A multi-provincial outbreak of Salmonella Newport infections associated with red onions: A report of the largest Salmonella outbreak in Canada in over 20 years

Abstract

An investigation into an outbreak of Salmonella Newport infections in Canada was initiated in July 2020. Cases were identified across several provinces through whole-genome sequencing (WGS). Exposure data were gathered through case interviews. Traceback investigations were conducted using receipts, invoices, import documentation, and menus. A total of 515 cases were identified in seven provinces, related by 0-6 whole-genome multi-locus sequence typing (wgMLST) allele differences. The median age of cases was 40 (range 1-100), 54% were female, 19% were hospitalized, and three deaths were reported. Forty-eight location-specific case sub-clusters were identified in restaurants, grocery stores, and congregate living facilities. Of the 414 cases with exposure information available, 71% (295) had reported eating onions the week prior to becoming ill, and 80% of those cases who reported eating onions, reported red onion specifically. The traceback investigation identified red onions from Grower A in California, USA, as the likely source of the outbreak, and the first of many food recall warnings was issued on 30 July 2020. Salmonella was not detected in any tested food or environmental samples. This paper summarizes the collaborative efforts undertaken to investigate and control the largest Salmonella outbreak in Canada in over 20 years.

Keywords: Epidemiology; Food-borne infections; Outbreaks; Salmonella.

Figure 1.

Distribution of Canadian Salmonella Newport outbreak cases (n = 515) by symptom onset, specimen collection or Salmonella isolation date from 15 June to 29 August 2020. Each box denotes a case.

Figure 2.

Red onion traceback distribution diagram for thirteen Canadian location specific case sub-clusters (restaurant or congregate living facility) with red onion exposures initially identified at the beginning of the investigation. The diagram depicts the number of cases associated with each identified sub-cluster and how they are linked to a single supplier, distributor and grower.