Ready-to-eat (RTE) fish products have been linked to almost 100 Listeria infections in an outbreak that started in 2011 and continued into this year, according to two European agencies.

The European Centre for Disease Prevention and Control (ECDC) and European Food Safety Authority (EFSA) said Austria, Belgium, Denmark, Finland, Italy, Germany, the Netherlands, Romania, Sweden, and the United Kingdom have had patients.

Data suggests a connection to smoked salmon and two establishments in Lithuania.

Of 94 confirmed cases, Sweden reported the first in 2011. Germany has the most with 45, followed by Belgium with 17 and Italy with 15. The UK had one patient. 

Data on age and sex were available for 76 cases from six countries. The age range of patients was 13 to 93, with a median of 78 years old. The most affected group was 75 to 84-year-olds. Of 54 cases with information, 52 were hospitalized, and 17 died due to or with a Listeria infection.

One active cluster
based on genomic similarity, cases can be divided into three sub-clusters. Sub-cluster 1 has 64 cases in five countries between 2016 and 2023, including 17 in 2022 and 2023. Ten people died, with five in 2020, two in 2019, and one each in 2021, 2022, and 2023. Sub-clusters 2 and 3 are historical, with 30 cases between 2011 and 2021.

In 2020, EFSA and ECDC prepared a Joint Notification Summary (JNS), which was not made public, of a Listeria cluster linked to fish reported by Germany. As cases in different countries continue to be recorded, the agencies publish an outbreak assessment.

National food investigations, traceability, and genomic data identified 34 Listeria monocytogenes isolates from 12 fish products and one isolates from the processing environment within the first sub-cluster.

Strain persistence over several years
Sequencing analysis found the outbreak strain in two processing plants in Lithuania and their sealed RTE fish products from 2016 to 2023. Between 2022 and 2023, contaminated products from these plants reached retail in Germany and Italy.

In mid-September 2023, following findings from an Italian investigation, the Lithuanian food safety agency said the implicated processor, which sourced fish from Norway, had stopped production. Cold smoked salmon tested negative for Listeria. One processing environment sample from a non-food contact surface was positive for Listeria monocytogenes. Sampling of sealed packs of smoked salmon collected chilled from a patient’s home in Italy tested positive for Listeria monocytogenes at 3,000,000 and 820,000 CFU/g.

A chilled smoked salmon sample collected from retail in Germany in May 2023 also tested positive for Listeria. Another factory in Lithuania manufactured this.

EFSA and ECDC experts said that further investigations were needed to identify the origin of contamination. This would allow national authorities to implement targeted control measures and mitigation actions.

“The interruption of the production of RTE fish products in one of the two processing plants is likely to reduce the risk of infections, but new cases are likely to occur in EU countries, particularly among immunosuppressed and people aged over 75 years, until all sources and sites of contaminations are properly controlled,” said ECDC.