Norwegian authorities found problems with the sampling plan of a company linked to a Listeria outbreak in 2022, according to a report.

At an inspection in October, the Norwegian Food Safety Authority (Mattilsynet) discovered the fish producer’s sampling plan was not sufficient, and that environmental samples had not been taken in line with Troll Salmon’s internal program.

During the visit, the agency took 14 environmental samples and the company also conducted sampling afterward. One of the authority’s tests was positive and the producer detected Listeria in two different drain samples.

The firm was ordered to take daily samples for an undefined time period and to updat the sampling plan. It also carried out additional cleaning and disinfection of premises and equipment as well as work to find the source of the outbreak strain.

Five people sick
The Veterinary Institute helped the Norwegian Institute of Public Health (FHI) and the Norwegian Food Safety Authority in the investigation.

From April to October 2022, the National Institute of Public Health detected listeriosis in five people. Sample dates ranged from February to October.

Cases were two women and three men aged 50 to 95 years old with a median age of 72. They lived in four counties: Nordland, Trøndelag, Viken, and Oslo, and all were hospitalized.

After patient interviews by the Norwegian Food Safety Authority in October, smoked fish from Troll Salmon was considered a possible source of infection. Three people reported smoked salmon or trout consumption and two named smoked salmon from this producer. Two patients were too ill to be fully interviewed, but one of them confirmed eating smoked salmon from this producer prior to becoming too sick.

Listeria had been found in two smoked salmon products from the company earlier in the year, but at a concentration below the legal limit, so they were not recalled. Items were obtained from a store in April and analyzed on the last day of shelf life in May 2022.

Samples were taken during the monitoring of ready-to-eat food in shops in 2022, a program under the Norwegian Food Safety Authority. Sequences were shared with FHI, who then compared them with patient isolates.

Possible past problems
The discovery of Listeria monocytogenes in environmental samples from the producer coincided with the detection of the fifth case in the outbreak, and the Norwegian Food Safety Authority ordered the firm to withdraw certain products from the market in October.

It was later found these isolates had a different profile to the outbreak strain but were similar to those taken from smoked salmon from the producer in the Norwegian Food Safety Authority’s monitoring program for ready-to-eat products, as well as historical patient isolates.

Listeria monocytogenes that resemble the outbreak strain were previously seen among cases in Norway from 2010, 2014, and 2018. Similar Listeria isolates have been found in one sample from the Norwegian Food Safety Authority’s monitoring program of sliced salmon, in environmental samples from the producer and patient samples from 2010 to 2015. However, a lack of epidemiological data from past patients means it is unclear if there is a link or not.

No Listeria monocytogenes was found in an unopened pack of Troll’s smoked salmon, taken from the refrigerator of one of the patients. Officials did not report whether the patient had consumed fish from a different package.

Discovery of Listeria in food or at production premises is not subject to mandatory reporting in Norway, but legislation sets criteria for the amount permitted in ready-to-eat food.

The low number of cases may be due to the fact that the detected sequence type has a low virulence and the concentration in food was initially low, said health officials.

No other countries that responded to a request for data had cases with the same genotype as the outbreak strain from Norway.