Report on Persistent Listeria Contamination Issue in a Swiss Cheese Facility

An article published in Emerging Infectious Diseases titled Listeriosis Caused by Persistence of Listeria monocytogenes Serotype 4b Sequence Type 6 in Cheese Production Environment discusses an ongoing Listeria outbreak in Switzerland that occurred in two waves, 2018 and 2020.

"Of the 34 human isolates, 30 were from blood samples and 1 each from an abscess, ascites, maternal placenta tissue, or stool sample (Table). One case of perinatal transmission and 10 deaths (29%) were reported."

A Swiss cheese manufacturer had reported the detection of Listeria monocytogenes in the facility and this was matched to the outbreak strain using whole genome sequencing.  "These findings prompted extensive environmental sampling on the production site of the manufacturer. A total of 50 swab specimens from locations, such as vats, cheese harps, skimming devices, sink drains, brushes, scrub sponges, trays, door handles, ripening cellar floors, and walls were obtained....L. monocytogenes was identified in 11 (22%) of 50 environmental samples, and all 5 sequenced isolates matched the outbreak strain CT (Table; Figure 2). These results lead to a recall on May 5, 2020, of 26 items, including brie, sheep and goat cheese, and organic cheeses; production was stopped immediately. " 

In conclusion, the report states "This outbreak highlights the risk for recontamination of pasteurized cheese products during manufacturing and emphasizes the need for routine sampling of products, manufacturing equipment, and the production environment. Routine quality controls should include WGS typing of environmental L. monocytogenes isolates to enable early recognition of potential food contamination and to ultimately mitigate the risk for listeriosis."