Canadian health officials published a report about seven cases of typhoid fever over four years (from 2018 to 2022) that were linked to an asymptomatic chronic S. Typhi carrier who worked irregularly as a food handler at various locations of a grocery store chain. Transmission occurred through food handling, shared workspaces and social and household networks. Although the outbreak was relatively small, six of seven cases were hospitalized with significant morbidity.
https://www.canada.ca/en/public-health/services/reports-publications/canada-communicable-disease-report-ccdr/monthly-issue/2024-50/issue-11-november-2024/locally-acquired-typhoid-fever-outbreak-linked-chronic-carriage-ottawa.html
Outbreak Report
Locally acquired typhoid fever outbreak linked to chronic carriage in Ottawa, Canada, 2018–2022
Janice Zhang1,2, Ann Jolly2, Tram Nguyen2, Monir Taha2, Christina Lee3, Antoine Corbeil3, Esther Dapaah2, Jeff Walker2, Curtis Cooper4, Jacqueline Willmore2
Abstract
Background: In Canada, Salmonella enterica serovar Typhi infections are uncommon and typically travel-related. In November 2021, Ottawa Public Health identified a link between two typhoid fever cases, with no recent history of international travel, to the same grocery store ready-to-eat counter
Background: In Canada, Salmonella enterica serovar Typhi infections are uncommon and typically travel-related. In November 2021, Ottawa Public Health identified a link between two typhoid fever cases, with no recent history of international travel, to the same grocery store ready-to-eat counter
.
Objective: This report describes the outbreak response to a rare occurrence of chronic S. Typhi carriage in Ottawa, Ontario, Canada and provides recommendations for investigations of small-scale protracted outbreaks.
Methods: We administered exposure questionnaires using a single interviewer approach, tested stool samples of contacts and food handlers, inspected food premises, collected food samples and reviewed takeout receipts. Social network, spatial and whole genome sequencing analyses were used to investigate additional possible links between cases.
Results: Seven people with typhoid fever and onset from October 2018 to May 2022 were linked to an asymptomatic chronic S. Typhi carrier. Whole-genome sequencing confirmed that all eight isolates matched the outbreak cluster. All cases and carrier resided within an eight km radius in Ottawa. The chronic carrier worked as a food handler at various locations of a grocery store chain, including the implicated ready-to-eat counter. Transmission occurred via food handling, shared workspaces and social and household networks.
Conclusion: The chronic carrier was excluded from food handling until successful completion of treatment and clearance testing. We overcame the challenges of a small but prolonged outbreak by identifying an asymptomatic carrier using a multi-method approach including whole genome sequencing and social network analysis.
Objective: This report describes the outbreak response to a rare occurrence of chronic S. Typhi carriage in Ottawa, Ontario, Canada and provides recommendations for investigations of small-scale protracted outbreaks.
Methods: We administered exposure questionnaires using a single interviewer approach, tested stool samples of contacts and food handlers, inspected food premises, collected food samples and reviewed takeout receipts. Social network, spatial and whole genome sequencing analyses were used to investigate additional possible links between cases.
Results: Seven people with typhoid fever and onset from October 2018 to May 2022 were linked to an asymptomatic chronic S. Typhi carrier. Whole-genome sequencing confirmed that all eight isolates matched the outbreak cluster. All cases and carrier resided within an eight km radius in Ottawa. The chronic carrier worked as a food handler at various locations of a grocery store chain, including the implicated ready-to-eat counter. Transmission occurred via food handling, shared workspaces and social and household networks.
Conclusion: The chronic carrier was excluded from food handling until successful completion of treatment and clearance testing. We overcame the challenges of a small but prolonged outbreak by identifying an asymptomatic carrier using a multi-method approach including whole genome sequencing and social network analysis.
