Listeria bacteria in smoked fish is an invisible threat


Listeria monocytogenes is a facultative aerobic Gram-positive bacterium. This bacterium is virulent, with 20% to 30% of clinical infections causing listeriosis leading to death.

In 2018, 701 cases of severe invasive listeriosis were reported to the Robert Koch Institute (RKI), i.e. 0.8 cases per 100,000 population.

Most diseases caused by the bacteria listeriosis are serious and are associated with blood poisoning, meningitis, or miscarriage. In 2018, the disease was fatal in 5% of cases. The elderly, people with weakened immune systems, pregnant women and infants are particularly vulnerable.

Listeria bacteria can be found in many foods of plant and animal origin. Cold-smoked or hot-smoked fish are often susceptible and, therefore, also suspected of being able to transmit this disease.

Other raw fish and seafood products, such as sushi, sashimi, oysters or processed products such as grated fish, may also be affected.

Raw, smoked or processed foods are often at high risk of listeria contamination, which can cause a number of dangerous diseases for users. (Illustrated image)

Professor Dr Andreas Hensel, President of the BfR, said: “Pregnant women, the elderly or people with weakened immune systems should only eat fish and seafood that has been thoroughly cooked.”

However, not all Listeria bacteria cause illness. Of the 20 described Listeria species, only Listeria (L.) monocytogenes is a significant cause of infection in humans.

Infection during pregnancy can lead to miscarriage, premature birth, stillbirth, or the birth of a sick baby. Furthermore, listeriosis mainly develops in people whose immune systems are weakened by old age, pre-existing medical conditions, or medications. They often develop blood poisoning, encephalitis or meningitis, or bacterial arthritis.

Listeriosis is associated with relatively high mortality in risk groups. In healthy individuals who do not fall into one of the risk groups, infection can lead to gastrointestinal inflammation plus fever, with a usually mild progression.

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L. monocytogenes bacteria are common in the environment and can be found in many foods. High detection rates are found in minced meat, raw meat and sausage dishes, and raw milk.

However, many other ready-to-eat foods of plant and animal origin, which are not subjected to further bactericidal treatment (eg heating) after processing, may also contain L. monocytogenes. Examples include cheese (made from raw or pasteurized milk), pre-cut vegetables, or sliced ​​sausage products.

This is because listeria bacteria can persist for a long time in food processing plants in areas that are hard to reach for cleaning and disinfection. Therefore, continuous germ entry during food production is possible.

Raw, smoked or processed fish and seafood products such as sushi, sashimi, oysters, hot or cold smoked fish and processed fish are often contaminated with listeria bacteria. 7-18% of samples of cold smoked or processed fish products tested by the food watchdog in Germany between 2007 and 2017, and 3-9% of samples of hot smoked fish products contained L. monocytogenes.

Even low bacterial concentrations pose a danger to risk groups. For example, when products are stored at home at a higher temperature than the manufacturer recommends or used after their use-by date. Furthermore, handling contaminated produce carries the risk of transferring listeria bacteria to other foods.

The BfR recommends that people at high risk for listeriosis in general should not avoid fish, but only eat fish or seafood that has been thoroughly cooked.

Listeria can be destroyed by heating food to a core temperature of 70°C for at least two minutes. High-risk groups should limit their intake of smoked or processed raw seafood products.

Huong Giang (via: Science Daily)

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