A multi-year Listeria outbreak in Switzerland that killed 10 people was linked to pasteurized cheese products, according to a study.

The incident highlights the risk for recontamination of cheese during manufacturing and emphasizes the need for routine sampling of products, manufacturing equipment, and the production environment, said researchers in the journal Emerging Infectious Diseases.

The two year outbreak of Listeria monocytogenes also caused 34 laboratory-confirmed cases. The nationwide incident was traced to persistent environmental contamination of a dairy with Listeria monocytogenes. Results of an investigation implicated the Käserei Vogel dairy that had sanitation shortcomings at the production site in Steinerberg, Schwyz.

Initial investigation didn’t find source
In 2018, the Swiss Federal Office of Public Health (SFOPH) recorded 52 cases of listeriosis. However, from March 6 to July 31, 2018, an increase of Listeria monocytogenes serotype 4b with 13 cases was recorded. Whole genome sequencing (WGS) was performed on these strains. Twelve of 13 isolates were closely related.

An outbreak investigation was started by the SFOPH and patients were contacted to assess food exposures by using a standardized questionnaire. However, the investigation did not lead to a suspect food, and the vehicle of infection remained unknown.

In a second wave, onset dates ranged from Jan. 22 to May 26, 2020. Another 27 infections with Listeria monocytogenes serotype 4b were recorded; four were hospital patients who had underlying conditions. Questionnaire-based data were not available to support a food hypothesis. A total of 22 strains grouped in a tight cluster on the basis of WGS.

Median age of the patients was 81 with a range of less than 1 to 99 years old. Eighteen of 34 affected people were women. One case of transmission from the mother to the baby close to the birth and 10 deaths were reported.

In late April 2020, Käserei Vogel reported to the cantonal laboratory it had detected Listeria monocytogenes from a sample of soft (brie) cheese made from pasteurized milk. Analysis was part of the manufacturer’s routine quality controls, which are mandatory in Switzerland. The cheese isolate matched the outbreak strain when compared by WGS.

WGS helped show close relatedness between isolates from cheese and the environment, and link the listeriosis cases from 2018 to the 2020 outbreak.

Isolation and WGS typing of Listeria monocytogenes from this quality control cheese sample provided crucial information that enabled the origin of contamination to be identified, according to researchers.

Sampling findings
Authorities traced the distribution chain of the dairy. The producer supplied several buyers who provided cheese to retailers throughout Switzerland. Buyers were told to stop delivery of products from the producer.

Findings prompted environmental sampling at the production site of Käserei Vogel. A total of 50 swab specimens from vats, cheese harps, skimming devices, sink drains, brushes, scrub sponges, trays, door handles, ripening cellar floors, and walls were taken.

Listeria monocytogenes was identified in 11 environmental samples, and all five sequenced isolates matched the outbreak strain. These results led to a recall in early May of 26 items, including brie, sheep and goat cheese, and organic cheeses and production was stopped.

After the recall of implicated products and a public warning by the Federal Food Safety and Veterinary Office, seven cases of listeriosis caused by the outbreak strain were recorded. The last known patient was sampled on May 20 and reported to SFOPH on May 25, 2020.

In August, the Schwyz Public Prosecutor’s Office opened criminal proceedings in connection with the incident to investigate allegations against the owner of the cheese firm.