Listeria monocytogenes contamination in ready-to-eat foods

Listeriosis is a foodborne disease caused by eating food contaminated with Listeria monocytogenes. This organism causes serious illness in vulnerable groups such as the elderly, pregnant women, individuals with weakened immune systems and the newborn. Listeria monocytogenes is well suited to growth and survival in foods, even if these are stored in the refrigerator, and is a major concern in ready-to-eat foods. Listeria are common in the environment and can be introduced into food by contact with contaminated surfaces.

What is the concern?

Listeriosis is a relatively rare but serious foodborne disease that is of concern because of the severity of illness caused and the high mortality rate. It particularly affects immuno-compromised people including the elderly, pregnant women and unborn or newborn infants. In these vulnerable groups, immunity is often suppressed and the microorganism responsible for causing illness, Listeria monocytogenes, is able to cause infection in other parts of the body, resulting in serious conditions including meningitis, encephalitis, blood poisoning and liver abscesses.

During the 1990s, listeriosis affected between 90–130 people per year in the UK. In recent years (2003–2007), the number of cases has increased to around 180–240 cases per year. This increase in cases has been linked to one group of vulnerable individuals, the elderly (those aged 60 years and over). Similar recent increases in cases of listeriosis in this age group have been reported in other countries in Europe but the reasons for this increase are not known. There is no evidence of increasing prevalence in foods.

It is recognised that most cases of listeriosis, including sporadic cases (affecting individuals and not linked to outbreaks) and those linked to common-source outbreaks, are caused by eating food contaminated with Listeria monocytogenes. The incubation time for listeriosis varies from 11–70 days and this means it is often difficult or impossible to identify the particular food that caused illness. However, using information from listeriosis outbreaks, it has been possible to identify the key types of foods responsible. These are predominantly ready-to-eat, long shelf-life delicatessen products such as certain meat products and soft cheeses.

Where can Listeria monocytogenes be found?

Listeria monocytogenes can be found in many natural environments, including soil, water, vegetation, sewage and farm environments. It can also be found in food manufacturing environments, in foods and in food preparation environments. In food processing environments, contamination of foods can occur after they are processed (such as cooking) but before they are packaged, and also when the products are handled in shops (eg, on slicing machines, delicatessen counters), in the home and in catering establishments.

Minimising risk

Listeria monocytogenes is destroyed by normal cooking processes (eg, reaching at least 70 °C at the centre of the food for 2 minutes), but can survive and grow at refrigeration temperatures. It is this characteristic of being able to multiply at low temperatures that enables Listeria to grow to levels that are high enough to cause infection.

Since many ready-to-eat foods are stored in the refrigerator before consumption and are not cooked before eating, they can be a potential vehicle for listeriosis. It is, therefore, very important that foods are stored at the correct temperature and that they are not kept in refrigerators for periods long enough to allow growth of Listeria.

Vulnerable groups can prevent Listeria infection by avoiding certain long shelf-life, ready-to-eat delicatessen products and by handling food properly. The European Food Safety Authority has identified ready-to-eat foods such as smoked fish, meat products and soft cheeses as priorities.

For more information about Listeria monocytogenes in ready-to-eat foods, read the Scientific Opinion of the Panel on Biological Hazards, EFSA, adopted by the BIOHAZ Panel on 6 December 2007.