Investigation Highlights How a 2024 Listeria Outbreak In Canada Was Linked to Pasteurized Plant-Based Beverages

Foodborne illness outbreaks happen when two or more people get sick from the same contaminated food or drink source. Solving an outbreak can be challenging and not all outbreaks are solved. Public health authorities gather evidence to identify what is making people sick, and if a source is found, remove it from the market to prevent further illness.

A newly published research paper describes how investigators identified plant-based beverages as the source of a 2024 Listeria monocytogenes (Listeria) outbreak that involved 20 confirmed cases and three deaths. In response to the outbreak, the Canadian Food Inspection Agency issued a food recall warning for certain plant-based beverage products.

What is Listeria?
Listeria is a type of bacteria that can cause serious illness, especially in people with weakened immune systems, including older adults and pregnant people. Infection often results from consuming contaminated food.

How the Outbreak Was Solved
Food sample collection was key in identifying the source of the Listeria outbreak. When an ill person is reported to public health, investigators may collect leftover food samples from the person’s home for testing. If a food sample tests positive for the same bacteria that made the person sick, this provides evidence pointing to a source of illness.

This investigation began when a leftover plant-based beverage sample collected by a public health unit inspector from the home of a listeriosis outbreak case tested positive for Listeria, pointing to a possible source of illness for the outbreak.

The evidence was strengthened by the testing methods used at Public Health Ontario’s (PHO) laboratory, in collaboration with partners at the National Microbiology Laboratory. Whole genome sequencing (WGS), a laboratory method that reads the genetic blueprint (DNA or RNA) of an organism, confirmed that the same strain of bacteria that caused the illness was also found in the beverage. Additional testing later found the same outbreak strain in five more leftover plant-based beverage samples collected from the homes of other outbreak cases.  WGS also linked illnesses dating back to 2023 and across four provinces, connecting a total of 20 cases to the outbreak.

The investigation found that all Listeria-positive beverage samples were produced at one manufacturing facility on the same production line. The outbreak strain was also found in post-pasteurization processing areas of the facility, suggesting contamination likely occurred after pasteurization. However, the source of the contamination was not identified.

The Search for Sources
This investigation highlights the importance of testing leftover food samples to support the timely identification of an outbreak source. In this investigation, the contaminated product was removed from the market just two weeks after the investigation began. It also reinforces that novel outbreak sources continue to be identified, and adds plant-based beverage products to the list of suspects for public health investigators to consider.

Looking Ahead
This investigation demonstrates how advances in laboratory testing and the collection of leftover food samples can help identify outbreak sources more rapidly. It also expands the range of foods that public health investigators may consider when investigating Listeria outbreaks, demonstrating the need to adapt to an evolving food landscape. Lessons learned from this outbreak will help inform future investigations and strengthen efforts to prevent foodborne illness in Ontario.