Salmonella in Ready-to-Eat Tofu: An Ontario Outbreak, 2021

Salmonella Typhimurium is usually linked to animal foods. In the summer of 2021, public health investigators in Ontario, Canada, traced an outbreak to something no one expected: ready-to-eat seasoned tofu.

Finding the cluster

On July 5, 2021, Public Health Ontario spotted three S. Typhimurium infections in different public health districts whose bacteria were nearly identical by whole genome sequencing — a sign of a common source. Six more were reported by July 9, and an investigation began with local, provincial and federal partners. The strain didn't match any previous cluster in PulseNet Canada, the national foodborne illness surveillance network.

A closely related case was also found in Quebec.

Illness onset and specimen collection dates. Credit: Centers for Disease Control and Prevention.

Tracing the tofu

Investigators interviewed patients about everything they ate in the week before falling ill and compared the answers with Foodbook, a national survey of what Canadians typically eat in a week. The evidence pointed to seasoned tofu eaten at restaurants — tofu that came from a single Ontario manufacturer and was served at all 14 restaurants involved.

• Local inspectors and the Canadian Food Inspection Agency (CFIA) collected 16 samples of the tofu from 10 restaurants and the manufacturer.
• S. Typhimurium was found in three opened packages at one restaurant location, closely matching the patients' strain. No other products from the manufacturer tested positive.

What went wrong at the factory

The tofu was sold in two sizes:

Inspectors also found poorly sanitized processing equipment and no food safety plan or sampling program. Investigators concluded that unsanitary conditions likely contaminated the tofu after it was made but before packaging — and that without a final heating step, nothing killed the bacteria.

The response

• CFIA recalled the 500-gram product at the hotel, restaurant and institution level.
• Inspectors made sure remaining product was pulled and destroyed at the restaurants and the plant.
• The manufacturer added a heat treatment step after seasoning and before packaging.

Why it matters

Soy products rarely cause foodborne illness, and no nontyphoidal Salmonella outbreak had been linked to tofu before, although lab studies have shown S. Typhimurium can grow on soy products. New outbreak foods can emerge as pathogens evolve or as diets change. This outbreak mostly affected people who ate vegan or vegetarian diets — followed by an estimated 5% of Canadians, and more often by women and younger adults, matching the patients' profile.

With plant-based proteins growing more popular worldwide, the authors call for:

• awareness among consumers and restaurants that ready-to-eat soy products can carry Salmonella;
• controls across production and the whole supply chain, with extra steps to stop bacterial growth;
• better guidance on processing and handling plant-based proteins.

Sources

Based on Osasah V, Whitfield Y, Adams J, Danish A, Mather R, Aloosh M, "An Outbreak of Salmonella Typhimurium Infections Linked to Ready-To-Eat Tofu in Multiple Health Districts — Ontario, Canada, May–July 2021," Morbidity and Mortality Weekly Report 72(32), Centers for Disease Control and Prevention; material in the MMWR series is in the public domain. Original report.