Listeria in different salmon batches suggests environmental contamination - study

By Joseph James Whitworth

- Last updated on GMT

Surveillance and WGS used in outbreak of listeriosis linked to cold-smoked salmon
Surveillance and WGS used in outbreak of listeriosis linked to cold-smoked salmon

Related tags L. monocytogenes Listeria monocytogenes

Contaminated salmon in two countries from different batches points to environmental contamination at the Polish factory, according to researchers looking at a listeriosis outbreak in Denmark.

Listeria in salmon processed in Poland by BK Salmon and sold in Denmark by Dansk Supermarked Group was linked to six illnesses earlier this year​.

Official controls at a French retailer found kosher chilled cured salmon was positive for Listeria monocytogenes and the isolate belonged to the same type as in the Danish outbreak.

Possible environmental contamination

Researchers said as salmon products in Denmark and France were from different batches it suggests environmental contamination at the production facility of BK Salmon.

BK Salmon measures

- Determining all potential threats along and in addition to implemented HACCP, among others

- Verification of machinery line-up and setup (including e.g. bearings, o-rings, etc.) to determine biofilm formation threats

- Testing and eventual implementation of central disinfection system (fogging) in main production halls (fresh processing and slicing departments)


- Testing of all reference samples from Danish recall (no threat determined)

- Consultancy with auditors, technologists, veterinary, laboratory analyst

- External training for our lab analysts confirming our methods and measures are complying to recent scientific findings

- Challenge tests on final products now in progress (as immediate effect of the recent training)

“It is too early to assess whether any measures taken at [BK Salmon] have been effective in controlling the outbreak,” ​they said.

“However, experiences from previous investigations suggest that once L. monocytogenes is detected in one product, the whole production site should be subject to a thorough inspection, and sampling with special attention to all the possible contamination/cross contamination issues before implementing corrective measures.

“Moreover, the risk for L. monocytogenes persistent strains in the production environment requires the close monitoring for several years to ensure the elimination of these.”

At the end of November three genetically linked isolates were also reported in Germany.

BK Salmon told us that action had been taken to determine and eliminate potential threat of contamination within the whole production environment at the earliest stage.

The company added it has undertaken measures to determine the various impacts of the challenges on the product outside the factory.

Investigation of outbreak in Denmark

In Denmark, in August Statens Serum Institut (SSI) identified a genetic cluster of four human L. monocytogenes sequence type (ST) 8 isolates by core genome multilocus sequence typing (cgMLST).

As of late August, the genetic cluster comprised six cases; five confirmed and one probable.

The age of cases ranged from 59 to 96 (median 80) and four were women. All patients had underlying illness and no travel history. One died within 30 days of diagnosis.

Epidemiological investigations including a standard questionnaire on exposures showed all confirmed cases had consumed cold-smoked and/or cured salmon in the 30 days before disease onset. Four had bought salmon in Dansk Supermarked Group.

A comparison between outbreak isolates and 16 L. monocytogenes ST8 food- and environmental isolates in Denmark from 2014 to August 2017 showed the human isolates clustered with a food isolate from cold-smoked salmon, cut and packaged at BK Salmon in Poland (zero to two allelic differences using cgMLST).

L. monocytogenes was detected in July at 110 CFU/g (threshold: 100 CFU/g) at the end of shelf life. The product was widely sold in Denmark and had been sampled by the DVFA in Dansk Supermarked Group as part of a consumer exposure survey.

Because L. monocytogenes concentration was just above the accepted limit and found at the end of product shelf life there was no recall of this batch. However, due to the positive finding, follow-up sampling was performed in August from the central storage unit of the retail chain. L. monocytogenes had been isolated from two batches analysed before end of shelf life.

In one sample from the same batches, which was analysed at the end of the shelf life, a L. monocytogenes level of 240 CFU/g was found. Isolates from the follow-up samples had zero to four allelic differences to the human outbreak isolates using cgMLST.

Dansk Supermarked Group recalled cold-smoked and cured salmon. The implicated batches were only sold in Denmark at the retail chain.

BK Salmon said the two exceeded levels were found in retail packs from a different lot of one product face and out of the 20 faces the company supplied to the retailer. Both were past the expiry on the day of the claim.

Contamination has been found in one face (Baadsmand Roget 125g) in two different lots.

“Massive testing of lots produced by BK Salmon SA, which were subsequently to the claim, taken from the Danish market and tested by the Danish food authorities eventually did not determine any poisoning threat.

"One positive lot determined by one test only (110 cfu/g) not five which is commonly acknowledged methodology. Also the very method of testing/count of cfu is not accredited. Second test was based on five tests (one positive = 240cfu/g) [but the] method [is] not accredited.

“After several week long multilevel investigation (by the authorities and external audits) our processing plant has been declared compliant with EU food safety systems.”

The company has, thanks to several years of cooperation with the retailer, developed more than 20 products, produced and delivered to the retailer two or three times a week.

A 'product' is a specific name and pack size (e.g. Budget Roget 160g; Budge Gravad 160g counted as two 'faces').

French link

The French National Reference Centre (NRC) for Listeria (Institut Pasteur) compared the sequences of the Danish human isolates against its database using cgMLST.

An isolate from a French resident belonged to the same cluster as Danish isolates. This probable case, a female in her mid-80s was diagnosed in June 2016. Santé Publique France was unable to find consumption history and information on travel to Denmark as the person had since died.

An official control by the Ministry of Economy at a French retailer sampled kosher chilled cured salmon. It was contaminated with L. monocytogenes at 460 CFU/g.

An isolate sent to the French NRC for typing belonged to the same cgMLST type as the Danish outbreak. Investigations confirmed it had not been further processed after production in Poland. The product was recalled and no illnesses were linked to it as of early December.

The EPIS-FWD platform allowed for the communication to link cases across borders.

“However, currently cross-border outbreaks are only detected when case numbers in at least one country exceed normal levels and are notified internationally,” ​said researchers.

“Therefore, a possible future system for easy exchange of and comparison of WGS data, e.g. by the use of an agreed cgMLST nomenclature, across borders will enable the identification of more dispersed outbreaks as well as cross-border links between food samples and human infections.”

Source: Eurosurveillance Volume 22, Issue 50, 14 Dec 2017

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

Authors: 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

Related topics Food Safety & Quality

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