Abstract
In August 2017, an outbreak of six listeriosis cases in Denmark was traced to cold-smoked salmon, using epidemiological investigations and whole-genome sequencing (WGS) analyses. Exchange of genome sequences allowed identification in France of a food isolate from a salmon-derived product and a human isolate from 2016 within the same cgMLST cluster as the Danish isolates (L2-SL8-ST8-CT771). The salmon product came from a third European Union country. WGS can rapidly link human cases and food isolates across Europe.
Highlights
In August 2017, an outbreak of six listeriosis cases in Denmark was traced to cold-smoked salmon, using epidemiological investigations and whole-genome sequencing (WGS) analyses
We investigated the relatedness of outbreak isolates further by single-nucleotide polymorphisms (SNP) analysis performed by both Serum Institut (SSI) and DTU using two analysis pipelines: Northern Arizona SNP Pipeline (NASP) [2] and CSI Phylogeny version 1.4 from Center for Genomics Epidemiology (CGE), DTU [3] leading to the same conclusion
The fact that the contaminated salmon products identified in Denmark and France were from different batches suggests environmental contamination possibly at the production facility at company Y
Summary
Human Denmark Food Denmark Human France Food France Poultry processing facilities Norway Salmon processing facilities Norway Human Denmark, unrelated to the outbreak. 1 isolate from a probable French case 2016 and food isolates sampled in Denmark (4 isolates; July-August 2017) and France (1 isolate; September 2017) from products produced by company Y. B Maximum parsimony tree based on whole-genome sequencing (WGS) data showing single-nucleotide polymorphisms (SNPs) within the L. monocytogenes ST8 isolates from patients and food sources in the genetic outbreak cluster. A human isolate from a French resident belonged to the same cluster (L2-SL8-ST8-CT771) as the Danish isolates. This French probable case, a female patient in her mid-80s, was diagnosed in June 2016. The other nine countries that replied to the EPISFWD UI-426 notification (Austria, Finland, Germany, Luxembourg, the Netherlands, Norway, Slovenia, Sweden, United Kingdom) did not report any human or food isolates linked to the Danish outbreak. After submission of this report, at the end of November, we were informed through EPIS about three genetically linked human isolates in Germany
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