Outbreak of gastroenteritis in a nursery school caused by a strain of Salmonella enterica serovar Typhimurium carrying the hybrid virulence-resistance plasmid pUO-StVR2

OBJECTIVE. Epidemiological and microbiological study of a salmonellosis outbreak, affecting 22 children in a nursery school in Oviedo (Spain). METHODS. Attack rates and epidemic curves were determined, and bacterial typing methods were applied. RESULTS. The outbreak was attributed to a Salmonella enterica serovar Typhimurium strain, belonging to an emergent type characterized by the presence of a hybrid virulence-resistance plasmid of 125-130 kb, named pUO-StVR2. The attack rate of confirmed cases vs. possible cases was 27.2% vs. 23.5% for the children and 0 vs. 26.5% for the staff of the affected center. The source of the infection could not be identified. Nevertheless, according to the evolution of the cases over time, the transmission route was likely to be personal contact between the staff and children, which facilitates fecal-oral dissemination. All but one of the 27 isolates analyzed (from 22 patients) showed identical features: R-profile, plasmid-profile, RAPD-type, PFGE-type; all were non-phage-typeable, with the exception of a DT104b isolate. pUO-StVR2 is probably a derivative of the virulence plasmid pSLT from the LT2 type strain that acquired an R-region complex (ACSSuT/bla(OXA)-catA1-strA/ B-aadA1-sul1-sul2-tet[B]), in which the bla(OXA)-aadA1 genes are part of the variable region of a class 1 integron. CONCLUSION. This outbreak is an example of how a Salmonella enterica serovar Typhimurium strain belonging to a type that is probably endemic in Spain can be transferred to the community and affect a susceptible population.