WGS-based analysis identified 12 patients with onset of symptoms between October 2015 and May 2018: six in Denmark, one in France and five in Germany. The first cluster of cases reported by Denmark in September 2017 was linked to the consumption of salmon products produced in Poland.

Initial investigations suggested that contamination may have taken place at a Polish processing company but the lack of recent WGS data on the L. monocytogenes -isolates collected at the processing plant make it impossible to confirm this is the case.

Although control measures were implemented following the Danish outbreak investigation in September 2017, the same strain was found in a salmon product in France in October 2017 and in a patient in Germany in May 2018. This suggests that the source of contamination may still be active and that contaminated products may have been distributed to more EU countries. Until the source of contamination is identified and controlled, new invasive L. monocytogenes infections associated with this outbreak may still occur.

It is likely that the extent of the outbreak is underestimated, since the use of sequencing to characterise L. monocytogenes isolates is only used by a few EU countries. A recent EU-wide study coordinated by ECDC revealed that more than half of the severe listeriosis cases in the European Union belong to clusters, many of which are not being picked up fast enough by the current surveillance system.

Listeriosis is a relatively rare but potentially severe food-borne disease that has been reported in increasing numbers in the EU/EEA countries since 2008. In 2016, 2 536 cases were reported, including 247 deaths. Pregnant women, the elderly and immunocompromised people are at higher risk of contracting listeriosis.