Accuracy of methods using somatic cell count and N-acetyl-beta-D-glucosaminidase activity in milk to assess the bacteriological cure of bovine clinical mastitis.

This study examined the capability of milk somatic cell count (SCC) and NAGase activity to discriminate between quarters that had been cured versus those that had not been cured at 4 wk after antimicrobial therapy for clinical mastitis. The distribution of microorganisms that were isolated before therapy from 630 quarters with mastitis was as follows: 225 strains of Staphylococcus aureus, 96 strains of coagulase-negative staphylococci, 152 strains of streptococci (Streptococcus dysgalactiae and Streptococcus uberis), and 157 strains of coliform bacteria. Bacteriological cure rates were 35% for mastitis caused by Staph. aureus, 75% for mastitis caused by coagulase-negative staphylococci, 66% for mastitis caused by streptococci, and 72% for mastitis caused by coliforms. Diagnostic accuracy of milk SCC and NAGase and their interquarter ratios for predicting bacteriological status of the control samples was assessed by calculating sensitivity, specificity, and accuracy and by means of receiver operating characteristic analysis. The efficiency of milk SCC and NAGase for predicting bacteriological cure was greatest for cows that had been infected with Staph. aureus. The main problem in detecting coagulase-negative staphylococci was low sensitivity, and the main problem in detecting streptococci and coliforms was low specificity. Receiver operating characteristic analysis is not completely suitable for the detection of mastitis because reference method bacteriology and indirect tests can never fully agree. To assess the recovery of cows from mastitis caused by Staph. aureus, bacteriology should be supplemented with an examination of milk SCC or NAGase activity at threshold values such as those presented here.