Weighing the evidence in foodborne illness investigations

In response to recommendations arising from the 2008 Listeria outbreak, Health Canada recently published a booklet entitled Weight of Evidence: Factors to Consider for Appropriate and Timely Action in a Foodborne Illness Outbreak Investigation. It is intended as “a general guidance document primarily for federal-level decision makers during foodborne outbreak investigations.”

The booklet is divided into 10 sections, starting with a graphical flowchart diagram suggesting possible scenarios and responses based on evidence gathered during the investigation. The authors point out that this is not a prescriptive, one-size-fits-all guideline, and that the weight of evidence “needed to proceed with action will vary with each outbreak…and would likely be based upon the experience of the investigator(s)…” all of which can add some uncertainty to the process and possible outcomes.

Inspectors investigating a possible outbreak are provided with sample forms to assist in information gathering from the victim, as well as recommendations on product sampling techniques. Although not exhaustive, the forms, at their very least, should ensure that all inspectors gather the same basic epidemiological information, which contributes significantly to the weight of evidence authorities will sift through to formulate an action plan.

Once samples are acquired, investigators must endeavour to determine the probable cause of the illness through various analyses. Naturally, foodborne pathogens top the list of probable causes. Investigators are encouraged to use pulse field electrophoresis, the gold standard analytical procedure for establishing the degree to which a pathogen in the food is a genetic match to the pathogen recovered from the victim. The results from pulse field and other genetic analyses are scrutinized with the aid of a large international database containing the genetic finger prints of thousands of other known pathogens.

If a match is confirmed, this result contributes heavily to the weight of evidence but isn’t essential to implicate a product or suggest a course of action on its own. For example, the pathogen isolated from the food could vary only slightly from the pathogen isolated from the victim, indicating a close genetic connection. It’s also possible that the pathogen may not be in the sample(s). Therefore, the agencies recommend that investigators gather other information to add to the body of evidence to aid them in their decision making.

Investigators are instructed to scrutinize the nature of the accumulated evidence using the following criteria:

• Plausibility – To determine if the food is a likely vehicle of the illness;
• Consistency – To verify that the same food is implicated in all cases;
• Specificity – To establish if a single specific food could be the source of infection;
• Association – To show that there is a strong correlation between the food and the outbreak;
• Temporal – To confirm that the timing of the outbreak is consistent with past outbreaks of the type in question;
• Dose response – To see if the severity of the infection is related to the quantity and frequency of consumption of the suspected food;
• Pulse electrophoresis – To look for a genetic link; and,
• Other sources – To eliminate other possible sources as possible causes.

Foodborne illness investigations are costly, stressful and complex affairs. Not knowing how regulatory agencies will react contributes immeasurably to the stress and the uncertainty food processors face in these situations. Consider this booklet to be an industry primer to understand how government agencies should be responding to foodborne outbreaks, something that could be very helpful if your company is implicated in an outbreak.

Dr. Ron Wasik, PhD, MBA (Dr. Fix It) is president of RJW Consulting Canada Ltd. Contact him at rwasik@rjwconsultingcanada.com