Genomic characterization of Salmonella enterica serotype Saintpaul isolates from horses.

Overview

• This research examines the emergence and genetic characteristics of Salmonella enterica serotype Saintpaul isolated from horses, highlighting its antimicrobial resistance and potential impact on animal and human health.

Background and Importance

• Salmonellosis is a contagious zoonotic disease affecting both animals and humans, making early identification of Salmonella serotypes critical.
• Understanding antimicrobial resistance patterns aids in controlling outbreaks in animals and reducing zoonotic transmission risks.
• Salmonella enterica serotype Saintpaul was previously rare in horses but has recently been detected more frequently, indicating an emerging health concern.

Study Scope and Methods

• Between 2018 and 2025, a total of 245 Salmonella enterica subsp. enterica isolates were collected from horses, from both ante-mortem (before death) and post-mortem (after death) cases.
• The predominant serotypes identified were:
  • Typhimurium (83 isolates)
  • Newport (28 isolates)
  • 1,4,[5],12:i:- (26 isolates)
  • Saintpaul (25 isolates)
  • Hartford (15 isolates)
  • Mbandaka (12 isolates)
• S. Saintpaul cases mainly surfaced in 2025, with 21 isolates from Central Kentucky associated with diseases such as enterocolitis, bronchopneumonia, omphalitis (infection of the umbilical region), and septicemia, primarily affecting young foals.
• Sixteen S. Saintpaul isolates underwent whole genome sequencing (WGS) to detail their genomic features.

Genomic Findings

• Antigenic profile analysis using the White-Kauffmann-Le Minor scheme showed the isolates had the profile 1,4,[5],12:e,h:1,2, consistent with known S. Saintpaul strains.
• All isolates were assigned the sequence type ST50 by multi-locus sequence typing (MLST), indicating a common lineage.
• Core genome MLST (cgMLST) revealed low genetic diversity among the isolates, suggesting they form a closely related cluster or outbreak.

Antimicrobial Resistance Characteristics

• Antimicrobial susceptibility testing demonstrated widespread multi-drug resistance (MDR), especially to:
  • Aminoglycosides
  • Beta-lactams
  • Tetracyclines
  • Sulfonamides
• Genotypic analysis confirmed the presence of several antimicrobial resistance genes, such as:
  • bla genes (beta-lactam resistance genes, including types not previously reported in equine Salmonella)
  • sul1 (sulfonamide resistance)
  • dfrA34 (trimethoprim resistance)
  • rmtE1 (aminoglycoside resistance via methylation)

Plasmid Analysis

• S. Saintpaul isolates contained multiple plasmids belonging mainly to:
  • IncI-gamma/K1 family plasmids
  • ColRNAI family plasmids, to a lesser extent
• The IncI-gamma/K1 plasmids consistently carried:
  • MOB relaxase, an enzyme involved in plasmid mobilization
  • MPF_I mating pair formation system, which facilitates plasmid transfer between bacteria
• These plasmids likely contribute to the spread of antimicrobial resistance within these bacterial populations.

Implications and Conclusions

• The emergence of multi-drug resistant S. Saintpaul in horses signals a potential new threat to animal health, particularly in young foals in Central Kentucky.
• Ongoing genomic surveillance and antimicrobial resistance monitoring are crucial to:
  • Detect and control outbreaks early
  • Limit the spread of resistant strains among animal populations
  • Reduce zoonotic risk to humans who come into contact with infected horses
• This study underscores the necessity of integrating genomic tools in veterinary pathogen surveillance to better understand epidemiology and resistance mechanisms.