Listeriosis frequently asked questions
What is listeriosis?
Listeriosis is an infection caused by the bacterium Listeria monocytogenes. It is usually acquired by eating food contaminated with this bacterium. Listeriosis is a notifiable disease in Ireland.
Who is at greater risk for listeriosis?
Anyone can become ill from eating food contaminated with Listeria monocytogenes, although the disease affects primarily the following groups of people:
- Pregnant women (and their unborn children)
- Adults with weakened immune systems (e.g. persons with HIV/AIDS, cancer, diabetes)
- The elderly
What are the symptoms of listeriosis?
Infection in a healthy adult is usually without symptoms or causes a mild flu-like illness. In immunocompromised and elderly individuals the infection can occasionally spread to the central nervous system causing meningitis and/or septicaemia, with symptoms such as headache, stiff neck, confusion, and loss of balance or convulsions.
Infected pregnant women may have no symptoms or experience only a mild flu-like illness. However, infection during pregnancy can lead to premature labour, meningitis in the newborn or even miscarriage.
How is listeriosis diagnosed?
Diagnosis is usually made by culture of blood or cerebrospinal fluid (the fluid bathing the brain and spinal cord). During pregnancy, a blood test is the most reliable way to find out if symptoms are due to listeriosis.
How does a person get listeriosis?
Infection occurs mainly through eating contaminated food. The incubation period (time between initial infection and first symptoms appearing) ranges between 3 and 70 days. The average incubation period is 3 weeks. If a woman eats contaminated food during pregnancy, the infection can be passed across the placenta to the baby.
How does Listeria get into food?
Listeria monocytogenes is widespread in the environment and can be found in soil and water. Vegetables can become contaminated from the soil or from manure used as fertilizer. Animals can carry the bacterium without appearing ill, and meat or dairy products from these animals can be contaminated. Foods may also be contaminated after processing, e.g. cheese.
The foods most often associated with infection are ready-to-eat refrigerated and processed foods such as: pre-prepared cooked and chilled meals, soft cheeses, cold cuts of meat, pâtés and smoked fish. Increasingly, outbreaks of listeriosis associated with fresh produce (particularly fresh vegetables that undergo little or no heat treatment) are being reported.
How is listeriosis treated?
Listeriosis can be treated with antibiotics. However even with treatment, infection can be severe and may result in death, especially in the elderly.
How can I protect myself from listeriosis?
Protecting yourself against listeriosis is particularly important for pregnant women, infants, the elderly and those with weakened immune systems. As with other food borne illnesses, there are several measures that will help to reduce your risk of infection with Listeria monocytogenes:
- Keep foods for as short a time as possible and follow storage instructions including 'use by' and 'eat by' dates
- Cook food thoroughly, especially meat, ensuring that it is cooked through to the middle
- Keep uncooked meats separate from vegetables and from cooked and ready-to-eat foods
- Wash salads, fruit and raw vegetables thoroughly before eating, or peel if appropriate
- Wash hands, knives, and cutting boards after contact with uncooked food
- Make sure that the refrigerator is working correctly
- When heating food in a microwave, 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 avoid eating high-risk foods such as:
- raw (unpasteurised) milk or foods made from raw milk,
- soft or mould-ripened cheeses (e.g. feta, Brie, Camembert, blue-veined cheeses),
- pâté and
- smoked salmon.
- If contact with ewes at lambing time is unavoidable for pregnant women, the elderly, or people with weakened immune systems, washing of hands after handling animals should reduce any possibility of infection.
safefood has excellent information on Listeria and pregnancy on their website, and it has this advice in a number of languages.
Last reviewed: 28 August 2017