Association between differential somatic cell count and California Mastitis Test results in Holstein cattle

Abstract

The California Mastitis Test (CMT) has been used to estimate total somatic cell count (SCC) levels in milk; however, milk with similar SCC levels occasionally shows inconsistent CMT results, which limits the use of the CMT. This observational study aimed to investigate how differential cell counts in milk influence the CMT in Holstein cattle through the novel parameters differential somatic cell count (DSCC) and macrophage proportion (MAC). We performed the CMT on d 0, 3, 5, 7, 14, and 21 after identifying mastitis, and simultaneously measured SCC, DSCC, and MAC at the quarter level. We followed 58 mastitis events occurring in 41 cows and obtained 307 quarter-level records after data cleaning. We transformed SCC to somatic cell score (SCS) and MAC to its logarithm to fit the normal distribution and analyzed the data using the cumulative logit mixed model. Results showed that both an increase in SCS (odds ratio: 3.66, 95% confidence interval: 2.89-4.64) and the logarithm of MAC (odds ratio: 4.35, 95% confidence interval: 1.91-9.91) can contribute to a higher CMT score. During the healing process of mastitis, MAC tends to increase as SCC decreases; thus, even samples with low SCC can cause positive CMT reactions. We recommend that practitioners avoid making treatment decisions based on the CMT alone. We also noted that the CMT is sensitive to subclinical mastitis with high MAC, hence it could be considered an alternative to detecting high MAC (chronic) mastitis.

Summary: The left panel shows the estimated California Mastitis Test (CMT) scores as a function of somatic cell score (SCS) and macrophage proportions. Because the SCS is a logarithmic conversion of somatic cell counts (SCC), the same CMT reaction may represent hugely varied SCC values depending on the macrophage proportions. As shown in the 2 right panels, elevated macrophage proportions have been identified in the resolution of mastitis and some subclinical mastitis cases. In such situations, the interpretation of the CMT scores requires more consideration.

Figure 1

Examples of how changes in SCS and macrophage proportions affect California Mastitis Test (CMT) results in 4 different cases (912B, 922B, 753D, and 781C). The 2 reference lines correspond to where SCS equals 4 and macrophage proportions equal to 35% (i.e., differential SCC equal to 65%). In the upper 2 panels, points are labeled with days (D) after identifying mastitis.