Researchers have shared details about how a Listeria outbreak in Switzerland in 2022 traced to fish was solved.

In the summer of 2022, the Federal Office of Public Health (OFSP) noticed increased reports of listeriosis cases, indicating a possible outbreak.

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

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

Initial signal

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

Interviews with patients four to eight revealed overlapping information on the consumption of smoked trout, the brand, producer, and retailer wher trout had been purchased, and provided the information that led to the audit of the suspected facility.

In total, 17 of the 20 outbreak patients mentioned consumption of fish. Smoked trout was confirmed for 15 patients, and nine named the same producer for smoked trout. According to information from physicians, another three people ate smoked trout from this firm.

Seven of 10 samples tested positive for Listeria monocytogenes, and the cantonal authority ordered a ban on production and distribution and a product recall. The Federal Food Safety and Veterinary Office (OSAV) issued a nationwide public alert for the smoked fish products concerned.

Kundelfingerhof supplied several of the main retailers in the area, ran a farm shop, and sold products directly to the public at a weekly and online market.

Whole-genome sequencing confirmed the relatedness of Listeria monocytogenes smoked trout product isolates and the patient-derived isolates. Following the ban on production and distribution and the recall, reporting of new outbreak-related cases dropped to zero.

Contamination at the plant

Sanitation measures at the manufacturing facility were informed by analyzing 60 swabs from the production environment and revising the company’s self-monitoring policy. Several months later, the ban on production and distribution was lifted after Kundelfingerhof took action 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 Centre for Epidemiological Outbreak Investigations (KEA) at the Swiss Tropical and Public Health Institute interviews patients diagnosed and reported with listeriosis using a standardized questionnaire.

“In the present outbreak, related cases were reported over nearly four months despite the short shelf life of the food item identified. This indicates a persistent contamination source in the production facility rather than contamination of a specific batch,” said scientists.

“Epidemiological surveillance through timely routine interviewing of all listeriosis patients in Switzerland played a key role in rapidly identifying the source of contamination and provided the basis for re-establishing food safety for consumers in this outbreak.”