Herd-level prevalence of contagious mastitis pathogens and insights into somatic cell count dynamics using bulk tank...

Mastitis continues to be among the top 3 challenges for dairy production globally, resulting in substantial economic losses. Current pathogen-specific prevalence estimates are crucial for effective surveillance and disease control. The objective of this study was to estimate the true prevalence of herds positive for major contagious mastitis pathogens in Alberta, Canada, and to describe bulk tank SCC (BTSCC) patterns and dynamics. Using a serial cross-sectional study design, bulk tank milk (BTM) samples were collected from all active dairy producers in Alberta in December 2021 (n = 484), April 2022 (n = 486), and October 2022 (n = 484) and tested for Staphylococcus aureus, Streptococcus agalactiae, and Mycoplasma bovis using a commercial multiplex quantitative-PCR kit. The BTSCC data for 2021 and 2022 were obtained for each herd for each milk pick-up and production-weighted geometric mean BTSCC (PWgBTSCC) was calculated. True prevalence was estimated using Bayesian latent class models. Mixed-effect modified Poisson and linear regression models were used to assess associations among pathogen positivity, herd characteristics, and PWgBTSCC. True prevalence of Staph. aureus-positive herds were 17.8%, 11.0%, and 13.8% in December, April, and October, respectively, whereas for Strep. agalactiae, true prevalence was consistent at 0.6%. For M. bovis, true prevalence was 2.0%, 4.7%, and 3.6% in December, July, and October, respectively. The prevalence of Staph. aureus-positive herds was higher (prevalence ratio [PR] = 1.31) among medium-sized herds (3,600-7,200 L/d milk delivered) compared with smaller herds (<3,600 L/d milk delivered). Hutterite colony herds were more frequently Staph. aureus-positive at least once (PR = 1.23) compared with non-Hutterite herds. Non-Hutterite colony herds in central and southern regions were less frequently Staph. aureus-positive compared with the north region; however, this trend was opposite among Hutterite colony herds. Annual PWgBTSCC for Alberta dairy herds from November 16, 2021, to November 15, 2022, was 154,000 cells/mL (median [first quartile-third quartile]: 118,000-202,000). Herds with a medium (100,000-200,000 cells/mL) or high (>200,000 cells/mL) annual PWgBTSCC were more frequently Staph. aureus-positive (PR = 1.58 and 2.40, respectively) compared with <100,000 cells/mL herds. Annual PWgBTSCC was elevated for herds testing positive for any major contagious mastitis pathogen at least once compared with consistently negative herds. Herds in the south region had an 11% lower annual PWgBTSCC compared with north-region herds. These findings provided updated prevalence estimates of major contagious mastitis pathogens and critical insights into BTSCC patterns across herd types and regions, supporting targeted within-herd diagnostic follow-up and evidence-based mastitis control strategies.