A major Listeria outbreak in Switzerland has sickened more than thirty people and killed seven people since 2022.
The first case was reported in April 2022 and the outbreak peak occurred in 2023. The last sick person became ill in June 2024.
An article in the Emerging infectious diseases magazine revealed that the average age of all 34 patients was 79, ranging from less than 1 to 93 years old. Eighteen were women and 16 men. The distribution was wide, with patients across different cantons.
According to the reporting data, listeriosis was the leading cause of death in all seven cases.
A questionnaire-based outbreak investigation by the Swiss Federal Office of Public Health revealed no specific food exposure.
In 2023, 74 Listeria monocytogenes infections were reported, compared to 78 the year before. This was mainly due to a nationwide outbreak, with six cases in 2022, 23 in 2023 and five in 2024.
Link to producer
All 34 isolates were genetically related to Listeria monocytogenes from baker’s yeast products from a commercial plant and its associated production lines.
Baker’s yeast products were supplied to the food industry throughout Switzerland and sold in stores. In 2022, the manufacturer found Listeria monocytogenes in yeast product samples as part of self-monitoring practices; However, due to the lack of regulatory requirements for non-ready-to-eat (RTE) products, the findings were not reported.
In July 2023, official random and risk-based checks by a cantonal laboratory revealed the presence of Listeria in a yeast product. WGS analysis confirmed this was the outbreak strain. This led to environmental sampling at the manufacturer’s production site and a retrospective WGS analysis of all available isolates.
From July 2023 to August 2024, the producer ordered microbiological tests at various production steps, and product samples or swabs were obtained from the yeast stream, vats, piping systems, drying filters, extruders, conveyors and cutting machines. All sequenced Listeria monocytogenes isolated from the production line and strains from yeast products matched the outbreak strain.
The laboratory identified a flow line connecting a starch vat to the yeast drying bed as the initial contamination point. The manufacturer began a thorough cleaning of all processing equipment at the production line site where the positivity was detected.
Risk of raw yeast and dough
No recall has been issued because baker’s yeast is not classified as an RTE food. However, only lots that had undergone extensive checks and contained less than 10 colony-forming units of Listeria monocytogenes per gram were released for sale. Further investigation showed no growth of Listeria monocytogenes during the shelf life of baker’s yeast.
Products made with baker’s yeast normally undergo a heating step to control contamination. However, as evidenced by the raw dough-related outbreaks caused by E. coli and Salmonella in Canada and the United States, contaminated dough poses a health hazard when consumed raw, and cross-contamination can occur during handling. In the Swiss case, the outbreak strain was also detected in food products in several restaurants and institutional catering companies in different cantons.
“This outbreak, which is likely linked to baker’s yeast, highlights the lack of data on the contamination of non-RTE products by foodborne pathogens, including Listeria monocytogenes, and the need for baker’s yeast manufacturers to consider this risk in their manufacturing processes to take. Furthermore, this outbreak should raise awareness that adhering to basic hygiene measures to prevent cross-contamination is essential when handling non-RTE foods,” researchers said.
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