Declaring the end of an enteric illness outbreak is an important step in the investigation. A specific end date helps assess and communicate risk to the public, re-assign resources, as well as remove temporary mitigation measures.
The criteria for declaring the end of enteric illness outbreaks are evaluated in “Criteria to Consider When Declaring the End of an Enteric Illness Outbreak” (Franklin K et al, unpublished), which expected to be published simultaneously in the Canada Communicable Disease Report (CCDR) and Eurosurveillance.
Three criteria were developed to guide the decision to declare the end of an enteric illness outbreak. This involves determining that illnesses have returned to baseline levels (criterion 1), identifying the last time that individuals may have been exposed to the outbreak source (criterion 2), and allowing enough time to pass to allow these individuals to become ill and be reported to public health authorities (criterion 3).
Identify the date that the outbreak could be declared over using the following steps.
- Identify the expected baseline levels as defined by the outbreak case definition.
- Identify the last date that individuals may have been exposed to the outbreak source.
- Identify the length of time required to allow individuals to become ill and be reported to public health authorities, and add this to the last date that an individual may have been exposed to the outbreak source to obtain the date the outbreak could be declared over. This length of time is the maximum incubation period for E. coli O157 + the 90th percentile of the reporting delay. Use the line list (Module 4 – Line list) to obtain the reporting delay estimate.
See the epidemic curve for this outbreak showing the three criteria (Module 4 – Declaring the outbreak over figure).
- Criterion 1: E. coli O157:H7 with the PFGE pattern combination of interest had not been seen since July 2010 Canada. Therefore the expected baseline is approximately zero (less than 1 case/year).
- Criterion 2: There are two possible dates that should be considered:
- Date of product recall: August 3, 2014 and
- Date of last onset: July 19, 2014.
- The date of the product recall is considered to be the last time individuals may have been exposed to the implicated source since other cases may have been exposed to the product between the last illness onset date and the date of product recall.
- Criterion 3: The time period (“lag window”) is the maximum incubation period for E. coli O157 (10 days – from review of the literature) + reporting to the lead investigating authority (see below). Only baseline levels should be observed in the “lag window”.
- The 75th-90th percentile of the reporting lag in the current outbreak: The reporting lag for each case was calculated by subtracting the “Illness Onset Date” from the “Report Date to OMD”; the median reporting delay is 18 days, the 75th percentile is 19 days and the 90th percentile is 19.9 days. In this case, the more conservative date of September 2, 2014 is used because this product has a long shelf life (i.e., it is frozen) and if consumers are not aware of the recall the product may remain in their freezers for months.
- Therefore, the lag window will start at August 3, 2014 and end on September 2, 2014: August 3, 2014 + 10 days + 20 days = September 2, 2014.
Note: This date is not static. The date should be re-evaluated if new, relevant information comes in (e.g., case reporting increases above baseline levels in the “lag window”, the CFIA reports that they have found some product on store shelves despite effectiveness checks or new traceback information suggests other products/product lots may be contaminated).