Listeriosis is caused by a type of bacteria called Listeria monocytogenes (listeria). Listeria is widespread throughout the environment and can be found in:
- soil
- wood
- decaying vegetation
- water
Listeria can also be found in food manufacturing environments.
It's thought that listeria may be present in the digestive systems of many animals, such as sheep and cattle, and that these animals pass stools that are contaminated with listeria.
It's estimated that up to 1 in 20 people may be carriers of listeria, but have no symptoms of listeriosis. Human carriers can also pass stools that are contaminated with listeria which can spread if, for example, the carrier doesn't wash their hands after going to the toilet, then handles food.
Food
Most cases of listeriosis are caused by eating food that's been contaminated with listeria. Listeria is most commonly found in unpasteurised milk and dairy products that are made from unpasteurised milk. You won't be able to tell that the food is contaminated because it will look, smell and taste normal.
Listeria is often found in food manufacturing environments and can contaminate food products after production. For example, contamination can occur:
- after the food is cooked but before it is packaged
- when food is handled in shops, such as on slicing machines or delicatessen counters
- in the home
Vegetables can be contaminated if they're grown in contaminated soil or fertiliser or if they're washed in contaminated water. Meat and dairy products can become contaminated if they're taken from animals that are infected with listeria.
Unlike most other types of bacteria, listeria can survive and often multiply in temperatures below 5ºC (41ºF). Therefore, listeria can still grow to potentially harmful levels in food that's stored in a fridge.
Listeria cannot multiply in temperatures below the freezing point of 0ºC (32ºF), but freezing food doesn't necessarily kill all of the listeria bacteria.
Listeria can be removed by cooking food thoroughly or, in the case of dairy products, pasteurising it (a heat treatment that's designed to kill bacteria). You should also wash raw fruit and vegetables thoroughly before eating them.
For foods that are ready to eat, make sure your fridge is at the right temperature (between 0ºC and 5ºC), follow storage instructions on food labels and don't use food that's past its ‘use by’ date.
Read more about preventing listeriosis.
