In Denmark, on 23 August 2017, Statens Serum Institut (SSI) identified a genetic cluster of four human Listeria monocytogenes sequence type (ST) 8 isolates by core genome multilocus sequence typing (cgMLST) [1]. The allele calling was performed in BioNumerics (v7.6.2, Applied Maths, Belgium). We initiated an epidemiological investigation and notified the Danish Central Outbreak Management Group (collaboration between the Danish Veterinary and Food Administration (DVFA), the National Food Institute at the Technical University of Denmark (DTU) and SSI). On 25 August, two additional human isolates were found to belong to the same genetic cluster.

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A confirmed case was defined as a person clinically diagnosed with listeriosis after 1 January 2017 with laboratory-confirmed L. monocytogenes ST8 clustering using cgMLST (≤ 5 allelic distance, single linkage). Cases diagnosed before 1 January 2017 with an isolate belonging to this cluster were defined as probable cases.

As of 25 August 2017, the genetic cluster comprised six cases; five confirmed and one probable. Laboratory sample dates ranged from 25 October 2015 to 21 August 2017. The age of the cases ranged from 59 to 96 years (median 80 years) and four were women. All patients had underlying illness and no travel history. One patient died within 30 days of diagnosis. Epidemiological investigations including a standard questionnaire on exposures showed that all five confirmed cases had consumed cold-smoked and/or cured salmon in the 30 days before disease onset. Four cases had bought the salmon in retail chain X. No other food-item was reported as consumed in high frequencies among cases. Epidemiological follow-up for the probable case did not include information on fish consumption.

Cross-border outbreak of listeriosis cause by cold-smoked salmon, revealed by integrated surveillance and whole genome sequencing (WGS), Denmark and France, 2015 to 2017

Eurosurveillance, 2017, Susanne Schjørring, Sofie Gillesberg Lassen, Tenna Jensen, Alexandra Moura, Jette S Kjeldgaard, Luise Müller, Stine Thielke, Alexandre Leclercq, Mylene M Maury, Mathieu Tourdjman ,Marie-Pierre Donguy, Marc Lecuit, Steen Ethelberg, Eva M Nielsen, https://doi.org/10.2807/1560-7917.ES.2017.22.50.17-00762

http://www.eurosurveillance.org/content/10.2807/1560-7917.ES.2017.22.50.17-00762