Four-year typhoid fever outbreak in Canada linked to chronic carrier

In a rare occurrence of locally acquired typhoid fever in Canada, health officials in Ottawa recently traced an outbreak back to a chronic Salmonella Typhi carrier working as a food handler. The outbreak involved seven confirmed cases from Oct. 2018 to May 2022 and was linked to the asymptomatic carrier through whole genome sequencing (WGS) and social network analysis.

Typically, cases of typhoid fever in Canada are linked to international travel, making this localized outbreak unusual. The investigation led by Ottawa Public Health underscores the potential for chronic carriers, who may shed bacteria without symptoms and unintentionally transmit the disease over an extended period. All eight outbreak-related isolates matched genetically, confirming the cluster.

Initial cases and detection
The outbreak investigation began when Ottawa Public Health identified two non-travel-related cases of typhoid fever in 2021. The patients, residing only 2.6 km (1.6 miles) apart, had a common link to a grocery store ready-to-eat counter. One patient was an employee, and the other was a customer. An outbreak was declared after initial interviews, and WGS analysis confirmed the cases were part of the same cluster.

The early stages of the investigation were challenging because of the infrequent and spaced-out patients. Public health inspectors initially focused on the grocery store’s ready-to-eat counter, where three food handlers were identified for testing. However, the food samples and stool tests for the handlers were negative, and no critical infractions were observed during the inspection.

Epidemiological and social network analysis
As cases continued to emerge, officials noted that most affected individuals resided within an eight km (five mile) radius of Ottawa. The confirmed cases ranged in age from eight to 50, with six of the seven requiring hospitalization. Contact tracing and spatial mapping revealed that four of the seven patients had connections to the implicated grocery store. Social linkages emerged as key factors for the remaining cases, suggesting transmission through shared social and workspaces.

Through additional interviews, the investigation identified an asymptomatic food handler, referred to as Case A, who has worked at multiple locations in the grocery store chain since 2017. This individual, who shared a country of origin with several of the infected individuals, tested positive for Salmonella Typhi, confirming the link. Ottawa Public Health hypothesizes that Case A contracted the infection before immigrating to Canada in 2017.

Public health response
Once identified as a carrier, Case A was temporarily excluded from food handling duties and referred for treatment. Following antibiotic therapy and successful clearance testing, Case A was eventually allowed to resume work. However, financial hardship became a concern during the exclusion period, highlighting the potential burden on employees who are excluded from work due to infectious diseases.

In response to the outbreak, Ottawa Public Health issued alerts to healthcare providers in the region about the risk of locally acquired typhoid fever in patients without a travel history. To prevent further transmission, contacts of infected individuals were provided with hygiene and food handling guidance. Testing was also offered to close contacts to ensure no additional cases were missed.

Challenges in investigation and lessons learned
The outbreak was difficult to detect because of the intermittent nature of typhoid shedding among chronic carriers and the multi-year span between the initial and final cases. Due to these extended time intervals, public health officials faced challenges in identifying common exposures and generating hypotheses.

The investigation demonstrated the value of integrating WGS and social network analysis, particularly in linking cases with no direct social contact but proximity in shared living spaces. Additionally, the need to coordinate with PulseNet Canada for WGS cluster tracking was crucial in confirming the genetic link between the isolates. The protracted nature of the outbreak highlighted the importance of monitoring typhoid cases in both endemic and non-endemic regions for potential local clusters.

This outbreak represents the first locally acquired transmission of Salmonella Typhi in Canada in more than 30 years. While typhoid fever is a rare illness in Canada, especially in non-travel-related cases, the event reminds us of the role asymptomatic carriers can play in prolonged, low-level disease transmission within communities.

This case provides a model for managing potential future outbreaks, emphasizing the need for awareness and preparedness even in non-endemic settings.

The full study can be found here.

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