. So it’s not surprising that it could spread quite quickly, and it is a bit higher this year compared to preceding years.”
Norovirus is a highly contagious virus that causes diarrhea, vomiting and stomach cramps. The virus is primarily spread through contaminated food or water, but can be transmitted through close contact with infected people or by touching contaminated surfaces.
While often labeled the “stomach flu,” norovirus is distinct from influenza viruses, the culprits behind the flu. Instead, norovirus manifests as a gastrointestinal illness.
Cases of norovirus are reported to the National Enteric Surveillance Program (NESP), and so far have revealed a higher-than-expected prevalence in 2024 compared to previous years across several provinces, notably Alberta and Ontario, PHAC said. There has also been a noticeable uptick in cases in Saskatchewan, New Brunswick, and Newfoundland and Labrador.
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Although norovirus cases are higher than the average of the previous five years, PHAC noted that they are “very similar” to the number at this time last year.
PHAC also said that comparing cases of reported norovirus in 2024 to a historical period, including the height of the COVID-19 pandemic (i.e. 2020-22), must be done with caution as several factors associated with the COVID-19 pandemic resulted in significantly reduced reporting of enteric pathogens, or germs that affect the intestines, including norovirus.
What's behind the rise?
Norovirus, known for its high contagiousness, particularly in crowded indoor settings, may have seen reduced exposure among Canadians during the pandemic due to lockdown restrictions, according to Tam.
Similar trends were observed with other viruses, such as influenza, which saw a decline during the pandemic and then surged once lockdown measures were lifted as people resumed more interactions.
“So we might just be seeing a slightly more unusual situation. And then we’ll get back to the usual kind of trends,” she said. “It’s always too early to tell. So I think we just need to monitor the situation carefully, but we can do something about it through good hygienic measures.”
Dr. Isaac Bogoch, an infectious disease specialist at the University of Toronto, told Global News, that he hasn’t seen a noticeable rise in cases of norovirus, but added that it is a seasonal virus, and tends to spike in the winter and early spring months.
“It’s not an uncommon infection, but it’s awful,” he said. “The colloquial name for norovirus is winter vomiting disease. We know there’s a seasonal component to it, although it is transmitted year-round.”
Bogoch added that without concrete numbers from PHAC, he’s unable to determine definitively whether there’s been an increase in cases or rates. The virus is not tracked at the provincial level. Given Canada’s growing population, he said the absolute number of cases may be higher, though it may not reflect an increased overall rate.
How to stay safe
Norovirus can spread from person to person, in food or water, or on contaminated surfaces. It’s so contagious, Bogoch said it can live on surfaces for hours or even days.
“We often see outbreaks in congregate settings, be it a daycare, be it a school, be it a cruise ship,” he said. “And typically, it’s direct contact with the virus and accidental ingestion of the virus, it can stick to surfaces.”
He mentioned that if you’re feeling unwell, classic symptoms include a sudden onset of nausea, vomiting, sometimes diarrhea, weakness and fatigue. However, he added that these symptoms typically subside within 24 to 48 hours and usually resolve on their own.
There is no medication to treat norovirus. Dehydration from vomiting and diarrhea is a chief worry, so those most at risk include young children, older people and those with weakened immune systems.
“The key thing is to watch fluid and electrolyte balance. Make sure people are getting the appropriate fluids and electrolytes. And then you typically will be able to recover in the comfort of your own home,” Bogoch said. “But if there are signs of dehydration, you should seek medical care.”
He adds the best defence against norovirus infection, especially during the peak winter season, is rigorous and frequent handwashing.
“So what do you do if you have this or if someone has this? Wash high contact surfaces down with bleach-based, commercially available cleaning products and get impeccable hand hygiene with soap and water,” Bogoch said. “And that will reduce the risk of transmission within the household.”
— With files from Global News’ Katherine Ward and the Associated Press
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