Shiga Toxin–Producing Escherichia coli Outbreak in Canadian Daycare Centers

Importance: Shiga toxin-producing Escherichia coli (STEC) infections have significant morbidity, with 15% to 20% of children developing hemolytic uremic syndrome (HUS), more than half of whom require kidney replacement therapy (KRT). Objective: To describe health care resource utilization, clinical outcomes, and outcomes associated with daily laboratory monitoring for thrombotic microangiopathy (TMA) in a large point-source STEC outbreak. Design, Setting, and Participants: This retrospective cohort study was conducted among a population-based cohort of individuals exposed to a contaminated food item served in 11 childcare centers in Calgary, Alberta, Canada, on August 29, 2023. Children and adults with confirmed STEC infection linked to the outbreak were included. Data were analyzed from June 9, 2025, to January 5, 2026. Exposure: E coli O157:H7 possessing genes encoding Shiga toxins 1 and 2. Main Outcomes and Measures: Health care utilization, symptoms and signs of infection, occurrence of HUS and KRT, and results of TMA screening. Results: There were 359 outbreak-linked confirmed STEC infections, including 285 children (median [IQR] age, 3.3 [2.3-4.2] years; 141 [62.4%] male) and 71 adults (median [IQR] age, 38.8 [31.9-46.4] years; 58 [81.7%] female) and 3 out-of-province cases; 326 primary and 33 secondary infections were identified. Of 359 infected individuals overall, 66 (18.4%) were asymptomatic. HUS occurred in 21 children (7.4%), all of whom had diarrhea (21 of 206 children with diarrhea [10.2%]). No adults developed HUS. This outbreak was linked to 508 and 395 visits by children with STEC to an emergency department and a dedicated STEC clinic, respectively; 591 visits occurred during a 7-day period, starting 6 days after exposure. Forty children were hospitalized, 9 children required KRT, and 3 children required intensive care. The most common symptoms in children and adults were diarrhea (206 children [91.2%]; 16 of 20 adults with data [80.0%]), abdominal pain (141 children [62.4%]; 11 of 20 adults with data [55.0%]), and bloody diarrhea (127 children [56.2%]; 7 of 20 adults with data [35.0%]). TMA testing had 100% (95% CI, 83.9%-100%) sensitivity, 95.7% (95% CI, 92.0%-98.0%) specificity, and a diagnostic accuracy of 96.1% (95% CI, 92.7%-98.2%) for development of HUS. Conclusions and Relevance: This cohort study reporting on a point-source STEC outbreak identified significant use of health care resources. Only 7% of children infected with STEC developed HUS, which is lower than reported in most prior outbreaks and sporadic case series. These findings highlight the importance of a coordinated public health response integrated with clinical care and suggest that proactive screening for TMA has the potential to mitigate adverse outcomes.