New report looks back on Salmonella outbreak tied to red onions in Canada

A recently published report has detailed the largest multi-provincial Salmonella outbreak in Canada in more than two decades, linking 515 confirmed cases to contaminated red onions imported from the United States.

The outbreak, which occurred between June and August 2020, affected residents across seven provinces, with 19 percent of those infected requiring hospitalization. The findings, outlined in a report from the Public Health Agency of Canada (PHAC) and other health agencies, underscore the complexity of foodborne illness investigations and the challenges posed by fresh produce contamination.

A separate report from the U.S. Food and Drug Administration in 2020, found that a sheep farm was likely a contributing factor in the contamination of the onions.

The U.S. finding came when investigators searched for the cause of the 2020 outbreak of Salmonella Newport foodborne infections associated with red onions from the Southern San Joaquin Valley and Imperial Valley in California.

“The outbreak, which caused 1,127 reported domestic illnesses and 515 reported Canadian cases, is the largest Salmonella outbreak in over a decade,” according to the U.S. research report. “This outbreak is also remarkable because the food vehicle, whole red onions, is a raw agricultural commodity that had not been previously associated with a foodborne illness outbreak.”

The FDA found that Thomson International Inc. was the producer of the onions.

According to the new report out of Canada, the outbreak, caused by Salmonella Newport, was first detected in that country in July 2020 when public health officials in Alberta and British Columbia began noticing a surge in cases linked to restaurants and congregate living facilities. Shortly afterward, the U.S. Centers for Disease Control and Prevention (CDC) notified Canadian health authorities of a similar cluster of Salmonella Newport infections spreading across several U.S. states. By the end of the investigation, both Canadian and U.S. officials had confirmed that red onions from a California grower, referred to in the report as “Grower A,” were the likely source of the contamination.

In Canada, the outbreak led to 515 confirmed infections, with ages ranging from 1 to 100 years old. According to the report, there were three deaths linked to the outbreak. The majority of cases, 71 percent, reported consuming onions in the week prior to falling ill, with 80 percent of those specifically identifying red onions as the likely culprit.

A collaborative effort between Canadian and U.S. health authorities played a critical role in identifying the source of the outbreak. Investigators used whole-genome sequencing (WGS) to analyze the genetic similarities between the Salmonella strains found in different patients, while traceback investigations relied on receipts, invoices, and import documentation to identify common suppliers.

Challenges in tracing the contamination source
Despite these efforts, no Salmonella bacteria were detected in any food or environmental samples tested, which the report attributes to the complexity of fresh produce contamination. The report notes that the distribution of onions, like other fresh produce, involves numerous steps between farm and consumer, making it difficult to isolate the source of contamination once a product has entered the food stream.

The investigation was notable for its focus on location-specific case clusters, which helped investigators generate hypotheses about the source of the outbreak before WGS results were available. These clusters were identified in restaurants, grocery stores, and congregate living facilities, where multiple people reported illness after eating similar foods. In many cases, traceback efforts were aided by detailed restaurant menus and supplier invoices, which revealed that sandwiches and burgers served at multiple outbreak locations all contained red onions.

The report also highlighted several challenges investigators faced during the outbreak, including recall bias, where patients may not accurately remember what they ate, and the ongoing COVID-19 pandemic, which strained public health resources. Despite these obstacles, public health officials were able to issue the first of several food recall warnings in late July 2020, urging consumers to avoid red onions imported from the U.S.

Although this was the largest Salmonella outbreak in Canada in more than 20 years, the report emphasizes that fresh produce has increasingly been recognized as a source of foodborne illness outbreaks globally. While Salmonella is more commonly associated with poultry, eggs, and meat products, this outbreak serves as a reminder that raw agricultural products like onions can also carry significant risks if not handled properly.

Lessons learned
The findings of the investigation offer valuable lessons for future foodborne illness outbreaks, particularly regarding the importance of collaboration between health agencies and the need for rapid response strategies. By leveraging the expertise of local, provincial, and international partners, the outbreak response was able to mitigate the impact of what could have been an even more widespread public health crisis.

At its conclusion, the investigation identified more than 18,000 tons of red onions that had been distributed across Canada and the U.S., leading to multiple product recalls. However, some cases were unable to be definitively linked to the California grower, suggesting that cross-contamination with other food items may have also contributed to the outbreak.

The full report can be found here.

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