Use of somatic cell counts and California mastitis test results from individual quarter milk samples to detect subclinical intramammary infection in dairy cattle from a herd with a high bulk tank somatic cell count.

Objective: To determine whether somatic cell counts (SCCs) or California mastitis test (CMT) scores for individual quarter milk samples could be used to detect subclinical intramammary infection among dairy cattle in a herd with a high bulk tank SCC.
Design: Prospective clinical trial.
Animals: 278 Holstein-Friesian dairy cattle from a single herd.
Procedure: Individual quarter milk samples were collected and submitted for bacterial culture, California mastitis testing, and determination of SCC. Additional milk samples were collected 34 days later and submitted for bacterial culture.
Results: During the initial visit to the herd, milk samples were collected from all 278 cows. However, because of blind mammary quarters or missing data, results for 1,057 quarter milk samples were included. Bacterial culture did not yield any growth for 622 (58.8%) of these samples. Regardless of the cutoff that was used, sensitivity of the CMT score was < or = 0.50 and sensitivity of the SCC linear score (SCS) was < or = 0.60. For 497 mammary quarters, results of bacterial culture of samples collected 34 days apart were concordant; bacterial culture did not yield any growth for 342 (68.8%) of these quarters. Regardless of the cutoff that was used, sensitivity of the CMT score was < or = 0.61 and sensitivity of the SCS was < or = 0.76 for mammary quarters with concordant bacterial culture results.
Conclusions and Clinical Relevance: Results suggest that neither CMT score nor SCC is sensitive enough to be useful as a screening test for identifying infected mammary quarters among dairy cattle in a herd with high bulk tank SCC.