USE OF COMPOSITE MILK SAMPLES FOR DIAGNOSIS OF STAPHYLOCOCCUS AUREUS MASTITIS IN DAIRY CATTLE

OBJECTIVE: To measure relative sensitivity and relative specificity for use of composite milk samples, compared with that of individual gland milk samples, for diagnosis of Staphylococcus aureus mastitis. ANIMALS: 505 cows suspected of having subclinical mastitis. Of these cows, 172 were considered infected with Sta aureus, based on the results from individual gland samples. PROCEDURE: Composite and individual gland milk samples were collected from cows suspected of having subclinical mastitis, and results of bacteriologic culturing of samples from the same cow were compared. Results were interpreted at the cow level. Relative sensitivity and relative specificity for composite samples were computed from 2 x 2 tables, using results from individual gland samples as references. RESULTS: Relative sensitivity for use of composite milk samples in diagnosing Sta aureus mastitis was 0.63. The relative specificity was 0.98. Factors influencing the relative sensitivity for composite samples were the number of infected glands per cow, the amount of Sta aureus shedding from infected glands, and the proportion of the composite milk obtained from each gland. CLINICAL IMPLICATIONS: Collecting composite instead of individual gland milk samples increases the number of false-negative results in diagnosing Sta aureus mastitis. By collecting consecutive samples from the same cow or by increasing the inoculum volume at culturing, this problem can be diminished.