Scientists detail the investigation into the listeria outbreak in Switzerland in 2022

Researchers have shared details of how a 2022 listeria outbreak in Switzerland that was traced to fish was resolved.

In the summer of 2022, the Federal Office of Public Health (BAG) noticed an increasing number of reports of listeriosis cases, which indicate a possible outbreak.

Twenty patient isolates of Listeria monocytogenes belonged to an outbreak cluster, 18 cases occurred in northeastern Switzerland. The average age of the patients was 77.4 years, with a range of 58 to 89 years, with both genders equally affected. Nineteen people were hospitalized and one person died.

In April 2022, four cases of listeriosis were reported living in the canton of St. Gallen. Samples from two patients were linked to each other and both ate fish. According to a study published in the Swiss Medical Weekly, the last patient tested positive at the end of July.

First signal

Analysis of the interview data revealed that smoked trout from a local producer called Kundelfingerhof was a suspected source of infection, triggering an on-site investigation of the production facility and sampling of the suspected products by the responsible cantonal food control team in mid-July 2022.

Interviews with patients four through eight revealed overlapping information regarding smoked trout consumption, the brand, manufacturer, and retailer from which the trout was purchased, providing the information that led to the investigation of the suspect establishment.

In total, 17 of the 20 outbreak patients reported eating fish. Smoked trout was confirmed in 15 patients and the same smoked trout manufacturer was mentioned in nine patients. According to doctors, another three people ate smoked trout from this company.

Seven out of ten samples tested positive for Listeria monocytogenes and the cantonal authorities ordered a production and distribution ban as well as a product recall. The Federal Office for Food Safety and Veterinary Affairs (OSAV) has issued a nationwide warning for the affected smoked fish products.

Kundelfingerhof supplied several of the region’s major retailers, operated a farm shop and sold products directly to the public at a weekly and online market.

Whole genome sequencing confirmed the relationship between Listeria monocytogenes isolates from smoked trout products and the patient-derived isolates. After the production and distribution ban and the recall, the reporting of new outbreak-related cases fell to zero.

Contamination in the factory

The hygiene measures in the production facility were determined by analyzing 60 swabs from the production environment and revising the company’s self-monitoring policy. A few months later, the production and distribution ban was lifted after Kundelfingerhof took measures to restore its legal status and listeria was no longer detectable in environmental samples and ready-to-eat products.

Listeriosis is a rare disease in Switzerland, with 40 to 60 cases confirmed every year.

All clinical Listeria monocytogenes isolates are whole-genome sequenced at the National Reference Laboratory for Enteropathogenic Bacteria and Listeria (NENT). The Competence Center for Epidemiological Outbreak Investigations (KEA) of the Swiss Tropical and Public Health Institute surveys patients with diagnosed and reported listeriosis using a standardized questionnaire.

“In the current outbreak, cases were reported over a period of almost four months, although the shelf life of the identified food was short. “This indicates an ongoing source of contamination in the production facility rather than contamination of a specific batch,” the scientists said.

“Epidemiological surveillance through timely routine surveys of all listeriosis patients in Switzerland played a key role in quickly identifying the source of contamination and provided the basis for restoring food safety to consumers in this outbreak.”

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