No 13/16 - 2026

Changed phone hours for the advisory telephone on Wednesdays

The Department of Infectious Disease Epidemiology and Prevention offers a daily telephone advisory service for healthcare professionals on vaccination programmes and the use of vaccines, including following exposure to infection.

We have decided to change the advisory service’s phone hours on Wednesdays, so that they will now be from 8:30–11:00, in line with the other four weekdays. The change takes effect on Wednesday, 6 May.

(Advisory Team)

Listeriosis in Denmark 2023–2025

Invasive listeriosis is a rare but serious foodborne infection caused by the bacterium Listeria monocytogenes, which primarily affects individuals with weakened immune systems due to disease or treatment (e.g. cancer, haematological disorders, kidney disease, and diabetes), advanced age, or pregnancy. The incubation period is longer than for most other foodborne infections, typically 2–3 weeks. L. monocytogenes can persist in food production environments for extended periods and may therefore give rise to outbreaks that can last for several years.

A report on listeriosis in Denmark for the period 2023–2025 is now available on SSI’s website.

In the period 2023–2025, an average of 66 cases were recorded annually [range 54–84], corresponding to an incidence of 1.1 per 100,000 population in Denmark per year. Listeria bacteria were detected in cerebrospinal fluid in 20% of patients; a further 78% of infections were identified in blood cultures, and 2% were identified in other sample materials, e.g. placenta. A total of nine cases of listeriosis in pregnant women were recorded in the period (1–5 per year).

Of the 199 individuals with listeriosis in the period 2023–2025, all were hospitalised in connection with their infection, and 45 (23%) died within 30 days of infection. This is in line with previous reports (Listeriosis – occurrence 2014–2018 and Listeria annual report 2019–2022).

In the period 2023–2025, 10 major genetic clusters/outbreaks with seven or more cases were identified using whole genome sequencing, corresponding to 38% of patients in the period. All patients are routinely contacted for an interview regarding possible exposures in the month prior to illness. This information is used continuously to identify and verify possible sources of infection in outbreaks. SSI works closely with the Danish Veterinary and Food Administration and the National Food Institute, DTU, under the auspices of the Central Outbreak Management Group, to investigate outbreaks and prevent further cases linked to the same food source.

The source of four of the major outbreaks was identified as ready-to-eat fish products and fish cakes, respectively. The outbreaks are described in more detail in the report on listeriosis in Denmark 2023–2025. With the Executive Order on notification of infectious diseases from 2023, listeriosis is no longer subject to clinical notification via SEI2. Instead, Listeria is now exclusively notifiable by laboratories to SSI, unless an outbreak is suspected. In addition, it is mandatory for Danish clinical microbiology departments to submit isolates to SSI. Continued close collaboration with clinical microbiology departments on rapid submission of isolates, as well as with hospital departments and patients on collecting relevant information on food consumption, remains essential and highly valuable.

L. monocytogenes is widespread in the environment and is therefore also found in many raw or minimally processed foods. The bacterium can grow at refrigerator temperatures, and risk foods are typically ready-to-eat products with a long shelf life under refrigeration. Known high-risk products include sliced deli meats, fish products such as cold-smoked and gravad fish, and soft cheeses, especially those made from unpasteurised milk. The Danish Veterinary and Food Administration provides guidance on how to avoid listeriosis during pregnancy or for individuals who are seriously ill and/or immunocompromised.

(P.M. Ryder, L. Espenhain, Department of Infectious Disease Epidemiology and Prevention; S. Schjørring, S. Bo Lassen, Department of Bacteria, Parasites and Fungi)