One Health Investigation of a Household Salmonella Thompson Outbreak in Italy: Genomic and Epidemiological Characterization of an Emerging Serotype

<i>Salmonella</i> is a Gram-negative enteric bacterium responsible for the foodborne and waterborne disease salmonellosis, which was the second most reported foodborne gastrointestinal infection in humans in the European Union in 2023. Animals represent the principal reservoir of this pathogen, with animal-derived food products serving as the main route of transmission to humans. In a household context, having numerous animals can be a crucial factor for contracting <i>Salmonella</i> spp. infection. In the present study, we report a case of a familiar outbreak of <i>Salmonella</i> Thompson that occurred in 2024 in central Italy, involving an infant and the companion animals (a dog, a cat and ten birds) of the family's farm. To support the epidemiological investigations, antimicrobial susceptibility testing and whole-genome sequencing (WGS) were conducted on strains from the human case and from animals. Eleven strains were isolated in total, from fecal samples collected from the child and the animals at different times. WGS confirmed the genetic relatedness between human and animal isolates, supporting the hypothesis of a shared source of infection, but genes or plasmid involved in antibiotic resistance were not found. Moreover, AST revealed that isolates were fully susceptible to major antimicrobial classes tested. Despite being an uncommon serotype, the involved <i>Salmonella</i> Thompson serovar 6,7: k:1,5 O:7 (C1) demonstrated a high pathogenic potential, emphasizing the need for vigilance even toward serotypes not typically associated with major public health concerns. Moreover, these findings underscore the critical need for an integrated One Health approach to effectively monitor, prevent, and control zoonotic infections.