Listeria can be found in salmon production environments and on fish, according to a study in Norway.

Researchers said fish with low levels of Listeria monocytogenes could enter the downstream supply so maintaining the cold chain during transportation and further processing is crucial to prevent growth in the final products.

The Institute of Marine Research screened for Listeria monocytogenes in salmon slaughter facilities between September 2020 and August 2021 with samples collected by inspectors from the Norwegian Food Safety Authority (Mattilsynet) as part of broader work to understand more about Listeria in the sector.

The EU has a maximum limit for Listeria monocytogenes in ready-to-eat foods intended for healthy adults of 100 colony forming units per gram of sample at the end of the expiry date.

Detections on fish and in environment
In total, 358 samples from 60 slaughter facilities, made up of 49 plants and 11 vessels, were examined. More than 100 samples were collected from the production environment and 250 from fish entering the facility and raw material at the end.

None of the 21 samples from the slaughtering vessels were positive for Listeria monocytogenes but 22 positives were detected in nine different slaughtering plants. In five of these sites, several positive samples were found.

Six slaughtering plants had positive samples at the end of the production line from either the fish surface or in raw material. However, at this stage fish is only ready for further processing and is not on its way to retail.

Four slaughtering plants had one positive sample, one had two positives, two had three positives and one had four positive samples, while one plant had six of seven samples positive for Listeria monocytogenes.

Listeria was detected in the production environment in the bleeding, fileting or packaging area. Nine positive samples were from a water hose handle, parts of the slaughter machine, manual slaughtering equipment, fileting machine parts, conveyor belts, knives and cutting boards, and a sorting table. Such situations can lead to cross-contamination of food products.

A higher prevalence was found when swabbing the fish skin and gills compared to the examined raw material. All raw material samples had numbers of Listeria monocytogenes below the quantification limit of 10 CFU/g.

Audit findings and future work
Inge Erlend Næsset, from the Norwegian Food Safety Authority, said as some fish are eaten without heat treatment and used for ready-to-eat products such as sushi and sashimi it is crucial that producers control Listeria.

“It is important that the salmon slaughterhouse continuously monitors Listeria and at all times has effective hygiene measures,” Næsset said.

This year, the Norwegian Food Safety Authority ran a campaign in which 63 establishments and vessels that slaughter salmon were inspected for measures and routines to prevent fish from becoming contaminated with Listeria.

This was done because of serious listeriosis outbreaks in Europe in 2018 and 2019 that affected 34 people and killed nine. They were linked to fish from processing sites in Poland and Estonia with Norwegian raw materials also possibly contaminated.

The majority of sites had good hygiene standards but 18 companies were warned about things such as their control system, including hazard analysis, sampling plan and measures to detect Listeria. One business was closed because of a lack of maintenance and cleanliness.

The Norwegian Food Safety Authority has also started inspecting approved producers of ready-to-eat seafood to see how well they control Listeria. The campaign began in November and lasts until April 2022.