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.

Listeriosis can be diagnosed with a blood test. Further testing is only required if it's thought that the infection has spread to your nervous system. These tests may include:

• a computerised tomography (CT) scan – a series of X-rays that are put together by a computer to produce a detailed image
• a magnetic resonance imaging (MRI) scan – where a strong magnetic field and radio waves are used to produce detailed images of the inside of your body
• a lumbar puncture (see below)

A lumbar puncture, also known as a spinal tap, is used to test a sample of fluid from the spinal cord. The fluid is called cerebrospinal fluid (CSF) and surrounds and protects the brain and spinal cord.

A hollow needle is inserted into the lower part of your back to draw out a sample of CSF. If the infection has spread to your nervous system, listeria will be present in the CSF.

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. Read more about the treatment of gastroenteritis.

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 given additional ultrasound scans to assess the health of your baby.

The best way to prevent getting listeriosis is to always ensure you follow good basic food hygiene as outlined below.

• Peel raw vegetables, salads or fruit or wash them thoroughly before eating.
• Wash your hands before preparing food, before eating and after going to the toilet.
• Wash kitchen surfaces and utensils regularly, particularly after preparing raw meat, poultry and eggs.
• Always separate raw foods from ready-to-eat foods. Don't store raw meat above ready-to-eat foods because there's a risk that juice containing harmful bacteria may leak from the raw meat.
• Always cook food thoroughly and check cooking instructions carefully, including the cooking time.

For foods that are ready to eat, the most important ways of reducing the risk of listeriosis are to:

• not use food after its ‘use by’ date
• make sure the temperature of your fridge is between 0ºC and 5ºC
• follow storage instructions on food labels

Advice for ‘at risk’ groups

If you're in a high-risk group for catching listeriosis – for example, you're pregnant or you have a weakened immune system (see below), you should avoid eating foods that are known to be at risk of listeria contamination. These include:

• ripened soft cheese, such as Camembert and Brie
• blue cheese, such as a stilton
• all types of pâté, including vegetable pâté
• unpasteurised milk
• undercooked food