Prevalence of udder infections and mastitis in 50 California dairy herds.

The California mastitis test (CMT) and bacteriologic culture were performed on samples of bulk-tank milk and cow-composite milk (n = 23,138 cows) from 50 California dairies, 19 of the 50 with known mastitis problems. Thirty-eight (76.0%) bulk-tank milk samples and 12,334 (53.3%) cows were positive by results of the CMT. Potential mastitis agents were isolated from 5,085 (22%) cows. Staphylococcus aureus was isolated from all 50 herds, Streptococcus agalactiae was isolated from 47 herds, and Mycoplasma sp was isolated from 24 herds. For cow-composite milk samples, the prevalences were 9.3% for Str agalactiae, 9.1% for S aureus, 0.9% for Mycoplasma sp, 1.2% for coliform bacteria, 0.9% for other streptococci, 0.8% for coagulase-negative staphylococci, and 1.3% for other organisms. The relative sensitivity and the relative specificity of the CMT performed on cow-composite milk samples were 83.4% and 55.2%, respectively, and the predictive value of positive CMT results was 34.2%.