Typing of Salmonella enterica serovar Saintpaul: An outbreak investigation

During the summer of 1993 an outbreak of human salmonellosis caused by Salmonella serovar Saintpaul occurred in Denmark. A total of 35 isolates originating from pigs, turkeys and imported foodstuffs, and 10 human isolates were compared following their characterization by agglutination of the O:5 factor, antibiogram typing, plasmid profiling, ribotyping and pulsed field gel electrophoresis, in order to identify the most probable source of infection. After typing, the source of the investigated outbreak remains obscure because so far no isolates with traits of the outbreak strain have been recovered from production animals. Presence of the O:5 factor and absence of plasmids in human and porcine isolates pointed to pork as the source of infection, whereas human isolates and all Danish isolates from turkeys had the same ribotype, indicating that turkey was the infection source. A possible explanation for the failure to find isolates with traits of the outbreak strain could be the presence of a third, but so far unidentified, source. The present investigation illustrates the necessity of using more than one epidemiological typing method for outbreak investigation. This is especially important when the organism involved is relatively uncommon and little is known about its diversity and distribution.