Prevalence, Antimicrobial Resistance Patterns, and Associated Risk Factors of Staphylococcus Aureus in Raw Milk From Dairy Cattle in Tilottama Municipality, Nepal

Staphylococcus aureus (S. aureus)-associated bovine mastitis is a serious economic and animal health concern in Nepalese dairy production. Yet, antimicrobial resistance (AMR) data from commercial dairy herds remain scarce. A cross-sectional study was conducted from January to March 2020 in Tilottama Municipality, Rupandehi District, Nepal. A total of 207 lactating cows from 69 commercial dairy farms were enrolled by two-stage cluster sampling. Composite milk samples were pre-enriched in Buffered Peptone Water and cultured on Mannitol Salt Agar. S. aureus was considered positive by a five-step biochemical protocol with the tube coagulase test as the definitive criterion. Antimicrobial susceptibility to six agents was determined by the Kirby-Bauer disk diffusion method, and phenotypic methicillin-resistant S. aureus (MRSA) was identified by cefoxitin (30 µg) disk diffusion. Animal-level S. aureus prevalence was 33.3% (69/207) and herd-level prevalence was 53.6% (37/69). Cefoxitin non-susceptibility was detected in 52.2% of isolates; multidrug resistance (MDR) in 26.1% and a multiple antibiotic resistance (MAR) index ≥0.25 in 71.0% of isolates. Chloramphenicol showed the highest susceptibility rate (92.8%). After Bonferroni correction, animal age ≥5 years (OR=3.88) and early lactation stage ≤6 months post-calving (OR=3.30) were significantly associated with S. aureus positivity. Cefoxitin non-susceptibility and MDR were common among S. aureus isolates from commercial dairy farms of Tilottama Municipality. Older cows as well as early-lactating ones carried the highest infection risk. Culture-guided antimicrobial therapy, improved antibiotic stewardship, and sustained antimicrobial resistance (AMR) surveillance with molecular confirmation are recommended to prevent antimicrobial resistance.