Case definitions

Case definitions are a set of standard criteria to determine who should be included as part of an outbreak and who should not.  The purpose is to ensure that cases are counted in a systematic and consistent manner in order to make comparisons and analyze trends.

In outbreak investigations, case definitions are only used for counting outbreak cases, not for establishing clinical diagnoses, or to make healthcare decisions for an individual patient. Existing guidelines, algorithms, and protocols should be followed as per local jurisdictional policies for appropriate clinical case management.

There are two components that should be included in case definitions:

  1. Clinical and/or laboratory criteria to assess if an individual has the illness of interest, and
  2. Restrictions by person, place, and time.

Case definitions are often refined to be more specific during the course of an investigation when new information becomes available (Further reading: Case definitions).

Question 1-5: Create case definitions for confirmed, probable and suspect cases for this outbreak.

ANSWER

Case definitions should be developed collaboratively with investigative partners. For this outbreak, the confirmed case definition will be very specific, as it includes a rare PFGE pattern combination. PFGE testing can take a few days to complete which introduces a time delay for classifying cases as confirmed; therefore, it is important that E. coli O157:H7 cases with PFGE pending still be captured under a probable or suspect case definition.

At this stage in the investigation, it is important to ensure that all cases that may be a part of this cluster, especially early cases, are included. Therefore, the time frame for these initial case definitions needs to go back two to four weeks before the first reported date of illness onset. This time frame is used because E. coli O157:H7 has an incubation period of up to 10 days, and initial laboratory sampling, shipping, and testing takes another two to three weeks. Since the earliest case in our cluster has an illness onset date of July 1, 2014, we can use June 1, 2014 in our initial case definitions.

Confirmed case: A resident of or visitor to Canada with

  • Laboratory confirmation of E. coli O157:H7 AND
  • PFGE pattern combination ECXAI.1398/ECBNI.1212 AND
  • Symptom onset or laboratory confirmation on or after June 1, 2014

Probable case: A resident of or visitor to Canada with

  • Laboratory confirmation of coli O157:H7 AND
  • PFGE pending AND
  • Symptom onset or laboratory confirmation on or after June 1, 2014

Suspect case: A resident of or visitor to Canada with

  • Symptoms* consistent with E. coli O157:H7 infection AND
  • An epidemiological link to a confirmed case AND
  • Symptom onset on or after June 1, 2014

*Diarrhea (three or more loose stools within 24 hours)

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