Listeria, what should I know?

Consumer Issues

By Guest Author on 03 April 2017

Over 80 years ago, L. monocytogenes was recognized as an animal pathogen that was widespread in nature, in soil, decaying vegetation and the bowels of many mammals.  But in 1983 the first human outbreak was reported in Canada in 1983, proving that indirect transmission from animals to humans was possible.  In that outbreak, cabbages, stored in the cold over the winter, were contaminated with Listeria through exposure to infected sheep manure.

There are six species of Listeria, but only L. monocytogenes causes disease in humans. These bacteria multiply best at 30-37 degrees Celsius, but also multiply better than all other bacteria at refrigerator temperatures. 


Update April 2016: It just got worse – our latest research gurus and friends have highlighted that there are now 17 species of Listeria. Please check out this link to the National Center for Biotechnology Information confirms.
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Thank you for your comments and keep them coming – we want FoodFocus information to always be 100% up to date and correct.
Update March 2018: The count rises to 18 species. 
"A bacterial strain isolated from a food processing drainage system in Costa Rica fulfilled the criteria as belonging to the genus Listeria, but could not be assigned to any of the known species.... Whole-genome sequence analyses based on the average nucleotide blast identity (ANI<80?%) indicated that this isolate belonged to a novel species. ... The name Listeria costaricensis sp. nov. is proposed for the novel species. " [2018]
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Update March 2019: Kaarin Goodburn MBE, of the Chilled Food Association has advised us that we are now up to 19 species of Listeria.
According to the National Center for Biotechnology Information "During a screening of Listeria species in food samples in Thailand, a Listeria-like bacterium was recovered from fried chicken and could not be assigned to any known species.... The name Listeria thailandensis sp. nov. is proposed for the novel species.". [2019]
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Foods associated with the spread of Listeria 

Certain foods are commonly associated with Listeria outbreaks, these include:

  • unpackaged ready-to-eat cold meats and packaged sliced ready-to-eat cold meats (such as those from delicatessen counters)
  • cold cooked ready-to-eat chicken purchased whole, in portions or diced
  • refrigerated pâté or meat spreads
  • pre-prepared or pre-packaged fruit or vegetable salads (for example salad bars)
  • chilled seafood, including raw seafood, for example oysters, sashimi or sushi
  • smoked ready-to-eat seafood
  • cooked peeled prawns (such as found in prawn cocktails, sandwich fillings and prawn salads)
  • soft, semi-soft and surface ripened cheeses such as brie, camembert, ricotta, feta and blue (both pre-packaged or from the delicatessen)
  • soft serve ice cream
  • other unpasteurized dairy products


When a person is, infected and develops symptoms of Listeria infection, the resulting illness is called listeriosis. Only a small percentage of persons who ingest Listeria fall ill or develop symptoms.


Symptoms of Listeria:

Listeria symptoms appear anywhere between 3 to 70 days after ingesting the foodstuff contaminated with Listeria monocytogenes, but usually around 21 days after the day of ingestion.


Typical symptoms include:

  • fever
  • muscle aches
  • nausea or diarrhea (less common)

 If infection spreads to the central nervous system, then symptoms include:

  • headache
  • stiff neck
  • confusion
  • loss of balance
  • convulsions

 For patients with a weak immune system, Listeria bacteria can invade the central nervous system and cause meningitis or a brain infection.


Infected pregnant woman experience mild, flu-like symptoms. However, infection during pregnancy can lead to miscarriage, infection of the newborn, or stillbirth. In some cases the Listeria infection is carried over into the fetus and symptoms can also appear in newborns in the first week of life, but can also occur later on. A newborn’s Listeria symptoms are often subtle, and include irritability, fever, and poor feeding.


Identifying Listeriosis

Doctors can only determine if a patient has listeriosis by taking a blood or spinal fluid sample. The typical method of identifying the bacteria cannot be done using a stool sample.


Who is at risk?


The following are most at risk:

  • The elderly
  • Pregnant women
  • Persons with weakened immune systems

 Persons with the following illnesses are also high risk:

  • HIV/AIDS or other autoimmune diseases
  • Cancer 
  • End-stage renal disease
  • Liver disease
  • Alcoholism
  • Diabetes

 If you are ill, displaying a fever or stiff neck, do not hesitate and consult your doctor immediately. By taking the correct prescribed antibiotics the infection can be cured and in pregnant women antibiotics will prevent the infection of the fetus.


How to prevent Listeria infection

As an individual, you can do the following to prevent infection by Listeria:

  • Do not drink raw (unpasteurized) milk, and do not eat foods that have unpasteurized milk in them.
  • Wash hands, knives, countertops, and cutting boards after handling and preparing uncooked foods.
  • Rinse raw produce thoroughly under running tap water before eating.
  • Keep uncooked meats, poultry, and seafood separate from vegetables, fruits, cooked foods, and ready-to-eat foods.
  • Thoroughly cook raw food from animal sources, such as meat, poultry, or seafood to a safe internal temperature.
  • Wash hands, knives, countertops, and cutting boards after handling and preparing uncooked foods.
  • Consume perishable and ready-to-eat foods as soon as possible.
  • Persons in higher risk groups should heat hot dogs, cold cuts, and deli meats before eating them.


The responsibility of the food industry


In summary, food manufacturers and food handlers have a responsibility to ensure a Listeria outbreak does not occur by applying the correct cleaning and sanitation protocols.


Microbiological testing of products for consumption for listeria and the rejection of contaminated food stuffs are crucial. Strict management and rigorous checking for this bacterium both in the process and in the final product is necessary since Listeria:


  • grows at such low temperatures
  • has a widespread range of sources from the environment,
  • symptoms may only present themselves long after ingestion


Listeria is a very real risk, and the food industry and the consumer need to be aware of these risks, and take the necessary precautions.



Michele Pietersen