Introduction
Typhoid fever is caused by direct or indirect fecal-oral exposure from an individual infected with Salmonella enterica serovar Typhi (S. Typhi). The primary incubation period is typically eight to 14 days but can range from three to more than 60 days. Symptoms can be non-specific, ranging from fever and mild illness to severe and potentially fatal illness requiring hospitalization Footnote1. While most infections resolve with treatment, approximately 2%–5% of those infected become chronic carriers who can continue to transmit the bacteria for many years, often without symptoms Footnote2.
Typhoid fever is a notifiable disease in Canada, and in Ontario, cases are reportable to the local Medical Officer of Health Footnote3Footnote4. Due to historic improvements to drinking water, sanitation and food safety, typhoid fever is uncommon in Canada Footnote1Footnote5Footnote6. Between 2012 and 2021, an average of 140 cases per year were reported (average rate 0.4 per 100,000 population) Footnote6. Most infections are associated with recent international travel, particularly visiting of friends and relatives in endemic regions Footnote7Footnote8Footnote9Footnote10Footnote11. The most recent published report of a locally acquired typhoid fever outbreak in Canada was in Ontario in 1990, linked to consuming raw contaminated imported shellfish Footnote12.
Between 2016 and 2020, an average of four cases of typhoid fever were reported per year in Ottawa, Ontario. In November 2021, Ottawa Public Health identified two locally acquired typhoid fever cases (cases D and E), residing 2.6 km apart, who had been reported two months apart, in September and November 2021, respectively. Case interviews identified linkage to a grocery store ready-to-eat food counter, one as a customer (case D) and the other as an employee (case E). An outbreak was declared in November 2021 and a multi-disciplinary team was assembled to identify the source of the outbreak and implement control measures. Preliminary whole-genome sequencing (WGS) confirmed these cases were related.
This report describes the outbreak response to a rare occurrence of chronic S. Typhi carriage in Canada and highlights the strengths of using a multi-method approach to overcome the challenges of investigating small-scale protracted outbreaks.
Results
Seven confirmed cases of typhoid fever with illness onset between October 2018 and May 2022 were linked to an asymptomatic carrier (Figure 1). All seven symptomatic cases were interviewed twice. The chronic carrier was interviewed first as a contact and then as a case. Twenty-eight close contacts were also interviewed but no additional cases were identified. Among the seven symptomatic cases, common symptoms included fever (n=7 cases), malaise (n=6), diarrhea (n=5), abdominal pain (n=5) and headache (n=4). Six cases were hospitalized for a median of nine days (range: 5–22 days) and one case received treatment in the emergency department; all recovered. All seven symptomatic cases and carrier resided within an eight km radius in Ottawa. Cases and carrier had a median age of 28 years old (range: 8–50 years) and six (75%) were male. Five of eight (63%) were immigrants to Canada (year of arrival ranged from 1997 to 2019), including four from the same country of origin, but no cases reported recent international travel.
Discussion
This is the first reported outbreak of locally transmitted typhoid fever in Canada since 1990. Seven cases of typhoid fever over four years (from 2018 to 2022) were linked to an asymptomatic chronic S. Typhi carrier who worked irregularly as a food handler at various locations of a grocery store chain. Transmission occurred through food handling, shared workspaces and social and household networks. Although the outbreak was relatively small, six of seven cases were hospitalized with significant morbidity. This investigation highlights some of the challenges of identifying and managing a typhoid fever outbreak as well as the strengths of using multiple epidemiological, laboratory and environmental investigation methods during outbreak responses.
Characteristics of this outbreak are similar to others reported from non-endemic, high-income countries Footnote5. Locally acquired typhoid fever outbreaks reported in the United States since the 1960s had limited secondary transmission and were often associated with a primary case of chronic S. Typhi carriage involved in food handling Footnote5Footnote17Footnote18Footnote19. This outbreak shares some of the challenges noted in these previous investigations. First, outbreaks caused by chronic S. Typhi carriage may be difficult to detect as carriers can shed bacteria intermittently for many years, potentially causing infections over a long period of time Footnote17. Second, a small number of cases and long incubation period can make it more challenging to generate source hypotheses. Finally, given that typhoid fever cases have become infrequent in non-endemic countries, public health representatives may have limited experience managing such outbreaks Footnote18.
