Locally Acquired Typhoid Fever Cases Spanning Four Years Linked to a Chronic Carrier in Ottawa, Canada, 2018-2022

Water or foodborne

Background: In Canada, Salmonella enterica serovar Typhi infections are typically associated with travel to endemic regions of the world. In November 2021, Ottawa Public Health identified that two non-travel typhoid fever cases were linked to the same grocery store ready-to-eat counter. Whole genome sequencing (WGS) confirmed these cases were related, prompting further investigation to identify the outbreak source and implement control measures.

Methods: Cases were identified through WGS of Salmonella Typhi isolates. Hypotheses were generated using case and contact interviews, contact testing, food premises inspection, food sampling, reviewing takeout receipts and public social media, and social network and spatial analyses.

Results: Eight confirmed cases, aged 8 to 50 years, were linked from October 2018 to May 2022. All cases resided within an 8 km radius in Ottawa, Canada and none reported international travel. Three cases were customers or employees of the same grocery store ready-to-eat counter. Stool testing of three ready-to-eat counter food handlers, linked to at least one case, did not detect Salmonella Typhi. Further contact tracing identified an asymptomatic carrier in May 2022 as the probable source of the outbreak; the pathogen was likely acquired before immigration to Canada in 2017. The chronic carrier worked as a food handler at multiple food premises, including the implicated ready-to-eat counter. All cases (n=7) were linked to the chronic carrier as customers, co-workers, neighbors, or through social or household relationships. The chronic carrier was excluded from food handling until successful completion of treatment and clearance testing.

Conclusions: This investigation led to detection of an asymptomatic carrier and to interventions that stopped the outbreak. The investigation illustrates the successful use of modern laboratory and epidemiological methods to solve a multi-year, initially unapparent typhoid fever outbreak in a non-endemic country.

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