More than half a dozen people have been infected by Listeria in Denmark since August.
Between Aug. 21 and Oct. 24, seven people infected with the same type of Listeria monocytogenes were registered at the Statens Serum Institut (SSI).
Five men and two women are ill. Patients range in age from 75 to 99 with a median of 84 years old. Four patients are from Hovedstaden, two from Syddanmark, and one is from Sjælland.
The National Food Institute at the Technical University of Denmark, Danish Veterinary and Food Administration (Fødevarestyrelsen), and SSI are investigating the outbreak.
The SSI is responsible for whole genome sequencing (WGS) of Listeria isolates from patients and interviews with them or their relatives to try to identify a possible source of infection.
WGS of the Listeria monocytogenes bacteria isolated from patients shows they are very closely related and belong to sequence type 403. That suggests a common source for the infections.
Outbreak update
The SSI has also updated the number of sick people in a different Listeria outbreak.
It was previously known that five people were ill. However, now 11 patients have been reported between Aug. 13 and Oct. 24.
Patients are five men and six women. They are aged between 30 and 95 with a median of 75 years old. Five cases are from Sjælland, three from Nordjylland, two from Syddanmark, and one from Midtjylland.
The outbreak investigation showed that fish cakes caused the illnesses. Comparison of samples from patients and product samples showed that the bacteria were identical.
Tenax Sild A/S, which produced the fish cakes, recalled them from stores in September after Listeria monocytogenes was detected. The latest best before date was Oct. 10, 2025.
WGS of bacteria isolated from patients showed they were closely related and are sequence type 7.
It can take up to 70 days after exposure to Listeria for symptoms of listeriosis to develop. Symptoms can include vomiting, nausea, persistent fever, muscle aches, severe headache, and neck stiffness.
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