This outbreak was difficult to identify due to the span of multiple years between cases. While the first clinical case was reported in 2018, the outbreak remained undetected until local public health nurses noted a common exposure between the two cases reported in 2021. Although WGS is routinely performed for Salmonella isolates in Ontario, the genomic linkage between cases was not flagged by the laboratory at the time due to the 60-day limit for initial PulseNet Canada cluster assignments. This illustrates the need to monitor and investigate any typhoid fever case for potential spatiotemporal and epidemiological linkages and to involve laboratory partners in surveillance and outbreak investigations to expand investigational options where relevant. It also highlights the potential benefit of expanding the PulseNet Canada relatedness analysis window beyond 60 days for S. Typhi, as recommended in another study examining typhoid fever outbreaks in the United States from 1999 to 2010 Footnote17.
During this outbreak investigation, multiple factors limited the information available to generate source hypotheses, including the protracted length of time between cases, the varying modes of infection acquisition and the small number of cases. A case-control approach would have been problematic as responses to questionnaires are greatly affected by recall bias. Once we employed a multi-prong approach including WGS, social network analysis, a single interviewer and asymptomatic contact screening, we were successful in tracing cases to the primary source. Other outbreak reports have also highlighted the importance of using multiple methods in typhoid fever investigation Footnote19Footnote20.
The precarious nature of food handling work also hindered the investigation. The initial public health inspection of the implicated ready-to-eat counter failed to identify the carrier as an employee due to employment across multiple locations of the grocery store chain. In typhoid fever investigations, given the potential long period of exposure and transient food handler workforce, we recommend taking an extensive employment history of past, present and temporary food handlers. The exclusion from work for typhoid fever treatment and clearance also caused financial hardship to the food handlers. The negative impacts of excluding infected persons from work duties is likely to be shared within social networks, thus discouraging further cases and contacts from being interviewed and tested. Recent full compensation for those on medically mandated leave, such as that made available due to COVID-19 illness, presents a potential mechanism to facilitate employment insurance for other notifiable infections requiring exclusion from work Footnote21.
Although there was no known direct contact, shared bathrooms or shared meals between case A (carrier) and case C (neighbour), we hypothesize that transmission potentially occurred through fomite contamination of common surfaces, such as doorknobs, railings or elevators. Likewise, although case H (the janitor) did not have known direct contact with case A or eat meals at the ready-to-eat counter, we hypothesize that acquisition likely occurred via common surfaces used by case A (carrier) at the grocery store (e.g., bathrooms). Training on hand hygiene and provision of proper personal protective equipment for janitors is essential to decrease the risk of enteric disease acquisition, as outside of healthcare and laboratory settings, janitorial work should not constitute an occupational hazard for infectious diseases Footnote22.
Although outbreaks of typhoid fever are rare in Canada, they remain a risk particularly with international travel and immigration from regions where typhoid fever remains endemic Footnote20Footnote23. In addition, the emergence of drug-resistant S. Typhi in South Asia, increasingly observed in cases diagnosed in Ontario, has made effective treatment more challenging and prevention more urgent Footnote8Footnote24. The United States has reported cases of drug-resistant S. Typhi among individuals with no history of recent international travel Footnote25. Surveillance and thorough case follow-up are essential to detect and control future outbreaks of typhoid fever Footnote19Footnote24.
Conclusion
This outbreak report describes a rare outbreak of typhoid fever associated with chronic S. Typhi carriage in Canada and contributes to the literature to inform future investigations. An interdisciplinary investigation was key to discovering the transmission source. This outbreak demonstrates the risk of infection and challenges in investigation among marginalized workers without comprehensive benefits or stable working conditions. The investigation also adds to the evidence for expanding the analysis window for S. Typhi WGS cluster assignment.
