“INTERPRETIVE SUMMARY: Accurate prevalence estimates of mastitis-causing bacteria are essential for designing effective mastitis control strategies. Bulk tank milk samples from all Alberta dairy herds (n= 495) were tested 3 times over a 12-month interval for major contagious mastitis pathogens. Overall, 57.0, 1.6 and 1.4% of herds were positive at least once for Staph. aureus, Strep. agalactiae and Mycoplasma bovis, respectively. Larger herds and those located in northern Alberta were more frequently positive for Staph. aureus, whereas herds in the southern region consistently had lower bulk tank somatic cell counts. These findings highlight regional and herd-level variation in pathogen detection and bulk tank somatic cell count dynamics that are relevant for mastitis surveillance and for targeting subsequent herd-level diagnostic work.”