Listeriosis

Causative agent

Listeriosis is caused by the bacteria Listeria monocytogenes which is commonly found in the natural environment (e.g. soil or water). The bacteria may 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 at low temperatures, and can even grow on refrigerated food. 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. Although pregnant women with listeriosis are mostly asymptomatic or only develop mild symptoms, they may transmit the infection to their foetuses or newborns, resulting in miscarriage, stillbirth, preterm birth and septicaemia or meningitis in the newborns.

Mode of transmission

Listeriosis is mainly caused by consumption of contaminated food. In pregnant women, it may also be transmitted from the mother to the foetus through the placenta, or to the newborn through an infected birth canal.

Incubation period

The incubation period may range from 3 to 90 days. Symptoms commonly occur 1-2 weeks after exposure to the bacteria.

Management

Patients with listeriosis often require hospitalisation and antibiotics are used to treat the infection.

Prevention

1. Maintain good personal hygiene

• Perform hand hygiene frequently, especially before handling food or eating; and after using the toilet.
• Wash hands with liquid soap and water, and rub for at least 20 seconds. Then rinse with water and dry with a disposable paper towel or hand dryer. If hand washing facilities are not available, or when hands are not visibly soiled, hand hygiene with 70 to 80% alcohol-based handrub may be considered.

2. Maintain good food hygiene

• Adopting the 5 Keys to Food Safety in handling food, 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, women preparing for pregnancy, infants, the elderly and people with weaker immune system, should avoid eating high-risk foods potentially containing Listeria. 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 and cold meats; cheese made from unpasteurised milk particularly soft and semi-hard cheese; pre-made or prepackaged salads, etc.
• High-risk individuals should make sure the food is freshly cooked. When reheating food at home, make sure it is steaming hot throughout. When eating out, they should avoid buffet-type meal and food that is served lukewarm. If this is not possible, choose hot dishes only.

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