Disease, responsible agent and role of ANSES

Listeriosis is a foodborne disease caused by a bacterium, Listeria monocytogenes. This bacterium is frequently associated with minor digestive problems including diarrhoea. However, serious forms can affect susceptible populations such as pregnant women, the elderly and immunocompromised people. Below is a description of this disease and advice on how to avoid it.

Listeriosis: some key figures

Every year, approximately 300 cases of listeriosis are recorded in France. These cases are isolated from one another (sporadic); no outbreaks have been identified in France since 2003. The number of recorded cases increased for no known reason in 2006 and then stabilised in 2008. In 2010, there were 4.9 cases of listeriosis per one million inhabitants and 5.2 cases per 100,000 births.

While rare, listeriosis is an infection that can become serious in certain susceptible populations who have predisposing factors that allow the bacterium to spread more easily. These include: 

  • the elderly
  • people with cancer or blood or liver disease (cirrhosis, etc.)
  • people with insulin-dependent diabetes
  • people who have undergone organ transplants
  • people whose immune system has been weakened (immunosuppressive therapy, autoimmune disease such as HIV, etc.)


For these people, listeriosis symptoms can worsen and last for several weeks, with manifestations including septicaemia, meningitis, brain abscess and local infection. Neurological damage can be observed and, depending on the study, the reported mortality rate associated with infection (fatality) ranges from 20 to 30%. 

Moreover, in pregnant women, infection can lead to flu-like symptoms (fever, chills, lower back pain) and result in miscarriage, premature labour, non-viable pregnancy or neo-natal infection (with a 20% infant mortality rate).


The responsible bacterium

Listeria monocytogenes is a bacterium that is widespread in the environment and resistant outdoors (soil, lakes, rivers, sewers, bays, primarily decaying vegetation, etc.).

Poorly prepared silage (moist fodder intended for animal feed) can contain large amounts of L. monocytogenes and cause infection in ruminants. The environment is primarily contaminated through the faeces of healthy and sick animals: 6 to 30% of cattle, sheep, pigs, goats and chickens naturally host this bacterium in their digestive tracts. These animals are the main sources of contamination in humans.

Because it is found in the environment, the bacterium can contaminate many types of foods. Thus, many people ingest small amounts of L. monocytogenes fairly often, with no severe symptoms appearing other than digestive problems that sometimes include diarrhoea. 

To date, approximately 70 outbreaks have been recorded around the world, including seven in France. The responsible foods were jellied pork tongue (1992 and 2000), rillettes (1993 and 2000), Brie cheese (1995), Pont l’Evêque cheese (1997), Epoisses cheese (1999), spreadable sausage pastes (2002) and Mortadella (2003).

In 2011, an outbreak was declared in the United States associated with contaminated cantaloupe melons. This event underlined the importance of remaining vigilant to microbiological risks and paying special attention to their emergence in plant products.


How to avoid contamination  

By adopting good hygiene practices at home, you can protect yourself against most microorganisms:

  • For foods that are intended to be chilled, set the refrigerator to no higher than +4°C.
  • Whenever surfaces have been soiled by foods, clean them without delay.
  • Do not place unpackaged foods directly on shelves.
  • Respect domestic hygiene practices: clean utensils and work surfaces before and after use. 
  • Wash hands after handling raw products.
  • Thoroughly wash vegetables and herbs before eating or cooking them.
  • Store leftovers in the refrigerator for less than 3 days, and for foods that are meant to be eaten warm, reheat them to an internal temperature above +70°C.
  • Respect Use-By Dates (UBDs) for packaged foods, and consume counter-cut products as quickly as possible.
Pregnant women and the most at-risk individuals are advised to avoid foods that are frequently contaminated with L. monocytogenes such as raw-milk cheeses (especially soft-rind cheeses), cheese rind in general, smoked fish, raw shellfish, taramosalata and raw meat products such as delicatessen meats. 


Role of ANSES

As National Reference Laboratory, the ANSES Laboratory for Food Safety in Maisons-Alfort and Boulogne-sur-Mer is responsible for coordinating the French network of laboratories in charge of monitoring Listeria in foods. In this capacity, the laboratory is responsible for developing methods for detecting this bacterium in foods and transferring them to the network it coordinates. It also regularly organises Inter-Laboratory Proficiency Tests in order to guarantee the reliability of the analyses undertaken by the network. The Maisons-Alfort Laboratory for Food Safety is also a European Union Reference Laboratory and as such plays the same roles at European level.

Furthermore, it carries out research into Listeria in order to improve their identification, quantification and characterisation in foods, which are essential to their surveillance. The laboratory's work also aims to describe and model their behaviour to improve quality and hygiene control during the production and preparation of foods and to better understand their mechanisms of action (virulence, toxicity, etc.).

For the assessment of risks related to bacteria in the Listeria family, the Agency, through its Risk Assessment Department and the Expert Committee for the 'Assessment of the biological risks in foods', has issued multiple Opinions and Reports on the assessment of risks related to Listeria. These publications deal with the state of knowledge on this bacteria, the classification of foods based on their risks of contamination with Listeria, surveillance methods and microbiological testing protocols for the various types of frequently contaminated foods (delicatessen meats, ready-to-eat foods, refrigerated foods and raw plant products) and the assessment of risks related to certain specific production processes (artisanal production of AOC Cantal and Salers cheeses).