Ready-to-eat salmon products, such as cold-smoked and marinated salmon, are the likely source of an outbreak of Listeria monocytogenes that has affected Denmark, Germany, and France since 2015, according to the European Safety Authority (EFSA) and the European Centre for Disease Prevention and Control (ECDC) which has used whole genome sequencing to identify the multi-country outbreak. By October 8, 2018, 12 cases, including four deaths, have been reported in the affected countries.

The first cluster of cases linked to the consumption of ready-to-eat smoked salmon produced in Poland was reported in Denmark last August. Control measures were implemented and other European countries and competent authorities were informed.

France reported the detection of the same strain of Listeria in marinated salmon originating from the same Polish processing company as identified in the Danish outbreak investigation in October 2017. The most recent case linked to the outbreak was notified in Germany in May 2018.

Lack of genome sequencing 
EFSA says that due to the lack of whole genome sequencing data from the environmental and food samples taken at the Polish processing plant, it’s currently not possible to confirm whether the contamination occurred in the suspected plant.

Moreover, until information on the Norwegian primary producers of the salmon used in the contaminated batches has been reported and assessed, the possibility of contamination at primary production level cannot be excluded.

The identification of the same Listeria strain in a salmon product in France and a new human case in Germany suggests that the source of contamination may still be active and that contaminated products have been distributed to other EU countries than Denmark, adds EFSA.

The abstract of the EFSA technical report says: “It is likely that the extent of this outbreak has been underestimated since the outbreak was identified through sequencing and only a subset of the EU/EEA countries routinely use this advanced technique to characterize L. monocytogenes isolates.”

“Although control measures were implemented following the Danish outbreak investigation in September 2017, the identification of the same strain in a salmon product in France and a new human case in Germany suggest that the source of contamination is still active and contaminated products have been distributed to other EU countries than Denmark.”

It adds that until the source of the infection has been eliminated, new invasive listeriosis cases may still occur.

“Pregnant women, the elderly and immunocompromised individuals are at increased risk of invasive listeriosis, which is associated with severe clinical course and potentially death.”