Listeria incubation period

Listeriosis is an infection caused by germs (bacteria) in the Listeria group (usually Listeria monocytogenes). Listeria bacteria occur worldwide. Most infections in adults just cause a mild flu-like illness and many people carry listeria without experiencing any illness at all. 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).

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.

• Listeria infection is rare. However, many cases are very mild and so don't get reported.
• There were 184 recorded cases of Listeria monocytogenes infection in England and Wales during 2016. That works out at about 3 people in every 1 million.
• However, listeriosis is one of the major causes of death due to food poisoning in England and Wales.
• Most of the reported cases occur in the elderly or in pregnant women.

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.

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

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 in those with weakened immune systems.

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

Symptoms of listeria in pregnancy

Listeriosis in pregnancy can cause a mild illness in the mother. However, infection during pregnancy can cause miscarriage, premature delivery, stillbirth or severe illness in a newborn child.

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.

Infection usually occurs after eating food contaminated with listeria germs (bacteria). See the 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 germs (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, magnetic resonance imaging (MRI) scan of the brain andan ultrasound scan of the heart (echocardiogram, or echo).

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 outlook. 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.