Typhoid fever is caused by direct or indirect fecal-oral exposure from an individual infected with Salmonella enterica serovar Typhi (S. Typhi). The primary incubation period is typically eight to 14 days but can range from three to more than 60 days. Symptoms can be non-specific, ranging from fever and mild illness to severe and potentially fatal illness requiring hospitalization Footnote1. While most infections resolve with treatment, approximately 2%–5% of those infected become chronic carriers who can continue to transmit the bacteria for many years, often without symptoms Footnote2.
Typhoid fever is a notifiable disease in Canada, and in Ontario, cases are reportable to the local Medical Officer of Health Footnote3Footnote4. Due to historic improvements to drinking water, sanitation and food safety, typhoid fever is uncommon in Canada Footnote1Footnote5Footnote6. Between 2012 and 2021, an average of 140 cases per year were reported (average rate 0.4 per 100,000 population) Footnote6. Most infections are associated with recent international travel, particularly visiting of friends and relatives in endemic regions Footnote7Footnote8Footnote9Footnote10Footnote11. The most recent published report of a locally acquired typhoid fever outbreak in Canada was in Ontario in 1990, linked to consuming raw contaminated imported shellfish Footnote12.
Between 2016 and 2020, an average of four cases of typhoid fever were reported per year in Ottawa, Ontario. In November 2021, Ottawa Public Health identified two locally acquired typhoid fever cases (cases D and E), residing 2.6 km apart, who had been reported two months apart, in September and November 2021, respectively. Case interviews identified linkage to a grocery store ready-to-eat food counter, one as a customer (case D) and the other as an employee (case E). An outbreak was declared in November 2021 and a multi-disciplinary team was assembled to identify the source of the outbreak and implement control measures. Preliminary whole-genome sequencing (WGS) confirmed these cases were related.
This report describes the outbreak response to a rare occurrence of chronic S. Typhi carriage in Canada and highlights the strengths of using a multi-method approach to overcome the challenges of investigating small-scale protracted outbreaks.
Results
Seven confirmed cases of typhoid fever with illness onset between October 2018 and May 2022 were linked to an asymptomatic carrier (Figure 1). All seven symptomatic cases were interviewed twice. The chronic carrier was interviewed first as a contact and then as a case. Twenty-eight close contacts were also interviewed but no additional cases were identified. Among the seven symptomatic cases, common symptoms included fever (n=7 cases), malaise (n=6), diarrhea (n=5), abdominal pain (n=5) and headache (n=4). Six cases were hospitalized for a median of nine days (range: 5–22 days) and one case received treatment in the emergency department; all recovered. All seven symptomatic cases and carrier resided within an eight km radius in Ottawa. Cases and carrier had a median age of 28 years old (range: 8–50 years) and six (75%) were male. Five of eight (63%) were immigrants to Canada (year of arrival ranged from 1997 to 2019), including four from the same country of origin, but no cases reported recent international travel.
Discussion
This is the first reported outbreak of locally transmitted typhoid fever in Canada since 1990. Seven cases of typhoid fever over four years (from 2018 to 2022) were linked to an asymptomatic chronic S. Typhi carrier who worked irregularly as a food handler at various locations of a grocery store chain. Transmission occurred through food handling, shared workspaces and social and household networks. Although the outbreak was relatively small, six of seven cases were hospitalized with significant morbidity. This investigation highlights some of the challenges of identifying and managing a typhoid fever outbreak as well as the strengths of using multiple epidemiological, laboratory and environmental investigation methods during outbreak responses.
Characteristics of this outbreak are similar to others reported from non-endemic, high-income countries Footnote5. Locally acquired typhoid fever outbreaks reported in the United States since the 1960s had limited secondary transmission and were often associated with a primary case of chronic S. Typhi carriage involved in food handling Footnote5Footnote17Footnote18Footnote19. This outbreak shares some of the challenges noted in these previous investigations. First, outbreaks caused by chronic S. Typhi carriage may be difficult to detect as carriers can shed bacteria intermittently for many years, potentially causing infections over a long period of time Footnote17. Second, a small number of cases and long incubation period can make it more challenging to generate source hypotheses. Finally, given that typhoid fever cases have become infrequent in non-endemic countries, public health representatives may have limited experience managing such outbreaks Footnote18.
