What is listeriosis, the disease that recently caused two deaths in Canada due to contaminated plant-based drinks? The Press takes stock with Lori Burrows, a microbiologist at McMaster University in Hamilton.
What is listeriosis?
Listeriosis is an illness caused by eating food contaminated with the bacteria Listeria monocytogenes. “This bacteria is not present in the food itself, basically. But it can contaminate the food during the processing or packaging process,” explains Lori Burrows.
However, unlike many others, “this bacteria tolerates cold and even salty environments well,” says the researcher. “So it will survive and proliferate even if the food is kept in the refrigerator.”
The bacteria Listeria can be found in:
- raw meat, like tartare
- raw fish, like sushi
- products made from raw or unpasteurized milk
- prepared foods – contaminated after cooking and before packaging – such as ready-to-eat products, smoked fish, soft and semi-soft cheeses made from pasteurized milk, cooked charcuterie (rillettes, terrines), hot dogs
- fresh produce, such as pre-packaged salads
Source: Quebec government website
Is this a rare disease?
Between 2 and 3% of delicatessen meats, soft cheeses and packaged salads are contaminated with the bacteria. Listeriaaccording to a meta-analysis published in 2019 and bringing together more than 100 studies. The Canadian Food Inspection Agency (CFIA) regularly recalls foods contaminated by this bacteria: in 2024 alone, it has recalled more than ten so far.
“Canadians likely regularly consume foods that may contain low levels of [Listeria] “, can be read on the Health Canada website. But listeriosis only occurs when you eat a food that contains a high concentration of this bacteria. So despite this, relatively few people get sick: across the country, there are three to five cases per million inhabitants each year.
Who is concerned ?
Listeriosis is treated with antibiotics and is not fatal most of the time. Symptoms include fever, muscle aches, nausea, vomiting, headache, and diarrhea or constipation.
But sometimes it can take a so-called “invasive” form, which is fatal in 20 to 30% of cases, according to figures from the World Health Organization (WHO). The people most likely to develop this serious form are the elderly, people with weakened immune systems (due to chemotherapy treatments, for example) and pregnant people.
“The bacteria can get into the bloodstream and reach the brain,” Burrows says. It can cause sepsis, encephalitis or meningitis. “In pregnant people, it can cross the placenta,” she adds. This can cause premature delivery or abortion. The baby dies in about 20 percent of cases.
How to identify the origin of an epidemic?
Symptoms most often appear between 3 and 30 days after eating the contaminated food, and sometimes up to 70 days later. This can make it difficult to identify the food that may have caused the illness. In addition, the symptoms are similar to those of other illnesses – flu, food poisoning, etc. It is likely that many cases go under the radar of doctors.
That’s why, Burrows explains, an outbreak can last for months before the product in question is identified. That’s the case now: the first cases were reported in August 2023, but the plant-based beverage recall wasn’t done until July 8, 2024, nearly a year later.
“It took a while to connect the dots and understand what the root cause was,” Burrows said. So far, 12 cases have been identified by the Public Health Agency of Canada, including nine hospitalizations and two deaths.1.
How to avoid catching the disease?
The bacteria is odorless, does not change the appearance of food and does not taste anything, so it is impossible to rely on your senses to spot it.
Pregnant people are generally advised to avoid eating raw meat or fish, or raw milk products. But no recommendations are usually issued for plant-based drinks.
Burrows says pregnant or at-risk individuals shouldn’t necessarily be wary of plant-based beverages in general. But in this case, and even for the general population, it’s important to get rid of any that you may have in your home—opened or not—that match the lot numbers on the CFIA’s list of recalled products.2.
“We must not forget that there is a long delay between the moment we consume contaminated food and the first symptoms. So if you have consumed it and you are at risk, if you are worried or if you have symptoms, do not hesitate to consult a doctor,” she says.
1. Read the article “Two deaths reported in listeriosis outbreak linked to plant-based milk recall”
2. Consult the list of plant-based drinks recalled by the CFIA