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Tracking the source of contamination in a food incident: how to find the smoking gun?

Published: 4 Mar 2013

By Dr Craig Shadbolt

© Lawmedia Pty Ltd, March 2013


 Dr Craig Shadbolt is our guest columnist this month. Dr Shadbolt is the Manager of the New South Wales Food Authority’s Foodborne Illness Investigation Unit.

Australia is fortunate to have access to a safe and wholesome food supply. However, the recent outbreak of listeriosis linked to the consumption of Jindi brand cheeses in Australia has highlighted the importance of food safety, and the consequences of what can happen if pathogenic bacteria find their way into foods.

But just how easy is it to attribute a particular food or company as the source of a food poisoning outbreak? What are the processes and tools at the disposals of outbreak investigators, who are often faced with significant time pressures to find and control the source of disease? In practice, outbreak investigations rely on highly specialised teams involving epidemiologists, laboratories, and food safety regulators who can act on, and react to, various pieces of information as they become available.

The role of the epidemiologist

Foodborne disease epidemiologists are typically the first people to detect an outbreak. In Australia, key foodborne bacteria such as Listeria monocytogenes and Salmonella are referred to as ‘notifiable’ pathogens. When these bacteria have been isolated from a patient (in a hospital or pathology provider) there are systems in place within state/territory jurisdictions and the Commonwealth for rapid notification to health departments. These notifications are carefully scrutinised for increases or clusters of detections above what may be considered as normal “background” levels. An unexpected spike will trigger further analysis and interviews of patients to determine any common threads to their illness.

Patients will be questioned around foods consumed in the last few days, including information on brand names and places of purchase or consumption. Epidemiologists look for patterns or trends in behaviour and consumption habits to search for a common link (such as a commonly distributed product) or food-source (a restaurant or wedding function). In the most recent outbreak associated with Jindi cheeses, epidemiologists were on high alert due to the increasing number of listeriosis cases notified across several Australian states.

While epidemiology may sound like a straight forward concept, in practice there are numerous challenges to be overcome. One of these is time, specifically the time taken for a person to eat, get sick, go to a doctor, have a sample collected and analysed, and the notification to a health department. Pathogens like L. monocytogenes typically have an incubation period of 1-2 weeks. Stated more plainly, it can take 2 weeks from the time a person consumed a food before any sign of illness becomes apparent. How many people can accurately say what foods they ate and where over such a long duration? Unless the food or place of consumption is very unique or specific, several people can fall ill before the smoking gun is found.

Fortunately there are other tools at the disposal of epidemiologists which can make their life considerably easier. One of these tools is the increasing sophistication and accessibility of DNA profiling.

The laboratory

In any outbreak investigation the laboratory is a key player. Specialist labs will analyse everything from human faeces or blood, a swab of a factory floor or meat slicer, right down to food samples obtained from a supermarket or someone’s fridge at home. Different samples and pathogens pose unique challenges. Some pathogens will not grow in the presence of oxygen and, as a result, require special anaerobic chambers in order to be identified. For certain food samples, particularly fresh produce such as fruit or lettuce, a pathogen like Salmonella may coexist with other non-harmful, soil-based organisms. As a consequence the food laboratory will need to employ specialist techniques and materials to detect the few cells of Salmonella that may be hiding in nooks and crannies.

These challenges are not new and will always be present. However, one of the greatest advances at the laboratory level that is assisting epidemiologists and food safety regulators alike is the wider use of molecular DNA profiling or “fingerprinting”. Just like humans, not all populations of L. monocytogenes or Salmonella are alike. Pathogenic bacteria will undergo subtle changes in their DNA fingerprint with time. By comparing the DNA fingerprint patterns of Salmonella from two infected individuals, for example, it is possible to determine whether these people may have been exposed to the same food or source. Similarly, it is also possible to compare these DNA fingerprints and match this up with Salmonella found on factory surfaces or from a specific food. For the epidemiologist, bacterial DNA fingerprinting makes it far easier to link cases of illness to a common source and hone in on the likely food. For the food safety regulator, matching Salmonella DNA patterns isolated from human cases to that found in a food or business provides strong evidence of the source of contamination.

The food safety regulator

Once a food commodity or event has been identified as a likely outbreak vehicle, the challenge for the food safety regulator is to trace the contamination to a source and ensure that corrective action is taken to prevent further illness. This can be a time consuming process depending on the level of evidence available. Questions to consider include:

-          how commonly consumed and how widely available is the product (e.g. chicken)?

-          is there sufficient traceability in the supply chain to implicate a single business (difficult with fresh produce which is often co-mingled at the wholesale level)?

-          what are the contributing factors through the supply chain likely to have caused the illness?

For a widely consumed product it is logical to commence early dialogue with major companies and peak industry bodies to assist with mitigation of the outbreak and determination of a common source. Tracing an outbreak to a specific factory or farm can often involve a lengthy paper trail, depending on the number of companies that may be involved in distributing the suspect food, and how this food has been handled or processed through the supply chain.

Where outbreaks result in people affected across several states, it is highly unlikely that multiple failures suddenly occurred at different places of purchase (such as a supermarket). A more logical explanation is a process failure at the place of manufacture or central distribution. Previous examples of this have included meat slicers contaminated with L. monocytogenes, or Salmonella-laden water used to wash rockmelons. Understanding how a product is manufactured and handled at every point in the supply chain is therefore a critical step for the food safety regulator.


While there are three distinct areas of expertise that are required for finding the ‘smoking gun’ in a food contamination event, these should never work in isolation. An outbreak investigation is rarely a linear sequence of events, from the time a cluster of illness is detected to when the eventual source of contamination is discovered and mitigated. In practice, foodborne disease epidemiologists, laboratory staff, and food safety regulators work together and exchange information on a regular basis in any successful outbreak investigation to trace a contamination event to a final source.