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Recently, an epidemiologist published a column about Listeria in Farsóttarfrettir, where it was stated that 5 people have been diagnosed with an infection caused by Listeria monocytogenes (listeriosis) so far this year . Now there are 6 cases. The source of the infection in food has not yet been found. The gestation period of listeria is usually long or as long as three weeks, and therefore it is often difficult to trace the source of infection as people do not remember what they ate 3-4 weeks before. Listeria monocytogenes (hereinafter referred to as Listeria) mainly causes disease among high-risk groups. Risk groups are persons with a weakened immune system, persons on immunosuppressive drugs or undergoing cancer treatment, pregnant women and the elderly. In healthy individuals, consumption of food contaminated with Listeria rarely causes symptoms or only mild flu-like symptoms. The Swedish Food Agency wants to increase the awareness of risk groups and food companies about preventive measures against Listeria.

What foods?

Listeria can multiply in refrigerated food at 4 °C, regardless of whether the packaging is vacuum-sealed, ventilated or not. Sensitive ready-to-eat foods, with a shelf life of several weeks, can be a risk if they have been contaminated with Listeria during the production process. However, Listeria cannot multiply in foods that are acidified so that the pH is below 4.5 or salted / dried so that the water activity is below 0.92. Dry ham such as parma ham, sausages, pepperoni, and sour milk products can be mentioned here.

Examples of foods in which Listeria can multiply if it is present.

• Smoked and buried fish.
• Meat toppings, which are cut and packed after cooking (eg ham, pâté, stew, and campfire jam).
• Prepared dishes, which are not heated enough before consumption (eg fish balls, meatballs, cooked chicken such as nuggets, strips, buns).
• Unpasteurized milk / raw milk and dairy products made from raw milk (eg soft cheeses).
• Frozen vegetables that are not heated before consumption.
• Cut fruits and vegetables with a shelf life of several days.
• Other ready-to-eat food products such as sandwiches that are not heated before consumption.

It is difficult to completely prevent Listeria in raw foods such as raw meat, raw milk, raw fish and raw vegetables, as listeria is widespread in the environment.

What can food companies do to reduce the risk of Listeria?

If Listeria is detected in cooked food, which is ready for consumption, it indicates that the food has been contaminated during the production process after heat treatment, for example from a cutting machine, packaging machine or conveyor belt, or the food has not received sufficient heat treatment.

If Listeria enters a food company through raw materials or through the interaction of employees, the bacteria can live in the production environment and hide in cracks, cavities, under tables and worn conveyor belts, becoming a kind of “house bacteria”. It can also form a biofilm on stainless steel and plastic surfaces if not properly cleaned. Biofilm is formed because the bacteria sticks to surfaces in places where it has not been possible to clean food residues. There, the bacteria can multiply and spread in the production environment. If a biofilm has formed, it can be difficult to remove and there is a risk of Listeria contamination of food. It is therefore extremely important that food contact surfaces and nooks and crannies are well cleaned in food establishments, and that maintenance is carried out well so that the bacteria cannot live in the premises.

Food companies that produce ready-to-eat foods must regularly monitor Listeria in the production environment and in products. The frequency of analysis depends on the assessment of the likelihood of listeria entering the food and the growth conditions in the food. See more information on listeria analysis for ready-to-eat foods.

What can consumers do to prevent illness caused by Listeria?

Consumers are encouraged to keep the following in mind when handling and storing perishable ready-to-eat foods.

• Ensure that the temperature in the refrigerator is not higher than 4 °C.
• Refrigerate food quickly after shopping.
• Keep in mind that if the temperature exceeds 4 °C for some time, the shelf life is not according to what it says on the packaging.
• Do not consume foods marked with a use-by date after that date.
• Heat the prepared dishes to 75 °C.
• Rinse vegetables and fruits well.

People in risk groups (see above) should not eat smoked and smoked fish, drink raw milk or dairy products made from raw milk, or eat cold cuts that are nearing their use-by date. When heating frozen vegetables and prepared dishes such as fish balls, meat balls, and chicken nuggets, it must be ensured that the temperature reaches at least 75 °C.

Advanced material:

• Listeriosis and symptoms
• Listeria analysis for ready-to-eat foods