What is Listeria?

Learn about the causes and symptoms of listeriosis, and how to prevent food-borne Listeria infections.
February 19, 2020

Listeria is a type of bacteria. There are many strains of Listeria, but only one of them, Listeria monocytogenes, is known to cause food-borne illness.

The term for the illness caused by infection with Listeria monocytogenes is listeriosis. According to the Public Health Agency of Canada (PHAC), Listeria is the leading cause of deaths related to food-borne illness each year in Canada.

Symptoms of Listeria infection

Symptoms of Listeria infection (‘listeriosis’) include fever, nausea, cramps, diarrhea, vomiting, headache, constipation and muscle aches.

In extreme cases, Listeria monocytogenes can spread to the nervous system and cause a brain infection or blood poisoning. Severe listeriosis can be fatal.

Symptoms of severe listeriosis may include confusion, headache, loss of balance and/or a stiff neck. These symptoms may appear up to 70 days after exposure to Listeria.

Who is most at risk from Listeria?

Listeria can cause illness in healthy people, but it is especially dangerous for vulnerable persons. Vulnerable persons are:

  • people with weakened immune systems
  • children under 5
  • adults over the age of 60
  • pregnant women

If Listeria spreads from a mother to her unborn baby, it can cause a miscarriage, stillbirth, premature birth or a life-threatening illness in the baby after birth.

Even if the mother experiences minimal or no symptoms, listeriosis can be fatal for her unborn baby or newborn.

What causes listeriosis?

Listeriosis is usually caused by eating food contaminated with high doses of Listeria monocytogenes.

The types of food that are most likely to become contaminated with Listeria include:

  • seafood (e.g. refrigerated smoked fish)
  • meat (e.g. pâté, deli meat, hot dogs)
  • raw fruits and vegetables (e.g. sprouts, romaine lettuce, cantaloupe)
  • unpasteurized dairy products and soft cheeses made with unpasteurized milk (e.g. queso fresco, feta, Brie)

Listeria can be spread between foods if proper food handling practices are not followed. Once Listeria is in an environment, it can be difficult to remove.

How to prevent Listeria

Listeria is an extremely resilient bacterium. Unlike most bacteria, Listeria can survive and sometimes grow on foods stored in the refrigerator. 

Cooking food properly and maintaining a high standard of hygiene is your best defence against Listeria and the serious consequences of causing a food-borne infection.

To minimize Listeria risks, be sure to do the following (or ensure the following are done in the facility):

  • Cook food to a safe internal temperature.
  • Use a food thermometer to confirm that the coldest part of the food has reached the required temperature. (Learn how to use and calibrate a probe thermometer).
  • Wash your hands with soap and warm water for at least 15–20 seconds before and after you touch raw meat, after using the toilet and any other time when hands are likely to have become contaminated. 
  • Keep raw and cooked or ready-to-eat foods separate and never reuse equipment or utensils that have touched raw food without cleaning and sanitizing them first.
  • Keep food out of the Temperature Danger Zone, which is the temperature range in which bacteria grow fastest.

It’s important to remember that food that is contaminated with Listeria will look, smell and taste normal, which is why the only way to prevent infections is through safe food handling practices and good hygiene.

The best way to ensure safe food handling and hygiene in a food premises is with food safety training and certification. In most provinces and territories in Canada, food handling certification is regulated and mandatory for a number of your staff.

To learn more about food handling certification, contact the Canadian Institute of Food Safety.