Effects of Outdoor Access and Season on Mastitis Pathogen Distribution and Milk Quality Parameters in Dairy Herds in Western Canada

The Western Canadian dairy sector faces distinct climates, geographic, and management constraints that influence decisions around cow housing, outdoor access, and milk quality. As producers balance productivity, welfare expectations, and environmental challenges, concerns persist regarding whether outdoor access increases the risk of clinical mastitis, particularly from environmental pathogens. However, little research has compared mastitis pathogen distribution across different housing systems in Alberta (AB) and British Columbia (BC). This thesis characterises mastitis pathogen distribution in Western Canada through: (1) a background review of dairy production systems, housing practices, and udder health determinants, (2) a multi-pathogen epidemiologic study evaluating associations between pathogen distribution and housing system, province, and season in AB and BC herds, and (3) a general discussion integrating biological mechanisms, regional considerations, and future research needs. A total of 117 commercial dairy herds (58 AB, 59 BC) participated in a 12-16-month cohort, submitting clinical mastitis milk samples for culture and bacterial identification. Mixed-effects logistic regression models evaluated whether pathogen distribution differed by province, housing type, and season, with random herd effects and adjustments for sampling compliance. A total of 2,143 CM samples were cultured, yielding 1,260 pathogen-positive isolates; the most common pathogens were non-aureus staphylococci, Escherichia coli, Staphylococcus aureus, Streptococcus uberis, Streptococcus dysgalactiae, and Klebsiella spp. Bulk tank somatic cell count geometric means fell within expected provincial ranges, supporting the representativeness of the study population. Key findings indicate that pasture access did not universally increase environmental pathogen detection, contradicting common producer concerns; E. coli accounted for a greater proportion of clinical mastitis cases in AB than BC and peaked in summer; Strep. uberis exhibited a winter peak independent of province and housing type; and Staph. aureus occurred more frequently in fully indoor systems than in outdoor/no pasture systems. Overall, the results demonstrate that mastitis pathogen risk reflects interacting influences of season, region, and housing practices, rather than outdoor access alone. These findings support pathogen-specific prevention strategies and highlight the need for future research incorporating environmental sampling, cow-level infection dynamics, and weather-based exposure metrics to refine management recommendations for Western Canadian dairy herds.