No 21/23 - 2022

Outbreak of invasive listeria infections in Denmark in May 2022

Since 13 May 2022, Statens Serum Institut (SSI) has received reports of eight patients infected with the same type of Listeria monocytogenes. More specifically, five men and three women aged 33-93 years were affected. This means that the total number of listeria cases recorded in May reaches 15, whereas we normally record an average of four monthly cases. The patients are primarily from the Capital Region of Denmark, except for one who resides in the Region of Southern Denmark. All patients had underlying conditions or other immunodeficiency prior to their listeria infection. All patients were admitted to hospital. Two had meningitis, five had sepsis, including a pregnant woman; and one had a cerebral abscess. Three elderly patients died within 30 days after being sampled. The SSI is investigating the outbreak in collaboration with the Danish Veterinary and Food Administration and the National Food Institute at the Danish Technical University to rapidly establish the source of infection and curb the outbreak. The SSI handles sequencing of listeria isolates from patients and conducts in-depth interviews on food purchases and food consumption in the month leading up to disease onset.

Invasive listeriosis is a rare but serious foodborne infection with the Listeria monocytogenes bacterium, which primarily affects immunocompromised persons, elderly people and pregnant women. Listeria monocytogenes may be widely present in the environment and therefore may also be found in many raw or lightly processed foods. The incubation period of listeriosis is typically 2-3 weeks but may vary down to only a few days. Listeriosis is a life-threatening disease, the mortality of which is approx. 25%. The mortality depends on age and the presence of other conditions.

Healthcare workers should be aware of listeriosis as a differential diagnosis when people present with relevant symptoms. The symptoms include a high fever, shivers, muscle aches, general malaise and neck and back stiffness or changed level of consciousness. Serious listeria infection may produce sepsis or meningitis. Listeria does not cause human-to-human infection, but a pregnant woman who has become infected may transmit the infection to her foetus during pregnancy or labour. Pregnant women often experience a milder course with fever, myalgia, diarrhoea, gastralgia or painful urination.

The SSI collaborates with the departments of clinical microbiology, which diagnose the listeria infections and submit listeria isolates to the SSI for whole genome sequencing. In this manner, new cases belonging to the outbreak are identified as rapidly as possible.

The listeria bacterium may reproduce at fridge temperature, and known risk products include cold meat cuts, fish cuts, e.g. cold-smoked and raw-spiced fish, and soft cheeses (particularly those produced from unpasteurised milk). The risk of becoming infected with listeria is higher if you ingest cold cuts past their expiration date or cold cuts that have been stored after opening for a long period of time. Rolled seasoned meat (Danish language: rullepølse) and cold-smoked and raw-spiced salmon have caused listeria disease outbreaks in recent years in Denmark.

Read more about the outbreak here (in Danish)

(L. Müller, L. Espenhain, E. Dibba White, Department of Infectious Disease Epidemiology and Prevention, S. Schjørring, Department of Bacteria, Parasites and Fungi)
8 June 2022