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

Objectives: We performed a retrospective analysis of MRSA isolates collected at the university equine clinic including clinical isolates from 2008 to 2021 and screening environmental, equine and personnel isolates from 2016.

Methods: Screening and clinical samples were cultured on Brilliance MRSA 2 and Columbia agar (Oxoid), respectively, with enrichment for environmental samples. Antimicrobial susceptibility was assessed by disc diffusion. All the isolates were characterized by spa typing. Eighteen selected isolates were subjected to WGS with subsequent wgMLST clonal analysis.

Results: Among 75 MRSA isolates, five spa types were identified, the majority (n = 67; 89.33%) was t011. All isolates were resistant to cefoxitin and ampicillin and carried the mecA gene. In addition, the isolates were resistant to tetracycline (n = 74; 98.67%), gentamicin (n = 70; 93.33%), enrofloxacin (n = 54; 72.00%), sulfamethoxazole-trimethoprim (n = 5; 6.67%) and lincomycin (n = 3; 4.00%) with corresponding genetic markers for the resistance detected in the sequenced isolates. All 18 sequenced isolates belonged to ST398, 16 carried SCCmec type IVa and two carried SCCmec type Vc (5C2&5). Further, isolates carried aur, hlgA, hlgB and hlgC virulence genes, and five isolates carried sak and scn genes, which are part of the immune evasion cluster. Close genetic relatedness was found between isolates from the staff of the clinic and clinical samples of horses.

Conclusions: Repeated introduction and long-term persistence of the equine LA-MRSA subclone (ST398-MRSA-IVa/Vc(5C2&5), t011) among the infected horses at the equine clinic with the colonization of personnel, and the environment contamination that might contribute to transmission were observed.