Danish officials are trying to find the source of a Listeria outbreak that has affected 17 people since 2019.

Two illnesses have provisionally been registered in 2024, said the Statens Serum Institut (SSI). 

Patients have been infected with the same type of Listeria monocytogenes since the summer of 2019.

They are 13 women and four men with a median age of 73 years old. One patient is in their 20s and four are in their 80s.

Two deaths reported

All have been hospitalized and two people died within 30 days of the infection being detected.

Two infections were recorded in 2019, one each in 2020 and 2021, four in 2022, and seven in 2023.

Seven cases live in Hovedstaden, five in Midtjylland, two each in Nordjylland and Sjælland, and one in Syddanmark.

The SSI, Danish Veterinary and Food Administration (Fødevarestyrelsen), and DTU Food Institute are investigating the outbreak.

In 2022, Denmark recorded 86 listeriosis cases with three large outbreaks.

SSI work involves whole genome sequencing of patients’ Listeria isolates and interviews with patients or relatives to try and identify a possible source of infection.

Whole genome sequencing of the Listeria monocytogenes bacteria isolated from patients has shown they are very closely related and belong to sequence type (ST) 1607.

The European Centre for Disease Prevention and Control (ECDC) and European Food Safety Authority (EFSA) are working on an assessment relating to a multi-country outbreak of Listeria monocytogenes ST1607 linked to fish products. This should be published next week.

The agencies are also dealing with another multi-country Listeria outbreak potentially caused by fish products. Details will be made public in mid-May.

about Listeria infections

Food contaminated with Listeria monocytogenes may not look or smell spoiled but can still cause serious and sometimes life-threatening infections. Anyone who has developed symptoms of Listeria infection should seek medical treatment and tell their doctors about the possible Listeria exposure.

People should monitor themselves for symptoms during the coming weeks because it can take up to 70 days after exposure to Listeria for symptoms of listeriosis to develop. 

Symptoms of Listeria infection can include vomiting, nausea, persistent fever, muscle aches, severe headache, and neck stiffness. Specific laboratory tests are required to diagnose Listeria infections, which can mimic other illnesses. 

Pregnant women, the elderly, young children, and people such as cancer patients who have weakened immune systems are particularly at risk of serious illnesses, life-threatening infections, and other complications. Although infected pregnant women may experience only mild, flu-like symptoms, their infections can lead to premature delivery, infection of the newborn, or even stillbirth.