Usefulness of differential somatic cell count for udder health monitoring: Diagnostic performance of somatic cell count, and differential somatic cell count for diagnosing of intramammary infections in dairy herds with automated milking systems

Mastitis poses significant economic challenges for dairy farms. Therefore, enhancing the accuracy of diagnostic methods for detecting intramammary infections (IMI) can potentially improve prevention, control and treatment strategies. The somatic cell count (SCC) is a well-established parameter for identifying inflammation resulting from IMI. Given the recent introduction of differential somatic cell count (DSCC) for routine milk sample screening, limited research has been conducted to assess its additional benefits for diagnosing IMI. Therefore, our main objective was to evaluate the diagnostic accuracy of SCC, DSCC and SCC-DSCC combinations in detecting IMI caused by any pathogen or by major pathogens using quarter milk samples. Five dairy herds using automated milking systems were selected using convenience sampling in Québec, Canada. Determination of SCC and DSCC was performed by Lactanet (Ste-Anne de Bellevue, QC, Canada) using a CombiFoss 7 DC instrument. A 5-populations 2-tests Bayesian latent class models was used, with bacteriological culture employed as the imperfect reference test. Posterior estimates for sensitivity (Se), specificity (Sp), and the predictive values for 2 hypotheticals IMI prevalences due to any pathogen or major pathogens were computed. The proportion of quarters positive for any pathogen or major pathogen using milk culture was 31.7% (5,125/16,176) and 5.4% (871/16,176), respectively. For the detection of IMI by any pathogen, using a serial interpretation for the combination of SCC ≥100,000 and DSCC at ≥65% increased the Sp from 0.71 (95BCI: 0.70, 0.72) to 0.84 (95BCI: 0.83, 0.86) compared with SCC alone at the cut-off ≥100,000 cells/mL, although resulting in a slight decrease in Se from 0.49 (95BCI: 0.43, 0.54) to 0.46 (95BCI: 0.42, 0.50). Moreover, for detecting IMI caused by major pathogens, combining SCC at the threshold of ≥100,000 cells/mL and DSCC at the threshold of ≥65% using serial interpretation increased the Sp from 0.68 (95BCI: 0.67, 0.69) to 0.80 (95BCI: 0.79, 0.81) compared with SCC alone at the ≥100,000 cells/mL threshold. Our findings suggest that DSCC could be combined with SCC to provide a modest improvement in Sp with minimal compromise in Se for identifying IMI caused by any or by major pathogens. In addition, DSCC combined with SCC provided a small improvement in Se for detecting any pathogen using the parallel interpretation. However, no improvements in Se were observed when using the combination of SCC and DSCC for detecting major pathogens.