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Listeriosis is an infection caused by germs (bacteria) in the Listeria group (usually Listeria monocytogenes). Listeria bacteria occur worldwide. Most infections in adults are mild and many people carry listeria without experiencing any illness. However, the elderly and people with weak immune systems (such as people with cancer or AIDS) may experience a severe illness, including infection around the brain (meningitis).

Listeriosis in pregnancy can cause a mild illness in the mother. However, the infection may affect the baby during pregnancy or the baby may become infected during the birth. This can cause miscarriage, premature delivery, stillbirth or severe illness in a newborn baby.

Listeriosis is an infection caused by germs (bacteria). It usually only causes a mild flu-like illness in healthy adults and children. However, it may cause a severe illness, particularly in the elderly and in people with a weakened immune system. Infection during pregnancy can cause miscarriage, premature delivery, stillbirth or severe illness in a newborn child.

Up to 1 in 20 of the population may be carriers of the disease and do not become ill.

Once infected, it can take from 1 to over 90 days for illness to develop. This is called the incubation period. The average incubation period is about 30 days.

Most healthy adults and children who develop listeriosis have only a mild flu-like illness or infection of the gut (gastroenteritis), with diarrhoea and sometimes being sick (vomiting). Severe infection with blood poisoning (septicaemia) or infection around the brain (meningitis) may occur, especially in the elderly and those with weakened immune systems.

Infected newborn babies may show signs of infection at birth or first show signs of infection during the first few days of life. Infection in newborn babies can be very severe and can include lung infection (pneumonia) and meningitis. Infected babies may have severe difficulty with breathing and feeding.

Vets and farmers may develop a skin infection on the arms or hands after contact with infected animals.

Infection usually occurs after eating food contaminated with listeria germs (bacteria). See separate leaflet called Food Poisoning. Foods most likely to be contaminated include ready-to-eat refrigerated meals and processed foods. For example, pre-prepared ready-to-eat cooked and chilled meals, soft cheeses, cold meats, pâtés and smoked fish.

It is impossible to tell from the appearance, taste or smell whether the food is contaminated with listeria.

A pregnant mother who is infected with listeria may transmit the infection to her baby during the pregnancy or when giving birth.

Listeria can also be found in raw food, soil, vegetation and sewage.

Listeriosis can be diagnosed by taking samples which are tested for listeria bacteria in the laboratory. For example, samples of blood, urine, spinal fluid or amniotic fluid from the womb (uterus). Stool (faeces) samples are not reliable.

Other investigations will depend on the severity of infection and may include a chest X-ray, lumbar puncture, MRI scan of the brain and a scan of the heart (echocardiogram).

Many healthy adults and children with mild illness only need treatment of the symptoms. For example, medicines to treat a high temperature (fever) or increased fluid intake if you have infection of the gut (gastroenteritis).

More severe infection should be treated with antibiotics. People with severe symptoms will need to be treated in hospital.

Most cases of listeriosis in healthy adults and children are mild and only last for a short time with full recovery.

Infection in babies often has a poor outcome. There is a significant risk of the baby dying, or long-term complications and delayed development may occur in those babies who do survive. This is why it is so important to make efforts to avoid listeria when you are pregnant.

  • Keep foods for as short a time as possible and follow storage instructions, including 'use by' and 'eat by' dates.
  • Cook food thoroughly, ensuring that it is cooked through to the middle.
  • Keep cooked food away from raw food.
  • Wash salads, fruit and raw vegetables thoroughly before eating.
  • Wash hands, knives and cutting boards after handling uncooked food.
  • When heating food in a microwave oven, follow heating and standing times recommended by the manufacturer.
  • Throw away left-over reheated food. Cooked food which is not eaten immediately should be cooled as rapidly as possible and then stored in the refrigerator.
  • Pregnant women, the elderly and people with weakened immune systems should not help with lambing or touch the afterbirth.

Further reading & references

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but makes no warranty as to its accuracy. Consult a doctor or other healthcare professional for diagnosis and treatment of medical conditions. For details see our conditions.

Original Author:
Dr Colin Tidy
Current Version:
Dr Colin Tidy
Peer Reviewer:
Dr Adrian Bonsall
Document ID:
13932 (v2)
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