Staphylococcus aureus Nasal Colonization and Risk Factors 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. Surveillance data are an important tool in the development of infection control policies. Hypothesis/Objectives – This study aimed to investigate the prevalence and risk factors for Staphylococcus aureus (SA) and MRSA nasal colonization and the duration of the 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 with MALDI-TOF, and confirmed with chromogenic agars and an oxacillin minimal inhibitory concentration testing. Horses colonized with MRSA were resampled until two negative cultures were obtained. Stabling management, activity, and medical history were obtained from owners via survey and medical files. Multivariate logistic regressions were used to model associations between risk factors and colonization. Results –The prevalence of SA and MRSA nasal carriage was 18.4% (95% CI 13.4-23.5%) and 5.7% (95% CI 2.7-8.7%), respectively. Of the 13 horses colonized by MRSA, only one tested positive after 3 months of follow-up. The risk factors associated with MRSA carriage were number of horses stabled with (more than 10 horses: OR 5.9 – 95% CI 1.1-63.3), previous hospitalization (OR 6.1 – 95% CI 1.0-35.7), and the year of admission (2022 vs. 2020-2021; OR 8.8 – 95% CI 1.7-90.9). Conclusions and clinical importance – The prevalence of MRSA nasal colonization is of concern, however, the carriage is transient. Environmental risk factors are important.