Assessment of the Zagreb mastitis test in diagnosis of subclinical mastitis in dairy cattle.

The objective of this research was to evaluate the Zagreb mastitis test (ZMT) as screening test for mastitis diagnosis in dairy cows. The study was carried out within the framework of subclinical mastitis monitoring in the Varaždin region. Research included 1549 quarter milk samples from 389 Simmental dairy cows. Milk samples for bacteriological examination, mastitis testing and SCC were taken in a sterile plastic tube during milking. Bacteriology examination of milk samples obtained 760 (49.06%) positive samples. The most common isolated pathogens, Staphylococcus aureus, Streptococcus spp. Lancefield group D, Pseudomonas aeruginosa, were found in 388 (51.05%), 84 (11.05%), 62 (8.16%) positive quarter samples, respectively. Using a cut-off of ZMT by scores ≥ 1 as positive, 63.20% (CI 95%, 60.80% - 65.60%) quarter milk samples were estimated as being positive for subclinical mastitis. This cut-off had quite high sensitivity (96.84%, CI 95%, 95.60% - 98.06%) and negative predictive value (95.79%, CI 95%, 94.14% - 97.44%), however specificity (69.20%, CI 95%, 65.98% - 72.74%) and positive predictive value (75.17%, CI 95%, 72.47% - 77.86%) were low. Using a ZMT cut-off ≥4×105 cell/mL as positive, 49.56% of the quarters were positive. Sensitivity (88.37%, CI 95%, 86.06% - 90.68%) and specificity (86.98%, CI 95%, 84.57% - 89.38%), as well as predictive values, were similar. On the basis of the results of this study, comparing SCC and ZMT in mastitis diagnosis, ZMT is a reliable diagnostic method for use in field conditions.