Investigating outbreaks of foodborne illness

You wake up and something’s not right. “Ugggh, my stomach is off!” Thirty seconds later, you’re running to the toilet.

Perhaps you didn’t cook the chicken enough the other night. Or maybe the fish taco food truck wasn’t the best idea for lunch this week. Either way, you may have symptoms of food poisoning, also known as foodborne illness.

While individual cases of foodborne illness are common in Canada, actual outbreaks – two or more cases linked by a common exposure within a set time frame – are more rare. Here’s a look at how the Public Health Agency of Canada (the Agency) works with its food safety partners to track and respond to large outbreaks caused by bacteria such as Salmonella, Listeria or E. coli.

Outbreak response – the big picture

Canada is fortunate to enjoy one of the safest food systems in the world, but foodborne illness outbreaks happen from time to time.

The Agency works with all the provinces and territories to monitor for a rise in cases of foodborne illness above regular levels. If a number of people appear to have the same illness in a given period and area, it’s called a cluster. When an investigation shows that persons in a cluster have food in common to explain why they all got the same illness, it’s called an outbreak.

Outbreak investigations can be led by local, provincial, territorial or federal health authorities depending on how widespread the illnesses are. In the event of a national outbreak, the Agency leads the response and coordinates with the Canadian Food Inspection Agency, Health Canada, and the provinces and territories with cases of illness. The Agency also provides laboratory support to all regions of the country.

Tracking the source

Responding to an outbreak is like putting together the pieces of a puzzle, except you don't know how many pieces the puzzle has, you don't know where to find them, and you don't know what the final picture will look like.

Lab expert testing bacteria samples
Lab expert testing bacteria samples

To investigate national outbreaks, lab experts test bacteria from people who got sick to find out if the bacteria are the same ones causing illness in other parts of Canada. Epidemiologists at the Agency identify and monitor lab-confirmed cases of illnesses and contact people who got sick. They then ask those people what they did, where they went and what they ate before they got ill. When many people mention eating the same food before they got ill, the epidemiologists work with food safety experts to track down where the food came from and to collect samples of the food for laboratory testing.

The lab experts then compare the results of the bacteria found in contaminated foods to the bacteria from people who got sick to see if they match. Matching bacteria means the investigators may have identified the source of the outbreak. If the source of the outbreak is found to be food, the product is removed from store shelves.

It can take several weeks from the time a person gets sick to the time a food source is identified, well after the person may feel better. Often people can’t remember what they ate, or the contaminated food is no longer available to test, so sometimes the source of the outbreak is never found.

The lab: genetics of bacteria

Just as people are genetically different from each other, so are bacteria. The Agency’s National Microbiology Laboratory (NML) – Canada's national infectious disease laboratory – has been using a method called pulsed field gel electrophoresis (PFGE), to get a "genetic fingerprint" for bacteria specimens from the people who got sick.

 Matthew Gilmour, Scientific Director General of the NML and Laboratory of Foodborne Zoonoses at the Agency

“First we cut the pathogen’s DNA into several pieces and place them in a gel,” says Matthew Gilmour, Scientific Director General of the NML and Laboratory of Foodborne Zoonosesat the Agency. “Then we run electricity through the gel, which causes the DNA fragments to separate based upon their lengths. The DNA fragments look like a bar code and represent the PFGE fingerprint. If different samples have identical fingerprints, then the infections may have been caused by the same food. If we then run the same test on bacterial pathogens from a suspect food sample and get the same fingerprint, we may have found the source of the outbreak.”

Dr. Celine Nadon, Chief of the NML's Enteric Diseases section adds, “While PFGE fingerprinting has been helping to quickly solve outbreaks for more than a decade, we are now also using ‘whole genome sequencing’, a technology that does this even faster and more accurately. Whole genome sequencing determines the entire genetic code of the bacteria, which gives us a lot more information to use in linking cases of illness to each other and a food source. We’ve been using it in all of our outbreak investigations in the past two years, and ultimately it will replace PFGE fingerprinting altogether."

Tips to protect yourself

If you’re reading this because you think you might have food poisoning, you may want to consider seeing your physician for further assessment. In the meantime, here are some key tips to avoid getting sick again.

Visit the Government of Canada’s Food Safety Portal or the Agency’s website for more information on food safety and how to protect yourself from foodborne illness. You can also download the free Recalls and Safety Alerts app, which sends up-to-date and reliable health and safety information right to your mobile phone.

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