Clinical mastitis was studied in 274 herds grouped into three categories based on bulk milk somatic cell count (SCC) -- low (less than or equal to 150,000); medium (150,000 to 250,000); and high (250,000 to 400,000 cells/ml).

The incidence rate of clinical mastitis did not differ among the three categories. Variance in the incidence of clinical mastitis among herds increased as bulk milk SCC decreased. Clinical mastitis caused by Gram-negative pathogens, such as Escherichia coli, Klebsiella spp., or Pseudomonas spp., occurred more often in herds with a low bulk milk SCC; whereas clinical mastitis caused by Staphylococcus aureus, Streptococcus dysgalactiae, and Streptococcus agalactiae occurred more often in herds with a high bulk milk SCC. Systemic signs of illness caused by clinical mastitis occurred more often in herds with a low bulk milk SCC. Both overall culling rate and culling rate for clinical mastitis were not different among groups categorized by bulk milk SCC. In herds with a high bulk milk SCC however, more cows that produced milk with a high SCC were culled. In herds with a low bulk milk SCC, more cows were culled for teat lesions, milkability, udder shape, fertility, and character than were cows in herds with a high bulk milk SCC. In herds with a low bulk milk SCC, cows were also culled more for export and production reasons.

Source: Journal of Dairy Science, Vol. 81, No. 2, pg. 411 (1998)