Health authorities in Denmark are investigating a Listeria outbreak that has infected seven people since late August, with the source of contamination still unclear.
According to the Statens Serum Institut (SSI), the cases were recorded between August 21 and October 24. The patients five men and two women are all elderly, ranging from 75 to 99 years old, with a median age of 84. Geographically, four cases are from Hovedstaden, two from Syddanmark, and one from Sjælland.
The SSI, together with the National Food Institute at the Technical University of Denmark and the Danish Veterinary and Food Administration (Fødevarestyrelsen), has launched an investigation to identify the contamination source. Whole genome sequencing (WGS) has revealed that all infections belong to Listeria monocytogenes sequence type 403, indicating a likely single source.
As part of the probe, SSI is conducting genomic analysis of patient samples and interviewing patients or their relatives to trace potential exposures.
In a separate outbreak, the number of confirmed cases has risen from five to eleven between August 13 and October 24. Patients include five men and six women aged 30 to 95 (median age 75), with cases reported from Sjælland (5), Nordjylland (3), Syddanmark (2), and Midtjylland (1).
Investigators have linked this outbreak to contaminated fish cakes produced by Tenax Sild A/S. WGS confirmed that the bacteria isolated from both patients and food samples were identical, belonging to sequence type 7. The company issued a recall in September after Listeria monocytogenes was detected, covering products with a best-before date up to October 10, 2025.
Consumers are reminded that symptoms of listeriosis can take up to 70 days to appear. Common symptoms include nausea, vomiting, persistent fever, muscle aches, severe headaches, and neck stiffness. Elderly individuals, pregnant women, and immunocompromised people are at higher risk of severe illness.
Authorities continue to urge caution and are advising anyone experiencing symptoms after consuming high-risk foods to seek medical attention promptly.