A listeriosis outbreak in Canada in 2024, which resulted in at least 20 illnesses and three fatalities, has been linked to contaminated plant-based milk products. This event is believed to be the first listeriosis outbreak associated with alternative dairy beverages, according to public health authorities.
A comprehensive investigation into the outbreak’s origin was recently published in Eurosurveillance. Listeriosis, caused by the Listeria monocytogenes bacteria, is a serious and infrequent foodborne illness that particularly impacts individuals with compromised immune systems. While ready-to-eat meats and certain cheeses are recognized sources of Listeria infections, new sources, such as plant-based milk, are continually being identified.
Unexpected Contamination in Pasteurized Food
Public Health Ontario investigators identified the outbreak in June 2024. Routine testing of leftover food from a patient’s residence revealed L. monocytogenes in an opened coconut-based drink. Genetic analysis confirmed a connection between the patient’s illness and the bacteria found in the beverage. Further examination uncovered several preceding cases with a similar genetic profile, dating from August 2023 to June 2024. The outbreak led to 15 hospitalizations and the three deaths. Affected individuals ranged in age from 7 to 89, with 14 of the 20 confirmed cases having pre-existing medical conditions.
Investigators noted that plant-based beverages are not typically associated with L. monocytogenes outbreaks, and contamination was not anticipated in a pasteurized product.
Traditionally, when listeriosis outbreaks occur, public health investigators use standardized questionnaires to pinpoint the source. However, these existing questionnaires did not include queries about plant-based dairy alternatives. "Plant-based beverages are not a known source of L. monocytogenes outbreaks and, because the implicated product was pasteurised, contamination was not expected," the study authors stated. The researchers developed a new, specific survey that inquired about the consumption of dairy alternatives. The initial eight patients in Ontario who completed this updated survey all reported drinking the same brand of plant-based beverages. Ultimately, 19 out of the 20 patients reported consuming products from that particular brand.
Precise Source of Contamination Remains Elusive
The investigation traced the outbreak to a single manufacturing facility in Ontario. Despite extensive swabbing of pre- and post-pasteurization areas, with L. monocytogenes detected in four non-food-contact zones (two of which were genetically linked to the outbreak strain), the exact source of the contamination was never definitively identified.
In July 2024, the brand voluntarily recalled 18 varieties of its refrigerated beverages with production dates extending through October 2024. A second brand, manufactured on the same production line but not linked to any confirmed cases, was also recalled. Following these recalls, production on that specific line ceased and was not resumed. The facility has since closed.
The authors emphasized the importance of testing leftover food samples during outbreak investigations and recommended that plant-based dairy alternatives should now be considered a potential source of listeriosis. They also advocated for the inclusion of questions about dairy alternatives in standardized follow-up questionnaires for future investigations.