Frequency and antimicrobial susceptibility of Staphylococcus aureus isolated from clinical bovine mastitis cases in British Columbia, Canada

Staphylococcus aureus is one of the most important bacteria responsible for clinical bovine mastitis globally, leading to significant economic losses in the dairy industry. Antimicrobials used to treat and prevent mastitis can lead to antimicrobial resistance (AMR) in S. aureus. We retrospectively evaluated AMR of S. aureus isolates from clinical bovine mastitis cases submitted to the Animal Health Centre in British Columbia from 2013 to May 2024. S. aureus was isolated from 15.0% of submitted bovine milk samples. Antimicrobial susceptibility testing was done on 611 of 1,347 S. aureus isolates. No methicillin-resistant S. aureus isolates were detected based on cefoxitin susceptibility test results, which is encouraging. The highest frequencies of resistance were found for penicillin (46.6%) and ampicillin (42.1%). The lowest frequencies of resistance were to ceftiofur and sulfamethoxazole-trimethoprim (0.2% each) and cephalothin (0.3%). AMR trends over our study period were generally stable, except for penicillin and ampicillin; penicillin resistance increased from 15.2% to 71.1%, and ampicillin resistance increased from 18.2% to 70.1%. The information in our study could help guide clinicians when choosing antimicrobial treatments to treat mastitis caused by S. aureus, particularly in the province of British Columbia. Because S. aureus has a broad host range and is of importance to both human and veterinary medicine, continued monitoring to detect the emergence of resistance is warranted.