Physiological Aspects of Milk Somatic Cell Count in Small Ruminants—A Review

The aim of this review was to focus on the physiological aspects of milk somatic cell count (SCC) in small ruminants (SM). The SCC is an important component naturally present in milk and is generally used as an indicator of milk quality and udder health in milk producing ruminants. SCC contains the following cells: polymorphonuclear neutrophils (PMN), macrophages, lymphocytes, and many milk epithelial (MEC) cells, cell fragments, and cytoplasmic particles/vesicles. PMN (40–80%) represent the major cell type in milk in healthy uninfected goats, whereas the macrophages (45–88%) are the major cell type in sheep’s milk. However, dairy goats and sheep have an apocrine secretory system that produces cytoplasmic cellular particles/vesicles and large numbers of cell fragments, resulting in the physiological SCC limit being exceeded. It is obvious that the SCC level in milk of SM can be affected by various influencing factors, such as milk fraction, breed, stage of lactation, parity, type of birth, milking system, and others. An increase in the SCC above the physiological level not only indicates an udder or general health problem but reduces milk production, changes the milk composition, and hence affects milk processing. Moreover, the milking machine plays an important role in maintaining udder health in SM and stable SCC at physiological levels in the milk obtained. So far, there are no healthy or pathological physiological SCC levels defined in SM milk. Furthermore, a differential cell count (DCC) or even a high resolution DCC (HRDCC), which were recently developed for cattle milk, could also help in SM to gain deeper insight into the immunology of the mammary gland and find biomarkers to assess udder health. In conclusion, SCC is an indication of udder health or exposure of the udder to infectious agents or mechanical stress and should therefore always be considered a warning sign.