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Listeria – What You Should Know

What is Listeria?

monocytogenes is an environmental pathogen that can contaminate foods and cause a mild, non-invasive illness (listerial gastroenteritis) or a severe, invasive illness (listeriosis). Listeriosis is characterized by a relatively high mortality rate compared to illnesses caused by most other foodborne pathogens (~20% compared to <1 % for Salmonella or E. coli O157), but can be treated with antibiotics.

  1. monocytogenes is found in soil, water, sewage, and decaying vegetation. It can be readily isolated from humans, domestic animals, raw agricultural commodities, and food packing and processing environments, in particular cool damp areas. Food safety concerns arise as L. monocytogenes can survive for long periods under adverse environmental conditions. It tolerates high salt concentrations such as non-chlorinated brine chiller solutions, it is able to survive acidic conditions in addition to surviving in frozen conditions for extended periods and growing at refrigeration temperatures. Listeria Monocytogenes can be killed through pasteurization and cooking at high temperatures.


Who is at risk?
Though anyone is susceptible, persons who have the greatest risk of experiencing listeriosis due to consumption of foods contaminated with L. monocytogenes are:

  • Pregnant women and their fetuses;
  • The elderly;
  • Persons with weakened immune systems; and
  • Organ transplant patients who are receiving drugs to prevent the body from rejecting the organ.

Incubation Period

  • 3-70 days


  • Fever, stiff neck, confusion, weakness, vomiting, sometimes preceded by diarrhea

Duration of Illness

  • Days to weeks

Common Sources

  • Ready-to-eat foods
  • Raw further processed meats, sausages, hot dogs, deli meats
  • Raw Vegetables
  • Refrigerated pâtés or meat spreads
  • Unpasteurized (raw) milk and dairy products such as ice cream
  • Soft cheese made with unpasteurized milk, such as Feta, Brie, Camembert
  • Refrigerated smoked seafood


Tips for the Consumer

Food producers and manufacturers are extremely vigilant and follow the highest of standards set to them in order to prevent contamination. To further assist in prevention there are a few guidelines (below) that consumers can follow:

  1. Do not drink raw (unpasteurized) milk, and do not eat foods that have unpasteurized milk in them.
  2. Wash hands, knives, countertops, and cutting boards after handling and preparing uncooked foods.
  3. Rinse raw produce thoroughly under running tap water before eating. Should you feel the need to take extra precautions, a vinegar solution of 1 tablespoon vinegar to 3,7 litres of water would be suitable.
  4. Keep uncooked meats, poultry, and seafood separate from vegetables, fruits, cooked foods, and ready-to-eat foods.
  5. If using the same utensils and work surface for the preparation of vegetables, fruits and uncooked meats ensure that they are washed thoroughly in between the preparation of each.
  6. Thoroughly cook raw food from animal sources, such as meat, poultry, or seafood to a safe internal temperature.
  7. Sanitize worktops and utensils and hands where possible
  8. Consume perishable and ready-to-eat foods as soon as possible.
  9. Persons in higher risk groups should heat hot dogs, and avoid cold cuts, and deli meats before eating them.
  10. Observe “Best before” and “Sell by” date of all products, especially perishables.
  11. Launder dishcloths when dirty an at least daily


Quick Summary

  1. Listeriosis is not a viral but a bacterial infection;
  2. Infection can take between 3 to 70 days to develop and last from days to weeks in severe cases;
  3. Some people are more susceptible to Listeriosis, while others can be carriers of the disease;
  4. Storing food in the fridge does not kill L. Monocytogenes;
  5. Monocytogenes is not limited to a single food source and can in fact be found in soil;
  6. Good food safety practices at home is important;
  7. Symptoms include: Fever, stiff neck, confusion, weakness, vomiting, sometimes preceded by diarrhea;
  8. Severe cases of Listeriosis can be treated with antibiotics;
  9. Listeriosis can lead to death in severe cases; and
  10. Listeriosis can be prevented.



  • Riedo FX, Pinner RW, Tosca ML, Cartter ML, Graves LM, Reeves MW, Weaver RE, Plikaytis BD, Broome CV. A point-source foodborne listeriosis outbreak: documented incubation period and possible mild illness. The Journal of infectious diseases. 1994;170(3):693-6.
  • CDC – February 27, 2012
  • Benjamin Silk’s key information on listeriosis in the aftermath of the large 2011 outbreak
  • Painter J & Slutsker L. Listeriosis in humans. In: E. T. Ryser & E. H. Marth., editor. Listeria, Listeriosis and Food Safety 3rd ed Boca Raton, Florida: Taylor and Francis Group; 2007. p. 85-110.
  • Tompkin RB. Control of Listeria monocytogenes in the food-processing environment. Journal of food protection. 2002;65(4):709-25.
  • Guidance for the Control of Listeria monocytogenes in Ready-To-Eat Foods, Part 4: Corrective Actions, July 2011, Ministry of Agriculture and Forestry: New Zealand

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