Taking the pulse of Listeria

In 2008, Canada saw one of its largest foodborne illness outbreaks in recent times. What was the cause? Listeria.

Dr. Franco Pagotto, Research Scientist and Co-Director of the Listeriosis Reference Service for Canada, recalls: “We worked closely with Public Health Ontario to screen for the disease, with the Canadian Food Inspection Agency to test the sanitizers used to clean the facility, and with the Public Health Agency of Canada who led the genome sequencing of the pathogen.” Unfortunately, during that time, a second foodborne illness outbreak took place. Health Canada researchers looked at the pathogens’ strains from each outbreak to distinguish their respective causes. In the end, numerous best practices were implemented to avoid another outbreak.

But… what exactly is Listeria?

The Listeria bacteria

Listeria is found in soil, sewage and untreated water everywhere. It can also be found in foods such as fish, meat, seafood, vegetables and fruits, and dairy products. Listeriosis is a foodborne illness that can spread when eating food or drinking water that has been contaminated, from contact with infected animals, or mom to baby during pregnancy. Despite the various ways the disease can spread, only about 130 cases have been reported annually in Canada in recent years. This is in part due to Dr. Pagotto’s work at the Listeriosis Reference Service.

The laboratory, which supports outbreak investigations and helps pinpoint their source, uses molecular epidemiology. Think of it as taking a pathogen’s fingerprint. As all pathogens are unique, researchers can match this print with those of contaminated individuals. If the prints match, they can find common elements between the individuals and track the cause of the outbreak. For example, if a dozen individuals that share the same pathogen print ate the same food product, we can assume that this product is likely the cause of the outbreak. This can be confirmed in the lab. These results are shared via PulseNet Canada, a Canadian database that includes the public health laboratories of all ten provinces, the Public Health Agency of Canada, and Health Canada. The data are also shared internationally to further allow the tracking of pathogens on a worldwide scale.

However, it’s not that simple. While most pathogens have an incubation of about 4 to 20 hours, symptoms of Listeria may appear up to 70 days after you have been exposed, making it particularly difficult to narrow down the source of the outbreak. Dr. Pagotto explains: “We try to figure out why a food product is contaminated by asking ourselves three questions. Is it the product itself? Is it how it’s manufactured? Or is it how it’s stored?” This means looking at where a food was grown, how Listeria contaminated it and how the pathogen persisted. By answering these questions, Dr. Pagotto can get a better understanding of the pathogen and help inform policy and guidelines, which help protect the health and safety of people in Canada.

What about us?

Is Listeria still an issue today? Very much so, says Dr. Pagotto. While Listeria can usually be found in deli meats, unpasteurized cheese and smoked salmon, it has recently been found in frozen vegetable products. Unlike most bacteria, Listeria can survive and sometimes grow on foods stored in the refrigerator. As such, cooking food properly and washing your hands often are some of the best ways to reduce your risk. People with a weakened immune system, those over the age of 60, and pregnant individuals are particularly at risk, and should also avoid eating certain foods. If you think you may have been exposed to Listeria, contact your health care provider. Take notes about your food consumption patterns, including types and brands of food, as well as travel and cannabis use. In the meantime, Dr. Pagotto continues his diligent work on Listeria: “We need to remain vigilant. Things are safe, and I strive to keep them that way and make them even safer.”