Denmark – Outbreak of invasive Listeria infection sequence type 7 in Denmark caused by fish meatballs

SSI

Between mid-August and December 2022, the Statens Serum Institut has registered 11 cases of Listeria monocytogenes . The Statens Serum Institut, the Danish Food Agency and the Danish Food Institute DTU have investigated the disease outbreak and identified fish cakes as the source.

About the disease outbreak

Between mid-August and December 2022, the Statens Serum Institut registered 11 people who are infected with the same type of Listeria monocytogenes (figure 1). Among the sick are 4 men and 7 women. In relation to age, there is one child and 10 adults (median age is 77). The patients are infected throughout the country (table 1). Interviews with six patients showed that all had eaten fish cakes.

Number of cases of Listeria monocytogenes sequence type 7 in Denmark per week, 2022 (n=11)

Table 1. Patients with the same type of Listeria monocytogenes in Denmark, August to December 2022, per region (n=11)

Region

Number of patients

The capital  4
Central Jutland 2
Northern Jutland 0
Zealand 2
Southern Denmark 3
Total 11

The investigation of the outbreak

The outbreak is handled by the Central Outbreak Group, which consists of SSI, the Danish Veterinary and Food Administration and the DTU Food Institute. SSI stands for whole-genome sequencing of listeria isolates from the patients and interviews with patients or relatives with a view to identifying a possible source of infection. The outbreak investigation showed that fish meatballs were the cause of the disease outbreak. The fish meatballs were recalled from stores on November 8, 2022.

The outbreak strain

The bacteria is of the type Listeria monocytogenes . By whole-genome sequencing of the Listeria monocytogenes bacteria isolated from the patients, it can be seen that they are very closely related and belong to sequence type 7.

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