Danish officials are investigating a years-long outbreak of Listeria infections that has affected nine people from 2018 through this month.

The Statens Serum Institut (SSI), Danish Veterinary and Food Administration (Fødevarestyrelsen) and DTU Food Institute are trying to find the source of the infections.

The same type of Listeria monocytogenes has been found in nine people from late 2018 to November 2021. The four patients this year have all only recently become ill. Two people fell sick in both 2018 and 2019 and one case was recorded in 2020.

Among those sick are seven women aged from 35 to 95 years old and two children younger than age 5. They live all over the country. All nine Danish patients have been hospitalized but there have not been any deaths.

Whole genome sequencing (WGS) showed samples from outbreak patients to be closely related, which means it is likely they came from the same source.

Interviews with patients or their relatives revealed that they had not been traveling, do not know each other and have not participated in the same events. Danish officials said this suggests a food sold throughout the country is the common source of infection.

The Statens Serum Institut has communicated about the outbreak via the European Centre for Disease Prevention and Control’s (ECDC) EpiPulse platform and so far nine countries have replied saying they have not registered Listeria cases with the same sequence type.

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 food poisoning 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.