title: Whole-genome Sequencing Used to Investigate a Nationwide Outbreak of Listeriosis Caused by Ready-to-eat Delicatessen Meat, Denmark, 2014.
abstract: Listeriosis is a serious foodborne infection. Outbreaks of listeriosis occur rarely, but have often proved difficult to solve. In June 2014, we detected and investigated a listeriosis outbreak in Denmark using patient interviews and whole-genome sequencing (WGS).
  
  
  
  We performed WGS on Listeria monocytogenes isolates from patients and available isolates from ready-to-eat foods and compared them using single-nucleotide polymorphism (SNP) analysis. Case patients had L. monocytogenes with ≤3 SNPs (the outbreak strain) isolated in September 2013-December 2014. Through interviews, we established case patients' food and clinical histories. Food production facilities were inspected and sampled, and we performed trace-back/trace-forward of food delivery chains.
  
  
  
  In total, 41 cases were identified; 17 deaths occurred (41%). An isolate from a delicatessen meat (spiced meat roll) from company A was identical to the outbreak strain. Half of the patients were infected while hospitalized/institutionalized; institutions were supplied food by company A. The outbreak strain was repeatedly isolated from further samples taken within this company and within companies in its distribution chain. Products from company A were traced and recalled from >6000 food establishments, after which the outbreak ended.
  
  
  
  Ready-to-eat spiced meat roll from a single production facility caused this outbreak. The product, served sliced and cold, is popular among the elderly; serving it at hospitals probably contributed to the high case-fatality rate. WGS used for patient isolates and isolates from food control inspections, coupled with routine epidemiological follow-up, was instrumental in swiftly locating the source of infections, preventing further illnesses and deaths.
authors.affiliation: Department of Infectious Disease Epidemiology Department of Microbiology and Infection Control, Statens Serum Institut.
authors.forename: Anne
authors.initials: A
authors.lastname: Kvistholm Jensen
authors.affiliation: Department of Microbiology and Infection Control, Statens Serum Institut.
authors.forename: Eva Møller
authors.initials: EM
authors.lastname: Nielsen
authors.affiliation: Department of Microbiology and Infection Control, Statens Serum Institut.
authors.forename: Jonas Torgny
authors.initials: JT
authors.lastname: Björkman
authors.affiliation: Department of Danish Veterinary and Food Administration, Copenhagen.
authors.forename: Tenna
authors.initials: T
authors.lastname: Jensen
authors.affiliation: Department of Infectious Disease Epidemiology.
authors.forename: Luise
authors.initials: L
authors.lastname: Müller
authors.affiliation: Department of Microbiology and Infection Control, Statens Serum Institut.
authors.forename: Søren
authors.initials: S
authors.lastname: Persson
authors.affiliation: Department of Danish Veterinary and Food Administration, Copenhagen.
authors.forename: Gitte
authors.initials: G
authors.lastname: Bjerager
authors.affiliation: Department of Danish Veterinary and Food Administration, Copenhagen.
authors.forename: Annette
authors.initials: A
authors.lastname: Perge
authors.affiliation: Department of Infectious Disease Epidemiology.
authors.forename: Tyra Grove
authors.initials: TG
authors.lastname: Krause
authors.affiliation: Department of Microbiology and Infection Control, Statens Serum Institut.
authors.forename: Kristoffer
authors.initials: K
authors.lastname: Kiil
authors.affiliation: Department of National Food Institute, Technical University of Denmark, Lyngby.
authors.forename: Gitte
authors.initials: G
authors.lastname: Sørensen
authors.affiliation: Department of National Food Institute, Technical University of Denmark, Lyngby.
authors.forename: Jens Kirk
authors.initials: JK
authors.lastname: Andersen
authors.affiliation: Department of Infectious Disease Epidemiology.
authors.forename: Kåre
authors.initials: K
authors.lastname: Mølbak
authors.affiliation: Department of Infectious Disease Epidemiology Department of Microbiology and Infection Control, Statens Serum Institut.
authors.forename: Steen
authors.initials: S
authors.lastname: Ethelberg
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D015331:Cohort Studies; D003718:Denmark / Q000453:epidemiology; D004196:Disease Outbreaks / Q000706:statistics & numerical data*; D005260:Female; D005517:Foodborne Diseases / Q000453:epidemiology / Q000382:microbiology; D016680:Genome, Bacterial / Q000235:genetics*; D059014:High-Throughput Nucleotide Sequencing; D006801:Humans; D008089:Listeria monocytogenes / Q000235:genetics*; D008088:Listeriosis / Q000453:epidemiology / Q000382:microbiology; D008297:Male; D008460:Meat / Q000382:microbiology*; D008875:Middle Aged; D020641:Polymorphism, Single Nucleotide / Q000235:genetics
keywords: Listeria monocytogenes; bacterial genome; foodborne diseases; high-throughput nucleotide sequencing; outbreak investigation