A Listeria outbreak in three European countries linked to salmon has sickened 12 and killed four people.

The European Centre for Disease Prevention and Control (ECDC) said the multi-country outbreak caused by Listeria monocytogenes sequence type (ST) 8 was identified through whole genome sequencing (WGS) analysis.

The agency also warned it was “likely” that the extent of the outbreak has been underestimated since it was identified through sequencing and only a few EU countries routinely use this technique to characterize Listeria monocytogenes isolates.

Listeriosis cases have been recorded in Denmark (six), Germany (five) and France (one). The first case was in October 2015 in Denmark and the most recent in May 2018 in Germany.

In August 2017, Denmark identified the first cluster of cases linked to ready-to-eat cold-smoked salmon processed in Poland by BK Salmon. Food Safety News has contacted the company for comment.

Five cases had the onset of disease between May and August 2017 and one died within 30 days of diagnosis. A historical confirmed case had disease onset of October 2015.

In October last year, France reported a matching Listeria monocytogenes strain in food isolates of marinated salmon from the same Polish processing company identified in the Danish investigation.

ECDC said this supports the hypothesis that contamination may have occurred at the processing company in Poland, which is currently in production activity.

“However, due to the lack of WGS data on the isolates found in the environmental and food samples taken at the Polish processing plant, it is not possible at present to confirm the contamination with the Listeria monocytogenes ST8 outbreak strain at the suspected Polish plant.”

The agency added that until detailed information on the Norwegian reared salmon exported by two firms used in the contaminated batches is known, contamination at primary production level cannot be excluded.

Last year, researchers in the journal Eurosurveillance said contaminated salmon in two countries from different batches suggested environmental contamination at the processing plant.

The French patient was a female over 80 years old who presented with meningitis in June 2016.

Five confirmed German cases were identified between August 2017 and May 2018. They were from four different federal states and had the onset of symptoms in August 2017 (three cases), December 2017 and May 2018 (one each). Three patients, including the one who fell ill in May 2018, died of listeriosis.

Four Listeria monocytogenes ST8 isolates closely related to the outbreak strain were found in three batches of sliced cold smoked salmon sampled at Dansk Supermarked Group and at a retail store belonging to the wholesaler in Denmark.

Official samples of products at BK Salmon were taken from September 2017 to September 2018. Twelve of 79 food and environmental samples were positive for Listeria monocytogenes but isolates were not typed. The pathogen was not found in any other official food (105) and environmental (35) tested samples collected in the same period.

In 2017, own-check sampling and testing found eight Listeria positives out of the 1,360 environmental swab samples analyzed. The firm has its own laboratory in which food samples are regularly tested. A total of 4,385 own-check tests in 2017 on finished products sampled did not find the pathogen.

Corrective measures by the company since September 2017 include production stopped from Sept. 3 to 14, 2017, added treatments for cleaning and disinfecting equipment and production rooms, thermal fogging, modernization of equipment on one of the three production lines at the slicing hall, which was considered by Polish authorities and the firm as the probable source of Listeria contamination and increased frequency of sampling and testing in its own and the official lab.

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