Danish officials have solved a Listeria outbreak that had been ongoing for three years.

Cold-smoked fish products imported from Estonia are the source of the long-lasting outbreak, which had made nine people sick and linked to two deaths since 2016.

Statens Serum Institut (SSI), the Danish Veterinary and Food Administration (Fødevarestyrelsen) and DTU Food – National Food Institute reported that two of the nine cases who became ill with the same type of Listeria were in 2019.

Luise Müller from SSI told Food Safety News that the first case fell sick in December 2016, five cases were reported in 2017, one in 2018 and two in 2019.

“Listeria was found in product from the Danish importer which is produced in Estonia, we don’t know what happened in the production facilities there. We know it is at the Estonian producer as it is not handled or repackaged in Denmark, it is only relabelled and sent to the Danish consumer. We cannot exclude that more cases could come due to the long incubation period of listeriosis,” Müller said.

Cold-smoked fish products have now been recalled by two Danish retailers. The Danish Veterinary and Food Administration advised consumers that the items should be discarded or returned to the store where they were purchased.

The Danish importer, Food With You, issued a recall of smoked trout after Listeria was found. All dates of “VORES Koldrøget Røget Ørred” 200-gram sold at Bilka and Føtex across the country are affected.

Dagrofa also recalled smoked trout due to the presence of Listeria. All dates from 14 to 27 March, 2019 of Blue Ocean røget ørred 200-gram sold at MENY stores throughout Denmark are involved.

Patients include four women and five men aged 52 to 90 years old who are spread across the country. Two people died. All patients in the outbreak had serious illnesses that put them at high risk for serious infections.

“We know they died within 30 days from the positive test for listeriosis but since they are all immuno-suppressed people or people with underlying disease we don’t know if they died from listeriosis,” said Müller.

Based on interviews with patients and after finding Listeria in samples taken from the Danish importer, the source of infection was traced to cold-smoked trout and salmon produced in Estonia. These items were sold in Denmark through two retailers; Salling Group and Dagrofa.

Müller said whole genome sequencing was used in the investigation.

“The outbreak was declared in January 2017 when we had three matching cases. It is a part of our regular surveillance that we sub-type all human isolates to see if any of them are within the same cluster and we interview all patients with listeriosis in Denmark regardless of the subtyping so all this information has been revised along the way but it is not until now that we have the exact source,” she said.

“We are in dialogue with the European Centre for Disease Prevention and Control and other countries to find out if there are any human cases or findings from food isolates in other countries but this is ongoing at the moment.”

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