Mastitis, mainly caused by bacterial intramammary infection (IMI), is still the costliest disease in milk production. The disease causes an elevated somatic cell count (SCC) in the milk. A commonly used cut-off to distinguish between cows likely to be infected and normal is a SCC at 200,000 cells/mL. However, a major problem of the spread and persistence of mastitis within dairy herds is still the subclinical form, a condition where the udder and the milk appear normal although the mammary gland is inflamed, infected, or both. Detecting these cows early is very important because they act as a reservoir for bacteria, resulting in an unnoticeable spread of mastitis to healthy herd mates (Halasa et al, 2007). Even when the SCC is under the 200,000 cells/mL cut-off level, it does not rule out that the cow is fighting an infection and is in an early stage of mastitis already.
Increased sensitivity to detect IMI
Since recently, differential somatic cell count (DSCC) is available in addition to SCC and the availability of new technology allows the simultaneous determination of both in milk-testing laboratories. The DSCC represents the combined proportion of polymorphonuclear neutrophils (PMN) and lymphocytes as a percentage of total SCC. Using a combination of both SCC and DSCC gives a better understanding of the udder health status and leads to an increase of sensitivity to detect IMI in udder health monitoring programmes. Earlier studies by Wall et al. (2018) showed that DSCC increased significantly following IMI even when SCC did not. Recent studies (Schwarz et al., 2019 and 2020) described test characteristics and predictive values for the combination of DSCC and SCC in connection with IMI at the end of the lactation period or during lactation. In both studies, sensitivity for detection of IMI by major pathogens increased with the combination of SCC and DSCC compared to SCC alone.
Newly defined udder health groups
Due to the novelty of the DSCC parameter, its associations with the performance of dairy cows (e.g. milk production) have yet to be described. A new study delved into this more. The study is based on data originating from routinely performed DHI testing in Austria (federal state Styria only), China (province Henan), Estonia, Germany (federal state Thuringia), and Spain (province Lugo, Galicia). In total, 961,835 test-day results generated in these countries between January 2019 and March 2020 were available for data analyses. Cows were categorised into four newly defined udder health groups (UHG) based on their test-day DSCC and SCC results as described in Table 1. The SCC and DSCC cut-offs used for categorisation were identified as ideal cut-offs in a previous study (Schwarz et al., 2020). Moreover, the SCC cut-off of 200,000 cells/mL is also recommended by IDF (2013). The main part of the study was focused on analysing the performance of dairy cows in these four different groups.