Characterisation of Salmonella Typhimurium from a fatal equine nosocomial outbreak and retrospective analysis of equine clinic salmonellosis cases (2010–2025)

In 2024, a highly fatal outbreak of equine salmonellosis occurred in a Hungarian equine referral hospital, resulting in the death or euthanasia of four out of five affected horses. Salmonella (S.) enterica subsp. enterica serovar Typhimurium was identified as the primary causative agent from equine faecal, reflux, and post-mortem intestinal content samples, while one case involved S. Coeln. Extensive environmental sampling during the outbreak also yielded multiple Salmonella serovars. Whole-genome sequencing revealed a high degree of genetic relatedness among the S. Typhimurium isolates, confirming nosocomial transmission. The source of the isolated S. Typhimurium was most likely a 3-year-old gelding imported immediately before the admission to the hospital. The isolates belonged to sequence type ST376 and exhibited multidrug resistance, including extended-spectrum β-lactamase and fluoroquinolone resistance genes. Retrospective analysis of microbiological records from 2010 to mid-2024 identified 23 Salmonella-positive equine cases involving eight serovars and three probable nosocomial clusters preceding the 2024 outbreak. Following the outbreak, enhanced passive surveillance was implemented between October 2024 and August 2025. During this period, 56 at-risk horses were examined using selective bacteriological testing of clinical and post-mortem samples, of which 11 (19.6%) were Salmonella-positive, representing eight different serovars. A distinct cluster of S. Martonos was detected, and six of the surveillance-associated cases resulted in fatal outcomes. These findings demonstrate that Salmonella is repeatedly introduced into the equine hospital environment and that serovars differ markedly in virulence and transmission dynamics. The exceptionally high case fatality observed during the 2024 outbreak underscores the importance of integrated genomic surveillance, rapid diagnostics, and sustained infection control measures to mitigate the risk of severe nosocomial salmonellosis in equine clinics.