Prevalence and Risk Factors of Staphylococcus aureus Nasal Colonization in Horses Admitted to a Veterinary Teaching Hospital.

BACKGROUND: Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of nosocomial infections, including in veterinary settings. HYPOTHESIS/OBJECTIVES: To investigate the prevalence, risk factors for Staphylococcus aureus (SA) and MRSA colonization, and the duration of MRSA colonization. ANIMALS: Elective cases admitted to the Veterinary Teaching Hospital were recruited (228 horses). METHODS: A cross-sectional study was conducted over 3 years. Nasal swabs were collected at admission and cultured for SA. Methicillin-resistant isolates were identified using matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) technology, oxacillin minimal inhibitory concentration (MIC), and PCR testing. Horses colonized with MRSA were resampled until two negative cultures were obtained. Stabling management, activity, and medical history were obtained from owners and medical files. Multivariable logistic regressions were used to model associations between risk factors and colonization. RESULTS: The prevalence of SA and of MRSA nasal carriage was 17.5% (95% CI: 12.4-22.7) and 6.2% (95% CI: 2.9-9.4), respectively. Of the 10 horses colonized by MRSA and monitored over time, only one tested positive after 3 months. More than 10 horses on the premises (OR 6.0 - 95% CI 1.1-64.2), previous hospitalization (OR 6.0 - 95% CI 1.0-35.2), and year of admission (2022 vs. 2020-2021; OR 9.0 - 95% CI 1.7-92.2) were associated with MRSA nasal carriage. CONCLUSIONS AND CLINICAL IMPORTANCE: The prevalence of MRSA nasal colonization is of concern; however, the carriage seems transitory. Apart from the medical risk factors, the importance of social interactions in MRSA transmission needs to be elucidated in horses.