Outbreak of Salmonella Reading in persons of Eastern Mediterranean origin in Canada, 2014-2015

Abstract

Background: Salmonella Reading (S. Reading) is a rare serotype of Salmonella subspecies (spp.) in Canada with less than nine cases reported each year (2011-2013). An increase in S. Reading was identified in several Canadian provinces in early 2015, prompting the initiation of a national outbreak investigation.

Objectives: To describe a multi-provincial S. Reading outbreak in Canada that affected over 30 people.

Methods: Cases were defined as laboratory-confirmed S. Reading with related pulsed-field gel electrophoresis (PFGE) patterns. Onset dates were between November 2014 and September 2015.​: Early in the investigation, investigators noted cases were predominantly of Eastern Mediterranean origin, mainly Afghan and Lebanese and many of those affected had consumed food items not typically captured on standard enteric outbreak hypothesis-generating questionnaires. An open-ended three day food consumption survey was conducted with a convenience sample of community informants to better understand food preferences of the affected ethnocultural populations. Results of the survey were used to design a focused questionnaire for case re-interviews and subsequent outbreak cases. Public health investigators obtained food samples from case homes and relevant food premises. Food safety authorities conducted traceback of suspected food items and collected food samples for laboratory testing.

Results: There were 31 confirmed cases (Ontario=23, Alberta=7, New Brunswick=1) and three probable (Ontario=2, Alberta=1) cases of S. Reading identified as part of the outbreak. The median age was 31 years (range less than one to 85 years) and 53% (18/34) of cases were female. Seven cases were hospitalized. No deaths were reported. Most cases were of Eastern Mediterranean origin (n=23) or had reported consuming Eastern Mediterranean foods (n=3). The predominant ethnic origins reported by cases were Afghan in Ontario (n=12) and Lebanese in Alberta (n=3). Genetic similarity of clinical isolates was further confirmed using whole genome sequencing.​: Three ethnic bakeries were identified as possible common exposures for the cases; however, traceback of foods of interest from these bakeries did not identify a common supplier and the source of the illness was not identified. In total, 227 food samples from retail premises (n=142), restaurants (n=13) and case homes (n=72) were tested; two food samples, kalonji seeds and tahini, were positive for S. Ruiru and S. Meleagridis. These products were recalled from the marketplace.

Conclusion: Despite extensive epidemiological, microbiological and food traceback investigations, a common source was not identified for this S. Reading outbreak. Challenges included lack of familiarity with the food items consumed in affected ethnocultural groups, as well as a lack of background data on expected food exposures in the outbreak population. Engaging local partners helped build understanding of food preferences in affected communities. Given Canada's ethnic and cultural diversity, culturally competent approaches to enteric outbreak investigations and food consumption surveys may be useful.

Figure 1. Confirmed and probable Salmonella Reading outbreak cases by week of illness onset and province, Canada, November 1, 2014 to September 11, 2015 (n=34)

* The cases were asymptomatic therefore the specimen collection date was used instead of the symptom onset date

Figure 2. Outbreak-related Salmonella Reading cases by ethnic origin and province, Canada, November 1, 2014 to September 11, 2015 (n=34)

Abbreviation: East. Med., Eastern Mediterranean

Figure 3. Whole Genome sequencing (WGS) for S. Reading cases, National Microbiology Laboratory -Public Health Genomics Unit

Reference: 15-0793. Method: NML Bioinformatics SNVPhyl Pipeline, Maximum-likelihood phylogenetic tree based on 642 high quality core genome single nucleotide variant (SNV) positions identified amongst 37 isolates over 93% of the reference genome. Context: US isolates, 1502Read12MP (ReadXAI.0012, ReadBNI.0005, ReadXAI.0011, ReadXAI.0014)