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Page last reviewed: 13/07/2011

Listeriosis is an infection that usually develops after eating food that's been contaminated by bacteria called Listeria monocytogenes, which is commonly known as listeria.

In most people, listeriosis is mild and causes flu-like symptoms or vomiting and diarrhoea (gastroenteritis). Read more about the symptoms of listeriosis.

Symptoms usually pass within three days without the need for treatment.

However, in rare cases, the infection can spread to other parts of your body and cause serious complications, such as meningitis (infection of the membranes surrounding the brain and spinal cord). Doctors describe this as invasive listeriosis.

If this is the case, you'll need to be treated with antibiotics in hospital for several weeks.

Read more about the treating listeriosis.

Where is listeria found?

The listeria bacteria have been found in a range of chilled 'ready-to-eat' foods, including:

  • pre-packed sandwiches
  • pâté
  • butter
  • soft cheeses, such as Brie or Camembert or others with a similar rind
  • soft blue cheese
  • cooked sliced meats
  • smoked salmon

Preventing listeriosis

The best way to prevent listeriosis is to ensure that you always practise good food hygiene. For example, you should:

  • not use food that's past its 'use by' date
  • follow storage instructions on food labels
  • make sure that the temperature of your fridge is between 0ºC and 5ºC
  • cook food thoroughly

If you're in a high risk group for listeriosis - for example, if you're pregnant or if you have a weakened immune system, you should avoid eating some foods, such as soft mould-ripened cheese or pâté.

Listeriosis and pregnancy

Pregnant women are at particular risk of developing listeriosis. This is because the body's natural defences against the listeria bacteria are weaker during pregnancy.

Pregnant women are almost 20 times more likely to develop listeriosis compared with the rest of the population.

A listeria infection in pregnancy doesn't usually pose a serious threat to the mother's health. However, it can cause pregnancy and birth complications, and can result in miscarriage. An estimated 22% of pregnancy-related cases of listeriosis will result in the death of the baby.


Types of listeriosis

Most listeria infections are limited to the digestive system and cause mild symptoms. Doctors call this non-invasive listeriosis. 

Invasive listeriosis occurs when the infection spreads to the blood or central nervous system, before spreading to the brain. This type of infection can be potentially fatal.

People with weakened immune systems are particularly vulnerable to invasive listeriosis. 
This includes:

  • people who are over 60 years old
  • pregnant women and their unborn babies
  • babies who are less than one month old
  • people with a health condition that weakens their immune system, such as HIV/AIDS or diabetes
  • people who are receiving medication that weakens their immune system, such as chemotherapy or radiotherapy

Read more about what causes listeriosis.

Immune system
The immune system is the body's defence system. It helps protect the body from disease, bacteria and viruses.

Page last reviewed: 13/07/2011

The time that it takes for the symptoms of listeriosis to develop after an initial listeria infection (the incubation period) can vary considerably from one to 90 days. The average incubation time is around 30 days.

Most cases of listeria infection are limited to the digestive system (non-invasive listeriosis). The symptoms include:

  • a high temperature (fever) of 38ºC (100.4ºF) or above
  • muscle pain
  • chills
  • feeling sick
  • being sick
  • diarrhoea

Invasive listeriosis

When the infection spreads into the blood (septicaemia) or the central nervous system (invasive listeriosis), the symptoms of fever, muscle pain and chills tend to be much more severe.

If the infection spreads to the nervous system and the brain, additional symptoms can include:

  • severe headache
  • stiff neck
  • changes in mental state, such as confusion
  • seizures (fits)
  • lack of physical co-ordination
  • uncontrollable shaking or twitching (tremor)

If listeriosis spreads to the brain, it can cause meningitis. This is an infection of the  membranes that surround the brain and spinal cord.

See the A-Z topic about Meningitis for more information.

Listeriosis in babies

Symptoms of listeriosis in babies can include:

  • lack of interest in feeding
  • irritability
  • seizures
  • breathing difficulties, such as rapid breathing or grunting when breathing
  • skin rash
  • a higher or lower temperature than normal

The normal body temperature for a baby is around 37ºC (98.6ºF).

Diarrhoea is the passing of frequent watery stools when you go to the toilet.
A fever is when someone's body temperature goes above the normal 37°C (98.6°F).
Jaundice is a condition that causes yellowing of the skin and the whites of the eyes. It is brought on by problems with the liver.
Pain is an unpleasant physical or emotional feeling and your body's way of warning you that it has been damaged. 
The sac-like organ of the digestive system. It helps digest food by churning it and mixing it with acids to break it down into smaller pieces.

Page last reviewed: 13/07/2011

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.


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.

Page last reviewed: 13/07/2011

Most cases of listeria infection remain in the digestive system (non-invasive listeriosis) and symptoms usually pass within three days.

Painkillers, such as paracetamol and ibuprofen, which are available over the counter from pharmacies, can be used to help relieve the symptoms of muscle pain and fever.

Diarrhoea and vomiting

If you have diarrhoea, it's very important that you drink plenty of fluids to replace those that have been lost. Read more about the treatment of diarrhoea.

If you feel that you can't eat - for example, because you feel sick or you're being sick, it shouldn't do you any harm. Make sure that you continue drinking fluids, and eat as soon as you can. Eat small, light meals. Avoid fatty, spicy or heavy foods.

Contact your GP if your symptoms don't improve within a few days.

Invasive listeriosis

Invasive listeriosis is where the infection spreads into the blood (septicaemia) or the central nervous system.

If you have invasive listeriosis, you'll be admitted to hospital so that you can be given injections of antibiotics (intravenous antibiotics) while your health is carefully monitored.

You may be given intravenous ampicillin/amoxicillin and/or gentamicin.

The length of time that you'll need to spend in hospital will depend on whether the infection has spread from your blood or nervous system to other organs, such as your brain.

Most people with invasive listeriosis require at least two weeks of treatment with intravenous antibiotics. However, in the most serious cases, at least six weeks of treatment may be needed.

Listeriosis in infants

Treatment for listeriosis in infants is the same as that for adults, although it's usually recommended that infants are kept in an intensive care unit (ICU) as a precaution.

Listeriosis in pregnancy

If you develop listeriosis during pregnancy, you'll be given antibiotics to help prevent the infection spreading to your baby. You may also be ultrasound scans given additional to assess the health of your baby.

Antibiotics are medicines used to treat infections caused by micro-organisms, usually bacteria or fungi. Examples of antibiotics include amoxicillin, streptomycin and erythromycin.
Intravenous (IV) means the injection of blood, drugs or fluids into the bloodstream through a vein.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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