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 milder form presents with typical gastroenteritis symptoms, called “febrile gastro-enteritis” 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).  The invasive form is the one to be concerned about as the bacterium leaves the intestines and enters the blood, liver and cerebro-spinal fluid.

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 it appears, tastes and smells normal.


SYMPTOMS

Symptoms usually appear from 1-4 weeks after eating food contaminated with Listeria monocytogenes. Some people have reported symptoms starting as early as 3 days and as late as 70 days after exposure.

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.


There are persons who are at greater risk of contracting listeriosis.  These are pregnant women, the elderly (over 65 years of age) and persons with weakened immune systems, for example, organ transplants patients, personswith HIV/AIDS, diabetes, cancer and other autoimmune diseases. Symptoms usually appear from 1-4 weeks after eating food contaminated with Listeria monocytogenes but symptoms can appear 70 days thereafter.

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 (ready to eat) foods, where possible, such as Viennas, Russians, Frankfurters and deli meat cuts before consuming them. Heating these foods to above 70 degrees C so that all parts are heated is important.

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 (ready to eat) foods mentioned above if not reheated thoroughly or if it is impractical to thoroughly heat RTE (ready to eat) foods before eating them. This is of particular importance to the susceptible sectors of the population.

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

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 and colder if at all possible.  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.

Which foods are normally associated with Listeria monocytogenes?

Globally, the foods most often implicated in listeriosis are RTE (ready to eat) 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 must be avoided. 

Manufacturers should aim to achieve the highest level of food safety to ensure our consumers are safe.
Listeria can continue to multiply in uncooked food kept in the fridge.
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.  Stick to the Use By dates for perishable foods. The NICD recommends the use of diluted bleach to clean areas in your fridge where you may have kept viennas or polony.