Listeriosis is a relatively uncommon disease caused by the bacteria Listeria monocytogenes. Listeria is widespread in nature and is commonly found in soil, water, mud, vegetation and sewage. It can also be found in raw meat, raw vegetables and unpasteurised dairy products. Some exposure to these bacteria is unavoidable. Every day, most healthy people eat foods that contain small amounts of Listeria with no apparent ill effects.
Listeriosis is of particular concern to pregnant women and people with a weakened immune system, such as diabetics, cancer and transplant patients, people who are HIV positive, and people with a history of alcohol abuse, as these people are at increased risk of contracting the disease.
Although the infection may cause minor or no symptoms in healthy people, including pregnant women, the risk of infection from a pregnant woman to her unborn child is high. Infection during pregnancy may lead to miscarriage, stillbirth, premature birth or a very ill newborn. The death rate in infected newborn babies is 20-30%.
Healthy people may show few or no symptoms. If symptoms are present, they may vary from minor complaints such as fever, headache, aches and pains, vomiting and diarrhoea to more serious forms of the illness such as meningitis (infection of the lining of the brain) and septicaemia (blood poisoning).
Listeriosis is mainly spread by eating contaminated food. The bacteria can also be transmitted from a pregnant mother via the placenta to an unborn baby or to the baby at the time of birth.
The average time from exposure to developing of the disease is about three weeks.
Listeriosis can be successfully treated with antibiotics if treatment is commenced early. However, newborn infants have a high mortality rate even with antibiotic treatment.
Pregnant women and other susceptible people are advised to avoid high risk foods.
What are high risk foods?
High risk foods are usually chilled ready-to-eat foods including:
- pate, uncooked smoked seafood, soft cheeses (eg. Brie, camembert, ricotta)
- cooked diced chicken (as used in chicken sandwiches)
- cold meat products (eg. cold roast meat, processed meats)
- pre-prepared and stored salads, raw seafood (eg. oysters)
- unpasteurised dairy products.
What foods are safe?
All freshly cooked foods, hard cheeses, fresh pasteurised milk and milk products, UHT milk, yoghurt, freshly washed vegetables and fruit, and canned foods are usually considered safe.
How can food be prepared safely?
- Refrigeration does NOT stop the growth of Listeria. High risk foods that have been prepared and then stored in a refrigerator for more than twelve hours should not be eaten by pregnant women or other susceptible people.
- Freshly cooked foods are safest. Conventional cooking destroys Listeria.
- Hot food should be thoroughly cooked and kept hot above 60 degrees C.
- Raw vegetables should be thoroughly washed before eating.
- Uncooked meats should be kept covered and separate from cooked foods and ready-to-eat food to avoid cross-contamination.
- Knives and cutting boards used to prepare uncooked foods should not be used to prepare cooked or ready-to-eat foods unless thoroughly washed first.
For further information, please contact your local doctor, community health centre or nearest public health unit.
Heymann, D., ed. 2008. Control of Communicable Diseases Manual, 19th edition. Washington, DC: American Public Health Association.
Mandell, Douglas, and Bennett’s 2009 Principles and Practice of Infectious Diseases, 7th edition. Churchill Livingstone.