The investigation began with the detection of a cluster of E. coli O157:H7 illnesses and initiation of an investigation (Module 1), followed by gathering of information from cases to generate hypotheses on the source of the illnesses (Module 2), compiling evidence from epidemiologic, microbiologic, and traceback investigations to implicate a potential source (Module 3), and concluding the investigation by declaring the outbreak over and disseminating the results of the investigation to relevant stakeholders (Module 4).
The likely sources of the E. coli O157:H7 illnesses in this outbreak were two different brands of frozen hamburgers produced at the same facility: two lots of Farmer Fred’s Burgers and one lot of Happy Burger. These products were recalled from the market.
A list of all documents used in the case study can be found here.
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This case study covered an outbreak investigation of a fictitious E. coli O157:H7 outbreak in the summer of 2014 from the federal perspective. All outbreaks are unique and the case study was not intended to be a guidance document for outbreak investigation in Canada. In order to improve the story flow, several creative liberties were taken. A selection of those creative liberties are summarized below:
|Section||Case study story||Comment|
|Module 1 – Enteric surveillance and outbreak detection||There is a very brief interval between the isolation/identification of organisms and routine reporting to NESP.||Such the timelines for detection of increases in reported case counts used in this case study are not reflective of the actual timeline for analysis of NESP weekly data.|
|Exercise 1: Creating an investigation line list||Date of illness onset was provided for all cases, even those lost to follow-up.||Date of illness onset is not typically available for all cases. When it is not available, the earliest date available should be used (illness onset date, specimen collection date, received date, isolation date, PFGE upload date, report date).|
|Exercise 3: Food frequency analysis in Excel||Eight hypothesis generating questionnaires are received within two days.||Turnaround time is often much longer than two days. It can take several days to get in touch with a case and find a convenient time to administer the questionnaire.|
|Module 3 – OICC call #2||In the case study, most of the information was shared during the OICC calls.||While OICC calls offer a great opportunity to provide updates, information is typically shared as it becomes available, which does not always align with the OICC call schedule.|
|Exercise 5: Epi Assessment||11/12 of cases report eating hamburgers prior to onset of illness.||% reporting implicated product is not always this high. Epi assessments are often made with less than 100% of cases reporting consumption of the product of interest.|
|Module 4 – Declaring the outbreak over||No cases are reported after the recall.||The recalled hamburgers were a frozen product. People are not always aware that they have recalled product in their homes, and therefore it is possible that additional cases may arise after the recall.|