Assessment of sensitivity and specificity of bacterial culture and the VetMAX™ MastiType Multi Kit in detecting Streptococcus uberis and Escherichia coli in milk samples from dairy cows with clinical mastitis in subtropical Australia

Mastitis, a prevalent and economically important disease in the dairy industry, poses substantial challenges to dairy cow health, milk quality, and farm profitability worldwide. Mastitis is predominantly caused by bacterial infections. The objective of this study was to estimate the sensitivity (Se) and specificity (Sp) of bacterial culture and the VetMAX™ MastiType Multi Kit PCR in identified clinical mastitis pathogens. A total of 396 quarter-level milk samples were collected from 396 cows with clinical mastitis on 29 farms in the subtropical dairy region of Australia between March and December 2021. These samples were cultured and tested by PCR, and analysed using Bayesian latent class analysis under the assumption of one population two tests and also of three populations two tests, by dividing the population into subpopulations based on regions. Informative priors used in the analysis were calculated from published evidence. Models were compared using the Deviance Information Criterion (DIC). Sensitivity analysis was performed to evaluate the impact of changes in priors. The most common isolates cultured and detected by PCR were Streptococcus uberis (17.4 % and 27.3 %, respectively) and Escherichia coli (12.6 % and 25.0 %, respectively). Under the assumption of one population two tests, the Se of PCR (at cycle threshold (Ct) ≤ 37) was higher than that of bacterial culture for both pathogens: for E. coli, the Se was 50.2 % (95 % posterior probability interval (PPI): 37.4; 74.1) for bacterial culture, and 93.7 % (95 % PPI: 85.5; 98.4) for PCR. For S. uberis, the Se was 50.4 % (95 % PPI: 40.9; 61.3) for bacterial culture, and 81.5 % (73.0; 88.9) for PCR. Conversely, the Sp of bacterial culture was higher than that of PCR for both pathogens: for E. coli, the Sp was 99.2 % (97.8; 100) for bacterial culture, and 95.1 % (87.8; 99.4) for PCR. For S. uberis, the Sp was 99.2 % (95 % PPI: 97.6; 100) for bacterial culture, and 96.7 % (95 % PPI: 92.1; 99.2) for PCR. Bayesian latent class analysis with three populations two tests was only performed for S. uberis. For E. coli, this could not be performed because there were no PCR-positive results in one subpopulation. Under the assumption of three populations two tests, for S. uberis, the Se was 49.6 % (40.6; 59.4) for bacterial culture, and 81.1 % (72.6; 88.6) for PCR; and the Sp for bacterial culture was 99.1 % (97.7; 100), and for PCR was 96.9 % (93.0; 99.3). The DIC for the one population two tests model was lower than the DIC for the three populations two tests model. The sensitivity analysis for the one population two tests model demonstrated that a 10 % reduction in priors led to substantial changes in Se of both bacterial culture and PCR tests for E. coli and S. uberis, with overlap percentages ranging from 80.6 % to 92.2 %. In contrast, the Sp of bacterial culture and PCR tests remained relatively stable despite changes in priors, except for the Sp of PCR test for E. coli. In summary, the VetMAX™ MastiType Multi Kit demonstrated higher Se compared to bacterial culture, suggesting its potential as a routine test for identifying mastitis pathogens in milk samples from cows with clinical mastitis. While the bacterial culture method offered higher Sp in pathogen detection; results obtained following bacterial culture and subsequent susceptibility testing remain valuable, particularly in guiding antimicrobial treatment for mastitis.