Evolving the Definition of an “Outbreak”

DE ANN DAVIS, Vice President Food Safety & Quality, Church Brothers Farms, True Leaf Farms

Gone are the days of the potluck supper being representative of a big outbreak. Not only did an outbreak in those pre-PulseNet days need to “big” for it to be identified, but pulsed field gel electrophoresis (PFGE), which was commonly used for detection, had distinct limitations. Today, whole genome sequencing (WGS) is revolutionizing investigations into foodborne illness in a way that goes beyond anything PFGE could do. But with this new technology, is it time to update our definition of an outbreak? For example:

1. Merriam-Webster defines an outbreak as: “a sudden rise in the incidence of a disease — an  outbreak  of measles.”
2. CDC’s definition takes a different twist. Its website states that an outbreak is  “the occurrence of more cases of disease, injury, or other health condition than expected in a given area or among a specific group of persons during a specific period. Usually, the cases are presumed to have a common cause or to be related to one another in some way. Sometimes distinguished from an epidemic as more localized, or the term less likely to evoke public panic.”
3. The Public Health Agency of Canada (PHAC) defines an outbreak as “an incident in which two or more persons experience similar illness after a common source exposure. An outbreak is identified through laboratory surveillance or an increase in illness that is unusual in terms of time or geography. An outbreak is confirmed through laboratory and/or epidemiological evidence.”

There are some nuanced differences between the CDC and PHAC definitions. Nevertheless, historically, the defining elements of an outbreak included having a common food, linked multiple illness, and a distinct period of time. Now, WGS is pushing back on how we describe the source of an outbreak (e.g., food vs. shared equipment, land, water, etc.) as well as the element of time. Even linked multiple illnesses span a timeframe — and potentially a geography — much wider than we’ve seen in the past. At what point do we stray so far from our conventional thinking of an “outbreak” that we call it something else?

What makes WGS extremely powerful, and what challenges the definition of an outbreak, is the ability of the technology to identify illnesses over the long haul. No longer do we need a “sudden rise” in the incidence of disease. And indeed, there are several examples of long-standing issues that have been uncovered using WGS. Additionally, WGS may untangle epidemiological investigations if the actual source is not a specific food product.

This can be illustrated through two types of “outbreak” scenarios:

• Environmental contamination of a facility. Environmental pathogens such as Listeria monocytogenes and Salmonella can lurk in manufacturing environments and contaminate food products, causing illness. They can be difficult to eradicate, meaning that food can be contaminated over a long period of time. Additionally, because it’s a facility issue, more than one food type can be contaminated. Has WGS moved us from linking an outbreak to a specific food toward a common source that could be a conveyor, water, field, harvest bin, tote, etc.? If WGS is able to show that illnesses are genetically linked, what would the role of epidemiology be if different people reported consuming different products? It’s quite possible that there would be no suspect food that could be traced. As the PHAC outbreak definition suggests, laboratory surveillance alone could be sufficient to call something an outbreak.
• “Natural” recurring contamination. Say you’re a farmer whose field happens to be below the path followed by migratory birds. Each year the same birds fly over, making sporadic deposits on your harvest. If you are growing fresh produce that lacks a kill step, someone (or maybe two, three, or more people) may become ill from this situation. For the sake of illustration, let’s say one person became ill in 2015. The next year, someone else becomes ill. But since it’s the same birds or flock, WGS shows a close genetic relatedness between the two illnesses. Does this now meet the definition of an outbreak? While this is an extreme example, it’s entirely possible — and plausible — that WGS would be able to detect this kind of occurrence.

Are we saying that illnesses related to these types of events shouldn’t be investigated? Absolutely not! It’s imperative that these issues be discovered so additional illness can be avoided. Further, public health agencies should get credit for recognizing these issues. But words matter, and in the past, calling something an outbreak elicited a focused consumer response aimed at avoiding illness. Given the potential of WGS to uncover long-standing issues that only occasionally cause illness, will consumers experience “outbreak fatigue” the way they have experienced recall fatigue? Calling linked cases outbreaks may dilute the urgency of a consumer’s response to take action.

Outbreaks that last for years even though the number of cases remain in the single digits (e.g., Blue Bell with 10 cases reported over five years, CRF Frozen Foods with nine cases over two-and-a-half years, etc.) represent situations so far from our classical definition of an outbreak that maybe we need a new term. It’s time to rethink what we mean by outbreak.