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INTRODUCTION
Salmonella is the most common bacterial cause of foodborne outbreaks and the second most common bacterial cause of foodborne infections in Denmark. In 2010, 1598 cases of salmonellosis were reported, with an incidence of 28·7/100 000 inhabitants, and an estimated 45% of all cases were acquired during travel outside Denmark [1]. The most common serotype of all registered cases was S. Typhimurium (40%) followed by S. Enteritidis (24%) [1]. The food vehicles most commonly associated with Salmonella transmission worldwide are poultry, pork, beef, eggs, and seafood; although in later years, Salmonella outbreaks have been increasingly linked to the consumption of vegetables and fruits [2, 3].
On 22 September 2011 in week 38, three cases of Salmonella enterica serovar Strathcona (antigen formula 6,7:lz13,lz28:1,7) infection were identified at Statens Serum Institut (SSI), Denmark. This was the first identification of this serotype in Denmark. A literature review plus an inquiry through international networks coordinated by the European Centre for Diseases Prevention and Control (ECDC), the European Food Safety Authority (EFSA) and the U.S. Centers for Disease Control revealed that the serotype had first been isolated in 1988 in a patient in the city of Strathcona, Canada. Since then the serotype has only been reported once globally, in 2006, when isolated from a French patient returning from Cameroon with a wound infection. Between 2007 and 2010, no cases were reported to The European Surveillance System (TESSy). Similarly, there were no reports of identification of this serotype from food, feed or animals worldwide. Following the identification of the Danish cases, an outbreak investigation was initiated to reveal the source in order to stop the outbreak.
METHODS
Laboratory characterization
In Denmark, salmonellosis is a laboratory notifiable disease and cases diagnosed by clinical microbiological laboratories must be reported within a week to the SSI. Further, Salmonella isolates are sent to the reference laboratory at SSI for serotyping and genotyping. Serotyping was performed according to the Kauffmann-White-Le Minor scheme [4] and pulsed-field gel-electrophoresis (PFGE) according to the PulseNet protocol using the XbaI restriction enzyme [5]. All isolates were tested for their antimicrobial susceptibility to a panel of antimicrobials by broth microdilution (Sensititre; Trek Diagnostic Systems Ltd, UK).
Descriptive epidemiology
For the Danish investigation, a case was defined...