This outbreak was difficult to identify due to the span of multiple years between cases. While the first clinical case was reported in 2018, the outbreak remained undetected until local public health nurses noted a common exposure between the two cases reported in 2021. Although WGS is routinely performed for Salmonella isolates in Ontario, the genomic linkage between cases was not flagged by the laboratory at the time due to the 60-day limit for initial PulseNet Canada cluster assignments. This illustrates the need to monitor and investigate any typhoid fever case for potential spatiotemporal and epidemiological linkages and to involve laboratory partners in surveillance and outbreak investigations to expand investigational options where relevant. It also highlights the potential benefit of expanding the PulseNet Canada relatedness analysis window beyond 60 days for S. Typhi, as recommended in another study examining typhoid fever outbreaks in the United States from 1999 to 2010 Footnote17.
During this outbreak investigation, multiple factors limited the information available to generate source hypotheses, including the protracted length of time between cases, the varying modes of infection acquisition and the small number of cases. A case-control approach would have been problematic as responses to questionnaires are greatly affected by recall bias. Once we employed a multi-prong approach including WGS, social network analysis, a single interviewer and asymptomatic contact screening, we were successful in tracing cases to the primary source. Other outbreak reports have also highlighted the importance of using multiple methods in typhoid fever investigation Footnote19Footnote20.
The precarious nature of food handling work also hindered the investigation. The initial public health inspection of the implicated ready-to-eat counter failed to identify the carrier as an employee due to employment across multiple locations of the grocery store chain. In typhoid fever investigations, given the potential long period of exposure and transient food handler workforce, we recommend taking an extensive employment history of past, present and temporary food handlers. The exclusion from work for typhoid fever treatment and clearance also caused financial hardship to the food handlers. The negative impacts of excluding infected persons from work duties is likely to be shared within social networks, thus discouraging further cases and contacts from being interviewed and tested. Recent full compensation for those on medically mandated leave, such as that made available due to COVID-19 illness, presents a potential mechanism to facilitate employment insurance for other notifiable infections requiring exclusion from work Footnote21.
Although there was no known direct contact, shared bathrooms or shared meals between case A (carrier) and case C (neighbour), we hypothesize that transmission potentially occurred through fomite contamination of common surfaces, such as doorknobs, railings or elevators. Likewise, although case H (the janitor) did not have known direct contact with case A or eat meals at the ready-to-eat counter, we hypothesize that acquisition likely occurred via common surfaces used by case A (carrier) at the grocery store (e.g., bathrooms). Training on hand hygiene and provision of proper personal protective equipment for janitors is essential to decrease the risk of enteric disease acquisition, as outside of healthcare and laboratory settings, janitorial work should not constitute an occupational hazard for infectious diseases Footnote22.
Although outbreaks of typhoid fever are rare in Canada, they remain a risk particularly with international travel and immigration from regions where typhoid fever remains endemic Footnote20Footnote23. In addition, the emergence of drug-resistant S. Typhi in South Asia, increasingly observed in cases diagnosed in Ontario, has made effective treatment more challenging and prevention more urgent Footnote8Footnote24. The United States has reported cases of drug-resistant S. Typhi among individuals with no history of recent international travel Footnote25. Surveillance and thorough case follow-up are essential to detect and control future outbreaks of typhoid fever Footnote19Footnote24.
Conclusion
This outbreak report describes a rare outbreak of typhoid fever associated with chronic S. Typhi carriage in Canada and contributes to the literature to inform future investigations. An interdisciplinary investigation was key to discovering the transmission source. This outbreak demonstrates the risk of infection and challenges in investigation among marginalized workers without comprehensive benefits or stable working conditions. The investigation also adds to the evidence for expanding the analysis window for S. Typhi WGS cluster assignment.
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