Tofu the Culprit in Canadian Salmonella Outbreak

A recent outbreak of Salmonella Typhimurium infections in Ontario, Canada had an unprecedented link to a plant-based protein, researchers found.

From May to mid-August of 2021, public health and food safety authorities investigated a cluster of 38 patients across multiple public health districts in Ontario; five patients were hospitalized, though no deaths were reported.

Ultimately, the outbreak was linked to the consumption of a ready-to-eat seasoned tofu that was distributed to multiple restaurants from a single manufacturer, reported Victoria Osasah, MPH, of Public Health Ontario and Johns Hopkins University Bloomberg School of Public Health in Baltimore, and colleagues in Morbidity and Mortality Weekly Report.

“Tofu is a novel outbreak-associated food vehicle for this pathogen and has not been implicated in previous outbreaks,” Osasah and colleagues wrote. “Soy products, including tofu, are uncommon vehicles for foodborne illnesses.”

Accordingly, Osasah and colleagues noted that, although S. Typhimurium is one of the most common serovars in Ontario, the outbreak strain was not related to any existing clusters or isolates in a national surveillance system. Patients with laboratory-confirmed infections related to the whole genome sequencing cluster were contacted by public health investigators for interviews.

Thirty patients were interviewed, and 63% reported being on a vegetarian or vegan diet, researchers reported. Among 25 patients who provided a response for “consumption of tofu,” 19 (76%) responded that they had consumed or probably consumed tofu. This represented “a significantly higher proportion” than was reported in a 2014-2015 survey conducted across all Canadian provinces focused on food consumed by Canadians during a 7-day period (3%; P<0.001).

Though other food items that were more likely to be consumed — such as non-dairy milk, vegetables, nuts, and avocado — were explored, they lacked specificity by product type, brand name, and place of purchase, researchers noted.

However, among the 19 patients who reported consuming tofu, 16 purchased seasoned tofu either at one of 11 restaurant franchise locations or at one of three non-franchise restaurant locations across Ontario before becoming ill.

As a result of the epidemiologic evidence, investigations were conducted at restaurants where patients reported having eaten seasoned tofu during the 7-day period before symptom onset, and additional investigations were conducted once a common manufacturer in Ontario was identified.

In total, 16 opened and closed specimens of the seasoned tofu were collected from 10 restaurants and the manufacturer. S. Typhimurium was isolated from three open specimens of seasoned tofu from one of the restaurant franchise locations, and the sequenced isolates were closely related by whole genome sequencing to those from outbreak-confirmed cases. Salmonella was not detected in other food specimens produced by the manufacturer.

Food safety investigations found that seasoned tofu from the same manufacturer was served across all 14 restaurants, researchers reported. The tofu was identified as a ready-to-eat food product, and restaurants purchased it as a 500-g vacuum-sealed package.

Investigations further identified the absence of a heat treatment process after the addition of seasoning to the packaged 500-g product, which was also sold online to other Canadian provinces. However, a packaged 250-g product did undergo additional heat treatment, and no illnesses were linked to that product.

Furthermore, several infractions were observed at the manufacturing plant, including poor sanitation of the processing equipment as well as the lack of a food safety plan or food sampling program, researchers reported.

Ultimately, the Canadian Food Inspection Agency issued a food recall for the 500-g tofu product, and existing products were removed from distribution and destroyed across the implicated restaurants and manufacturing plant. Additionally, a heat treatment step after the addition of seasoning and before packaging was added at the manufacturing plant.

“Novel outbreak-associated food vehicles can emerge because of evolution of a pathogen or a change in dietary trends,” Osasah and colleagues wrote. “This outbreak largely affected patients who had adopted a vegan or vegetarian diet.”

Of patients affected by the outbreak, the median age was 27 years, Osasah and colleagues noted. Twenty-five (66%) patients were 24 or older, and 21 (55%) identified as female.

“The implication of detecting S. Typhimurium in tofu as a novel outbreak-associated food vehicle is of public health importance because of the global increase in the consumption of plant-based proteins and the associated high disability-adjusted life years associated with S. typhimurium infection,” Osasah and colleagues concluded. “Improved guidance regarding the processing and handling of plant-based proteins in the supply chain is warranted to eliminate the growth and transmission of foodborne disease pathogens.”

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