The circulation of methicillin-resistant Staphylococcus aureus between humans, horses and the environment at the equine clinic.

This research examines the relationship between humans, horses, and their environment in the transmission and persistence of Methicillin-resistant Staphylococcus aureus (MRSA) at a university equine clinic. It identifies that a particular subclone of MRSA often found in livestock (LA-MRSA) is the most common variant among the isolated MRSA samples from clinic staff, horses, and the environment.

Methods of the Research

• The research was executed through a retrospective analysis of MRSA samples collected from the university equine clinic over almost a decade. Clinical samples were derived from 2008 to 2021, whereas environmental, equine, and personnel samples were obtained in 2016.
• Both clinical and screening samples were cultured with various agar, and environmental samples went through an enrichment process. Antimicrobial susceptibility testing was performed via disc diffusion.
• All the collected MRSA isolates underwent spa typing, a conventional method used to categorize Staphylococcus aureus strains based on variation in the polymorphic ‘spa’ gene.
• Eighteen selected isolates also underwent Whole Genome Sequencing (WGS), followed by clonal analysis through whole-genome, multi-locus sequence typing (wgMLST).

Results of the Research

• Out of 75 MRSA isolates, five spa types were identified with the majority (89.33%) being t011. All isolates displayed resistance to cefoxitin and ampicillin and carried the mecA gene, a determinant of MRSA.
• The isolates were also resistant to other antimicrobials such as tetracycline, gentamicin, enrofloxacin, sulfamethoxazole-trimethoprim, and lincomycin. Genetic markers corresponding to these resistances were identified in the sequenced isolates.
• All 18 sequenced isolates belonged to the sequence type ST398 – a type commonly associated with livestock-associated MRSA. Among these, 16 carried SCCmec type IVa, and two carried SCCmec type Vc (5C2&5). These SCCmec types refer to the Staphylococcal Cassette Chromosome mec, a mobile genetic element that carries the mecA gene.
• The isolates also harbored virulence genes such as “aur”, “hlgA”, “hlgB”, and “hlgC”. Some strains also carried “sak” and “scn” genes, which are associated with immune evasion.
• A significant genetic relatedness was observed between isolates from the clinic staff and clinical samples from horses, indicating possible transmission.

Conclusions of the Research

• There was evidence of recurring introduction and long-term persistence of the equine LA-MRSA subclone (ST398-MRSA-IVa/Vc (5C2&5), t011) among the infected horses at the clinic.
• The research also noted colonization of this MRSA subclone in personnel and the contamination of the environment, both of which may contribute to spread and transmission of this bacteria.