Skip to main navigation menu Skip to main content Skip to site footer

Original article

Vol. 154 No. 5 (2024)

Rapid detection of the source of a Listeria monocytogenes outbreak in Switzerland through routine interviewing of patients and whole-genome sequencing

  • Cornelia Speich
  • Roger Stephan
  • Nicola Dhima
  • Florian Hollenstein
  • Jule Horlbog
  • Giulia Delvento
  • Ekkehardt Altpeter
  • Meike Zuske
  • Michelle Raess
  • Helena Greter
Cite this as:
Swiss Med Wkly. 2024;154:3745


AIMS OF THE STUDY: Listeriosis is a notifiable disease in Switzerland. In summer 2022, the Swiss Federal Office of Public Health noticed an increase in reports of listeriosis cases, indicating a possible ongoing outbreak. Here we present the approaches applied for rapidly confirming the outbreak, detecting the underlying source of infection and the measures put in place to eliminate it and contain the outbreak.

METHODS: For close surveillance and early detection of outbreak situations with their possible sources, listeriosis patients in Switzerland are systematically interviewed about risk behaviours and foods consumed prior to the infection. Listeria monocytogenes isolates derived from patients in medical laboratories are sent to the National Reference Laboratory for Enteropathogenic Bacteria and Listeria, where they routinely undergo whole-genome sequencing. Interview and whole-genome sequencing data are continuously linked for comparison and analysis.

RESULTS: In summer 2022, 20 patient-derived L. monocytogenes serotype 4b sequence type 388 strains were found to belong to an outbreak cluster (≤10 different alleles between neighbouring isolates) based on core genome multilocus sequence typing analysis. Geographically, 18 of 20 outbreak cases occurred in northeastern Switzerland. The median age of patients was 77.4 years (range: 58.1–89.7), with both sexes equally affected. Rolling analysis of the interview data revealed smoked trout from a local producer as a suspected infection source, triggering an on-site investigation of the production facility and sampling of the suspected products by the responsible cantonal food inspection team on 15 July 2022. Seven of ten samples tested positive for L. monocytogenes and the respective cantonal authority ordered a ban on production and distribution as well as a product recall. The Federal Food Safety and Veterinary Office released a nationwide public alert covering the smoked fish products concerned. Whole-genome sequencing analysis confirmed the interrelatedness of the L. monocytogenes smoked trout product isolates and the patient-derived isolates. Following the ban on production and distribution and the product recall, reporting of new outbreak-related cases rapidly dropped to zero.

CONCLUSIONS: This listeriosis outbreak could be contained within a relatively short time thanks to identification of the source of contamination through the established combined approach of timely interviewing of every listeriosis patient or a representative and continuous molecular analysis of the patient- and food-derived L. monocytogenes isolates. These findings highlight the effectiveness of this well-established, joint approach involving the federal and cantonal authorities and the research institutions mandated to contain listeriosis outbreaks in Switzerland.


  1. Bundesamt für Gesundheit (BAG). Listeriose - Jährliche Fallmeldungen und Inzidenzen der letzten 10 Jahre und aktuelles Jahr bis Woche 39/2022 [updated 04.10.2022].
  2. Krämer J, Prange A. Lebensmittel-Mikrobiologie. 7th ed. Stuttgart: Utb GmbH; 2016.
  3. Werber D, Hille K, Frank C, Dehnert M, Altmann D, Müller-Nordhorn J, et al. Years of potential life lost for six major enteric pathogens, Germany, 2004-2008. Epidemiol Infect. 2013 May;141(5):961–8. 10.1017/S0950268812001550 DOI:
  4. Barton Behravesh C, Jones TF, Vugia DJ, Long C, Marcus R, Smith K, et al.; FoodNet Working Group. Deaths associated with bacterial pathogens transmitted commonly through food: foodborne diseases active surveillance network (FoodNet), 1996-2005. J Infect Dis. 2011 Jul;204(2):263–7. 10.1093/infdis/jir263 DOI:
  5. Malinverni R, Bille J, Perret C, Regli F, Tanner F, Glauser MP. Listériose épidémique. Observation de 25 cas en 15 mois au Centre hospitalier universitaire vaudois. Schweiz Med Wochenschr. 1985 Jan;115(1):2–10.
  6. Lamond NM, Freitag NE. Vertical Transmission of Listeria monocytogenes: Probing the Balance between Protection from Pathogens and Fetal Tolerance. Pathogens. 2018 May;7(2):52. 10.3390/pathogens7020052 DOI:
  7. Angelo KM, Jackson KA, Wong KK, Hoekstra RM, Jackson BR. Assessment of the Incubation Period for Invasive Listeriosis. Clin Infect Dis. 2016 Dec;63(11):1487–9. 10.1093/cid/ciw569 DOI:
  8. Vivant AL, Garmyn D, Piveteau P. Listeria monocytogenes, a down-to-earth pathogen. Front Cell Infect Mi; 2013. p. 3. DOI:
  9. Baumgartner A, Schmid H. Listeria monocytogenes in genussfertigen Lebensmitteln: eine Auswertung der amtlichen Untersuchungen in der Schweiz der Jahre 2006-2008. Journal für Verbraucherschutz und Lebensmittelsicherheit / Journal of Consumer Protection and Food Safety. 2013;8:109-17. DOI:
  10. Nüesch-Inderbinen M, Bloemberg GV, Müller A, Stevens MJ, Cernela N, Kollöffel B, et al. Listeriosis Caused by Persistence of Listeria monocytogenes Serotype 4b Sequence Type 6 in Cheese Production Environment. Emerg Infect Dis. 2021 Jan;27(1):284–8. 10.3201/eid2701.203266 DOI:
  11. U.S. Food & Drug Administration. Water Activity (aw) in Foods
  12. Kiczorowska B, Samolińska W, Grela ER, Bik-Małodzińska M. Nutrient and Mineral Profile of Chosen Fresh and Smoked Fish. Nutrients. 2019 Jun;11(7):1448. 10.3390/nu11071448 DOI:
  13. Lachmann RA-O, Halbedel SA-O, Lüth SA-O, Holzer A, Adler M, Pietzka A, et al. Invasive listeriosis outbreaks and salmon products: a genomic, epidemiological study. (2222-1751 (Electronic)).
  14. European Food Safety Authority (EFSA). Foodborne outbreaks - dashboard
  15. European Food Safety Authority; European Centre for Disease Prevention and Control. The European Union One Health 2020 Zoonoses Report. EFSA J. 2021 Dec;19(12):e06971. DOI:
  16. Halbedel S, Sperle I, Lachmann R, Kleta S, Fischer MA, Wamp S, et al. Large multi-country outbreak of invasive listeriosis by a Listeria monocytogenes ST394 clone linked to smoked rainbow trout, 2020-2021 [in revision]. Microbiol Spectr. 2023 Jun;11(3):e0352022. 10.1128/spectrum.03520-22 DOI:
  17. Hoffman AD, Gall KL, Norton DM, Wiedmann M. Listeria monocytogenes contamination patterns for the smoked fish processing environment and for raw fish. J Food Prot. 2003 Jan;66(1):52–60. 10.4315/0362-028X-66.1.52 DOI:
  18. Swiss Federal Act on Foodstuffs and Utility Articles, Stat. SR 817.0 (20Jun2014, 2014).

Most read articles by the same author(s)