Updated on 25 February 2018

What is Listeriosis?

Listeriosis refers to the invasive form of illness caused by the bacterium called Listeria monocytogenes.  This form of the illness is the more severe of two types of illnesses caused by this organism.  The other milder form presents with typical gastroenteritis symptoms i.e. fever and diarrhoea.  This latter form is usually not serious and most people heal quickly.  However, the invasive form is far more serious causing 20-25% mortality (deaths). 

Where does the organism come from?

Listeria monocytogenes is naturally present in the environment where crops are grown and animals are reared. It is present in the soil, decaying vegetation, rivers etc. It can therefore realistically be found in raw meats and crops in low numbers, which are generally, not a problem. It is when higher numbers are found or the organism grows to infectious levels and the food is eaten without a "Listeria kill step" (technical term: listericidal step) or it has been re-contaminated after a listericidal step and then consumed, that illness can occur.  The susceptible sectors of the population are more at risk of contracting listeriosis (see below).

How does one contract Listeriosis?

The illness listeriosis is contracted when a person consumes food that is contaminated (infected) with sufficient levels of the organism Listeria monocytogenes.  An illness that is transmitted via food in this way is commonly known as a "foodborne illness" and the causal organism as a "foodborne pathogen".  Unfortunately, one will not know if the food is contaminated as the food appears, tastes and smells normal. 

Who can contract Listeriosis?

Everyone can contract the illness, but there are persons who are at greater risk.  These are pregnant women (20 times more likely to contract listeriosis) and their foetuses, the elderly (over 65 years of age) and persons with weakened immune systems, for example, undernourished people, those who have had organ transplants, those with HIV/AIDS, diabetes, cancer and other autoimmune diseases. 

How does Listeriosis affect pregnant women and unborn babies?

Pregnant women typically experience only fever and other flu-like symptoms, such as fatigue and muscle aches.  However, infections during pregnancy can lead to miscarriage, stillbirth, premature delivery or other life-threatening infection of the newborn, such as meningitis. The bacterium is transferred to the unborn baby via the placenta and the blood system.

Can Listeriosis be transmitted to a new born baby via breast milk?

The organism cannot be transmitted via breast milk to new born babies.

How does Listeriosis affect other vulnerable people?

Symptoms can include headache, stiff neck, confusion, loss of balance and convulsions in addition to fever and muscle aches.  In Canada in 2008, elderly people in retirement homes were infected with listeriosis via contaminated deli meats and 22 people died.

Can listeriosis be transmitted from person to person?

Listeriosis is not transmitted from person to person (like influenza for instance), with the exception of pregnant woman to baby transmission via the placenta and the blood system.

How long does it take for symptoms to show?

Symptoms usually appear from 1-4 weeks after eating food contaminated with Listeria monocytogenes. Some people have reported symptoms starting as late as 70 days after exposure or as early as the same day of exposure (although the latter is very rare). 

Which foods are normally associated with Listeria monocytogenes?

Globally, the foods most often implicated in listeriosis are Ready-To-Eat (RTE) foods such as:

  • Ready-to-eat deli meats (ham, polonies, other similar cold meats) and hot dogs
  • Refrigerated pates or meat spreads
  • Unpasteurized (raw) milk and dairy products
  • Soft cheese made with unpasteurized milk, such as queso fresco, Feta, Brie, Camembert
  • Refrigerated smoked seafood
  • Pre-packaged salads
  • Raw sprouts

There have been cases attributed to fresh cut melons (cantaloupes) and ice cream in the USA as well.  At food processing level, contamination of any food after a step that kills Listeria (listericidal step) must be avoided. 

What can I do to prevent contracting Listeriosis?

Vulnerable sectors of the population (such as pregnant women and other groups mentioned above) should take special precautions:

  • Cook food thoroughly before eating it, especially all meats and fish
  • Thoroughly heat RTE foods, where possible, such as hot dogs and deli meat cuts before consuming them
  • Do not drink unpasteurized (raw) milk
  • Consume only soft cheeses made from pasteurized milk
  • Wash all fruit and vegetables thoroughly before consumption
  • Practice good hygiene in the kitchen - wash hands, working surfaces and utensils thoroughly between handling raw and cooked food - do not cross-contaminate
  • Avoid consuming any of the RTE foods mentioned above if not reheated thoroughly or if it is impractical to thoroughly heat RTE foods before eating them. This is of particular importance to the susceptible sectors of the population.

Does cooking kill Listeria?

Thorough cooking of food to the core, does kill Listeria.  Temperatures used for pasteurization of milk also kill Listeria.

What about refrigeration of food?

Listeria is unlike most other bacteria in that it can grow at refrigeration temperatures.  It is important to monitor the temperature of your refrigerator.  It should be below 4 degrees C.  Perishable and Ready-To-Eat foods that are refrigerated should also be eaten as soon as possible.  Stick to the Use By dates for perishable foods.

 Copyright 2017 - Dr LE Anelich