Listeria monocytogenes is a pathogenic bacterium ubiquitous in nature that can be found in water, soil and in human and animal faeces. It can survive and multiply in environment at temperature as low as 0°C but it is readily killed during thorough cooking. Sometimes it is hard to identify the causative food item since symptoms caused by Listeria monocytogenes may only appear within 3 to 70 days after consumption of contaminated food.

What are its routes of transmission?

       Listeria monocytogenes is mainly transmitted through food-borne route. It is commonly found in undercooked, contaminated meat and vegetables and in unpasteurised milk. Some processed food products could be contaminated with the bacterium during their processing or storage, such as soft cheeses (e.g. Brie and Camembert), cold meat, salad, seafood intended to be eaten raw and cold smoked seafood products (e.g. smoked salmon).

What are the symptoms of infection caused by Listeria monocytogenes?

       Whether an infected individual will develop a disease depends primarily on the bacterial amount (in the causative food item) as well as the age and immune status of the host. Symptoms typically appear within 3 to 70 days after infection. In general, people in good health are less susceptible to the bacterial infection. However, pregnant women, newborns, elderly adults and individuals with weakened immune systems are susceptible to infection-induced complications, such as septicaemia, meningitis and encephalitis. In the case of infected pregnant women, Listeria monocytogenes can be transmitted to the foetus through the placenta, resulting in miscarriage, stillbirth, premature labour or causing the newborns to suffer from severe diseases.

How to avoid getting infected with Listeria monocytogenes?

1. Practice good personal and food hygiene;
2. Choose food wisely. Check the expiry date of food products and see whether their packaging is intact and undamaged, especially refrigerated ready-to-eat food with a long shelf life;
3. Handle food with caution. Handle raw and cooked food separately to avoid cross-contamination;
4. Food must be thoroughly cooked and promptly consumed;
5. High risk individuals, such as pregnant women and those with weakened immune systems, should avoid consumption of high risk food, such as unpasteurised milk, soft cheese, pre-prepared or stored salad and cold meat. 

Advice for the trade

1. Purchase from reputable suppliers to ensure the supplies come from reliable sources;
2. Pay attention to food, environmental and staff hygiene. Develop a cleaning and disinfection schedule;
3. Cook food thoroughly for safe food preparation;
4. Prevent the prepared food from contamination by raw food, utensils, people and vectors;
5. Maintain prepared food at proper temperature (cold food at below 5°C and hot food at above 60°C) to ensure food safety.