A comparative evaluation of four detection tests and the isolation of coagulase-negative staphylococci from subclinical mastitis cases in south Indian cattle

Subclinical mastitis (SCM) is an important condition affecting quality milk production. Several coagulase negative staphylococci (CoNS) have recently been identified to be associated with SCM throughout the world. Studies on comparative evaluation of tests to detect SCM or the contribution of CoNS to SCM in India are lacking. This study evaluated 313 milk samples from organized and unorganized sectors in Somatic Cell Count (SCC), Electrical Conductivity (EC), California Mastitis Test (CMT) and Bromothymol Blue (BTB) tests to detect SCM, and correlated SCM status with isolation of CoNS. Considering standard cut-off values, 44.7%, 48.2%, 21.4% and 26.8% of the samples indicated SCM status by SCC, EC, CMT and BTB tests, respectively. Whereas sensitivity was poor (32.86–41.42%) for all the tests, specificity was poor for EC (45.09%) but good for CMT (87.86%) and BTB (84.97%). When assessed by both of a pair of tests, or all the three tests, specificity was very high (>90%) but sensitivity was very poor (14–30%). When tested by either of a pair of tests, both sensitivity and specificity were poor to average (44–57% and 36–73%, respectively). Isolation of CoNS and SCM status correlated in the case of SCC and EC but not CMT and BTB tests. The results suggest that CMT and BTB tests are the best field tests. Furthermore, a combination of tests is not advisable given the high rate of both false and negative results to identify SCM, and that CoNS may or may not play a role in SCM.