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20 April 2017

Causative agent

Listeriosis is caused by the bacterium Listeria monocytogenes which is commonly found in the natural environment (e.g. soil or water). The bacteria may also be found in some contaminated raw foods, such as vegetables and uncooked meats as well as unpasteurised milk. It is also found in processed foods that become contaminated during or after processing, such as soft cheese and cold cuts. Listeria monocytogenes is able to survive under low temperature, and can multiply in refrigerated food that has been contaminated. The bacteria can be killed by cooking.

Clinical features

A person with listeriosis usually presents with fever, headache, and sometimes gastrointestinal symptoms such as nausea, vomiting and diarrhoea. Some patients may develop severe complications such as meningoencephalitis or septicaemia. Pregnant women, newborns/infants, the elderly and persons with chronic diseases or weakened immunity are at higher risk of being infected. Pregnant women with listeriosis, though mostly develop mild symptoms only, may transmit the infection to their foetuses/newborns resulting in miscarriage, stillbirth, septicaemia or meningitis in the newborns.

Mode of transmission

Listeriosis is mainly caused by consumption of contaminated food. It may also be transmitted through placenta from mother to foetus, or through an infected birth canal to her newborn.

Incubation period

The incubation period may range from 3 - 70 days. On average symptoms occur 3 weeks after exposure to the bacteria.


Patients with listeriosis often require hospitalisation and antibiotic is used to treat the infection.


  • To prevent listeriosis, you should maintain good personal, food and environmental hygiene.
  • Proper food handling by applying 5 Keys to Food Safety, i.e. Choose (Choose safe raw materials); Clean (Keep hands and utensils clean); Separate (Separate raw and cooked food); Cook (Cook thoroughly); and Safe Temperature (Keep food at safe temperature), can help prevent listeriosis.
  • High-risk individuals, such as chronic disease patients, pregnant women, infants, the elderly and people with weaker immune system should avoid eating high-risk foods. High risk food items for listeriosis include refrigerated ready-to-eat food with a long shelf life (greater than 5 days) (e.g. smoked salmon, smoked ham, deli meats, cold-cuts), cheeses made from unpasteurised milk including some soft and semi-soft cheeses, pre-prepared and stored salads, etc.

* Please visit the website of Centre for Food Safety for more information on food safety.