Distribution of mastitis-causing pathogens isolated from subclinical mastitis cases in Wisconsin dairy herds with low bulk tank somatic cell counts

The objective of this cross-sectional study was to characterize the distribution of mastitis-causing pathogens ( MCP ) in cases of subclinical mastitis ( SM ) across 14 Wisconsin dairy farms with a history of low bulk tank somatic cell counts ( BTSCC ). Herds were eligible for enrollment if their average BTSCC during the previous 12 mo was ≤ 150,000 cells/mL. A total of 455 cows were enrolled based on recent DHI(A) records indicating SCC ≥ 200,000 cells/mL. Milk samples were collected from functional quarters (n = 1,811), and SCC was measured in 1,756 samples due to insufficient milk volume or sample loss. Only quarter milk samples with SCC ≥ 200,000 cells/mL were used for microbiological culture. The quarter-level prevalence of SM was 33.3% (585/1,756). Among SM cases, 22.6% (132/585) were contaminated, 28.4% (166/585) were negative for microbial growth ( NG ), and 49.0% (287/585) had an identifiable microbe. After excluding contaminated samples, NG and non- aureus staphylococci and mammaliicocci (NASM) (n = 120) were the most frequent diagnoses. Enterococcus spp. ( ECS , 8.8%) and Lactococcus spp. ( LAC , 6.8%) were identified more frequently than environmental streptococci ( ENV ; 6.6%). Among NASM, Staphylococcus chromogenes (13.9%) was the most frequently identified species. This study did not have enough power to identify significant variability for MCP distribution between farms. These findings highlight that, although NG and NASM remain the predominant microbiological culture results, ECS and LAC are distinct and important contributors to SM that may be underestimated by traditional phenotypic methods. Overall, these findings emphasize the importance of pathogen-level surveillance for developing targeted mastitis control strategies in high-performing dairy systems.