Case definitions

Following the activation of the OICC, investigation partners must agree on 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 2-2: Create a draft confirmed case definition for this outbreak.

Case definitions should be developed collaboratively with investigative partners. For this outbreak, the confirmed case definition can be very specific, as it includes cases genetically linked by whole genome sequencing (WGS).

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. Since the earliest case in our cluster has an illness onset date of April 22, 2020 and Salmonella has an incubation period of up to seven days, we could use April 15, 2020 in our initial case definition. However, in this situation it would be more appropriate to use April 1, 2020 to account for possible reporting delays. For multi-jurisdictional enteric outbreak investigations, the reporting delay refers to the time from the case’s symptom onset (or their specimen collection date if no onset date is available) to the time the case is reported to the Public Health Agency of Canada’s Outbreak Management Division (PHAC-OMD). After the case seeks health care, many steps must be taken before PHAC-OMD is notified of the case, all of which take time. These steps include collecting the specimen, testing the specimen submitted by the case for pathogens, conducting molecular subtyping on positive specimens, and reporting the case to the national data base. For Salmonella outbreaks this delay is usually between three and six weeks.     

Confirmed case: A resident of or visitor to Canada with

  • Laboratory confirmation of Salmonella Newport AND
  • Isolate matching PulseNet Canada cluster 2005NEWWGS-1ON-MP by whole genome sequencing* AND
  • Symptom onset or laboratory confirmation on or after April 1, 2020.

*Guidelines for whole genome sequencing interpretation are determined by the Public Health Agency of Canada’s National Microbiology Laboratory. 

Question 2-3: It is decided that a confirmed case definition is all that is required for this investigation. However, other investigations may include ‘probable’ and ‘suspect’ case definitions. Why might you want a probable case definition for this investigation? Draft a probable case definition that could be used.

Probable and suspect case definitions can be used when laboratory results are pending or not available. In this investigation, you could consider adding a probable case definition to account for Salmonella Newport cases that are pending whole genome sequencing:

Probable case: A resident of or visitor to Canada with

  • Laboratory confirmation of Salmonella Newport AND
  • Whole Genome Sequencing results pending AND
  • Symptom onset or laboratory confirmation on or after April 1, 2020.

This would allow investigation of probable cases and their food exposures earlier, which helps improve case recall and the possibility of leftovers being available for sampling. However, it may also introduce noise into the investigation if the probable cases do not become confirmed cases. Therefore, it is important to weigh the advantages and disadvantages of using multiple case definitions in each investigation. A probable case definition like this would not work for common Salmonella serotypes that are reported frequently (e.g., Salmonella Enteritidis), but can be considered for rare or less common Salmonella serotypes like Salmonella Newport. 

Further information on case definitions, including probable and suspect case definitions, is available here.


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