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What is listeriosis?

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

In most people, listeriosis is mild and causes symptoms including a high temperature (fever), vomiting and diarrhoea. These symptoms usually pass within three days without the need for treatment.

However, in rare cases, the infection can be more severe and spread to other parts of your body, causing serious complications, such as meningitis. Common signs of severe listeriosis include a stiff neck, a severe headache and tremors.

Pregnant women are at an increased risk of developing listeriosis.

Read more about the symptoms of 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

Read more about what causes listeriosis.

Seeking medical help

If you are pregnant and show signs of listeriosis, or if you have a young child who shows signs of the illness, you should seek immediate medical advice.

If you are not pregnant and are an otherwise healthy adult, you should seek medical help if your symptoms are severe.

Listeriosis is usually diagnosed with a blood test. If it is thought that the infection has spread to the nervous system, further tests may include a MRI scan and a lumbar puncture.

Mild cases of listeriosis usually do not need treatment. However, if the infection has spread to the nervous system, you'll need to be treated with antibiotics in hospital for several weeks.

Read more about the treating listeriosis.

Preventing listeriosis

The best way to reduce your chances of developing 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.

Read more about preventing listeriosis.

'At-risk' groups

Some people are particularly vulnerable to severe listeriosis.
This includes:

  • people over 60 years of age
  • pregnant women and their unborn babies
  • babies who are less than one month old
  • people with a weakened immune system, such as those with HIV/AIDS or those receiving some types of medication such as chemotherapy

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.

Listeriosis symptoms

Symptoms of listeriosis in most healthy adults are mild. They usually develop from 3-70 days after the initial infection.

They are similar to flu and gastroenteritis, including:

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

These symptoms will usually pass within a few days, even without treatment.

Severe listeriosis

If 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.

Listeriosis in infants

Symptoms of listeriosis in infants 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). For more information, see [what is a high temperature in children?]

When to seek medical help

You should seek immediate medical help if:

  • You show signs of severe listeriosis
  • Your child shows signs of listeriosis
  • You are pregnant with a fever and chills

If you need help outside normal surgery hours, you can use your local out-of-hours service.

Causes of listeriosis

Listeriosis is caused by a type of bacteria called Listeria monocytogenes (listeria). It is mainly spread through contaminated food.

Listeria is widespread throughout the environment and can be found in soil, wood, decaying vegetation and water.

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 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 contaminated with listeria which can spread if, for example, the carrier doesn't wash their hands after going to the toilet, then handles food.

Contaminated food

Most cases of listeriosis are caused by eating food contaminated with listeria. Listeria is most commonly found in unpasteurised milk and dairy products made from unpasteurised milk.

Listeria can also be 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 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 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 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.

Treating listeriosis

Most listeria infections don't need specific treatment, as the symptoms usually pass within three days.

Over the counter painkillers, such as paracetamol and ibuprofen, can offer some relief for muscle pain and fever if you need it.

Diarrhoea and vomiting advice

If you have diarrhoea, it's very important that you drink plenty of fluids to replace those that have been lost. There are also several medications available, but these are rarely necessary. Read more about the treatment of diarrhoea.

If you've been vomiting or feeling sick, it should be fine to avoid eating for a short while. However, make sure that you continue drinking fluids, and eat as soon as you can. Eat small, light meals and avoid fatty or spicy foods.

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

Severe listeriosis

If listeriosis spreads into the blood (septicaemia) or the central nervous system, you'll be admitted to hospital so that you can be given injections of antibiotics (intravenous antibiotics) while your health is carefully monitored.

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 severe 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.

Preventing listeriosis

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

This includes:

  • Peeling raw vegetables, salads or fruit or washing them thoroughly before eating.
  • Washing your hands before preparing food, before eating and after going to the toilet.
  • Washing kitchen surfaces and utensils regularly, particularly after preparing raw meat, poultry and eggs.
  • Always separating 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 cooking food thoroughly and checking 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 0-5ºC
  • follow storage instructions on food labels

Advice for ‘at risk’ groups

If you are in a high-risk group for catching listeriosis - for example, if you are pregnant or you have a weakened immune system, you should avoid eating foods that are known to be at risk of listeria contamination.

Foods to avoid include:

  • soft mould-ripened cheese, such as Brie, Camembert and chèvre (a type of goat's cheese)
  • soft blue-veined cheese, such as Danish blue and gorgonzola
  • all types of pâté, including vegetable pâté
  • unpasteurised milk
  • undercooked food

It is safe to eat hard blue-veined cheese during pregnancy, such as Stilton, as well as other types of hard cheese, including Cheddar and Parmesan - even if these are made from unpasteurised milk.

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