The objective was to determine the proportions and bacterial counts of major mastitis pathogens in samples of bulk tank milk (BTM), as well as to clarify the relationship between these bacteria counts and bulk tank somatic cell count (BTSCC). The purpose was to judge the importance of the counts of mammary pathogens for BTSCC at the population level. Samples of BTM were collected from 268 randomly selected anonymous dairy herds (with approximately 29 000 dairy cows). Staphylococcus aureus, other coagulase-positive staphylococci, Streptococcus agalactiae and Streptococcus dysgalactiae were grouped as contagious pathogens, and Streptococcus uberis, Escherichia coli, Pseudomonas aeruginosa, Enterococcus faecalis and Enterococcus faecium as environmental pathogens. Based on assessment of the dominant pathogen in each herd, environmental pathogens were found to dominate. Counts of specific mammary pathogens in BTM samples did not exceed a geometric mean of 1000 cfu/ml. Significant differences were documented in BTSCC between BTM samples containing Str. agalactiae, Staph. aureus, and Str. dysgalactiae and BTM samples that were pathogen-free. Geometric means of BTSCC associated with these pathogens were noticeably higher than the overall BTSCC (Str. agalactiae 243 628; Staph. aureus 205 610; Str. dysgalactiae 203 978; overall 173 000 cells/ml). It follows that the somatic cell count (SCC) associated with these pathogens contributed substantially to the overall BTSCC. Environmental pathogens predominated in samples of BTM with SCC <300×103/ml and contagious mastitis pathogens predominated in BTM samples with SCC >300×103/ml. No correlation was detected between bacterial counts of specific pathogens and BTSCC. This study revealed that the assessment of bacterial counts of mammary pathogens in samples of BTM in relation to BTSCC is applicable for the monitoring of changes in the occurrence of major mastitis pathogens in dairy herds at a national level.
Consequence of alternative standards for bulk tank somatic cell count of dairy herds in the United States
