Swiss authorities have revealed a Listeria outbreak sickened 20 and killed one person in 2022.

The Federal Office of Public Health (OFSP), Federal Food Safety and Veterinary Office (OSAV), and cantonal, or regional, authorities detected an outbreak of listeriosis in July 2022 and identified smoked trout as the source.

In early July, an unusually high number of Listeria monocytogenes cases were reported to OFSP. Whole genome sequencing (WGS) confirmed they were linked.

As part of an investigation carried out with OFSP, patients were interviewed and evidence pointed to smoked trout produced by one company.

Cantonal food authorities carried out an inspection of the firm in the canton of Thurgovie, also known as Thurgau, which found the bacteria in smoked trout and in the production environment. WGS matched the isolates from food samples and patients, confirming the link.

In July, smoked fish from the company Kundelfingerhof was withdrawn from sale and recalled from stores such as Coop, Migros, and Volg.

Of the 20 people affected, 10 were men and 10 were women. They were between 58 and 89 years old but half of them were older than 75. The first two cases were in April 2022 but the majority were reported in June and July.

Zurich was the canton with the most infections with nine while 18 patients came from the northeast of the country. Nineteen people were hospitalized and one person died.

Production stopped and improvements made
During patient interviews, 17 people mentioned eating fish before falling sick, 11 had consumed meat products, 15 had eaten smoked trout and a dozen people mentioned the same company.

In mid-July, OFSP informed OSAV about the outbreak, who passed information to the cantonal laboratory of Thurgovie, which then checked Kundelfingerhof. Listeria was detected from various product samples. They were later found to match those from patients in the outbreak.

Products were recalled, OSAV issued a public warning, and production was temporarily stopped. Authorities said the cause of the contamination was determined and eliminated. Kundelfingerhof has since adapted its self-checking measures and restarted production.

Kundelfingerhof reported the financial impact was more than a quarter of a million Swiss francs (around $271,000), which was higher than first estimated but part of it was covered by insurance.

Operations at the fish smokehouse were halted for about two months but the all-clear was given in September 2022 after measures that included lab testing.

With the involvement of external experts, structural changes were made, the fish processing site was cleaned and decontaminated plus monitoring was intensified.