Investigation of an outbreak of human salmonellosis caused by Salmonella enterica ssp enterica serovar Infantis by use of pulsed field gel electrophoresis

Analysis of chromosomal DNA restriction patterns produced by pulsed field gel electrophoresis (PFGE) was used to investigate an outbreak of human salmonellosis caused by Salmonella enterica ssp. enterica serovar Infantis (S. Infantis) involving more than 500 registered human cases. The outbreak had been tentatively traced back to a single pig slaughterhouse. A total of 135 isolates from various sources produced 21 different PFGE patterns with the restriction endonuclease XbaI. All human isolates from the outbreak belonged to a single type, the 'EPI-type', whereas human isolates recovered before and after the outbreak belonged to several different types. All isolates investigated from the suspect pig slaughterhouse and its supplier pig herds belonged to the EPI-type. Isolates from pork from the central meat market in Copenhagen, which received most of the carcasses from the suspect slaughterhouse, also belonged to the EPI-type. This was furthermore, the case for isolates from beef from the same market, indicating that cross-contamination had taken place. All isolates from pork and some, but not all, isolates from beef, collected in butchers shops during the outbreak belonged to the EPI-type. The typing results supported that the outbreak was a common source outbreak, probably originating from a limited number of supplier pig herds supplying animals to a single slaughterhouse.