Somatic cell count (SCC) is related to the total number of cells per millilitre of milk. Primarily, SCC is composed of leukocytes that are produced by the cow’s immune system during an inflammatory process in the mammary gland, commonly referred to as mastitis (Looper, 2012). This inflammation results from the introduction and multiplication of pathogenic microorganisms in the mammary gland, and a complex series of events leading to reduced synthetic activity, compositional changes and an elevated SCC in milk; the magnitude and temporal relationships of these responses vary with nutritional status, other animal factors and the pathogen involved. However, because an elevated SCC is a response to an infection in the mammary gland by pro-inflammatory mediators, the major factor influencing SCC is the infection status. The effects of the stage of lactation, age, season and various stresses on SCC (Rupp et al., 2000) are minor if the gland is uninfected. Except for normal diurnal variation, few factors other than infection status have a significant impact on milk SCC (Harmon, 1994).
The predominant cell type present in milk, besides shedding epithelial cells, is leucocytes, which includes macrophages, polymorphonuclear neutrophil cells (PMNs) and lymphocytes (Boutinaud and Jammes, 2002). Macrophages are generally the predominant cell type in healthy cow milk. They can quickly fight bacterial invasion by phagocytising the antigen and releasing messengers, which are recognised by the PMNs (also phagocytic cells) that will be recruited to the infection site (Li et al., 2014). When PMNs arrive at the site of infection, they will phagocyte microorganisms and kill them using a combination of oxidative and non-oxidative mechanisms (Pham, 2006).
The SCC in milk naturally increases after calving when colostrum is produced before the cow starts lactation, and tends to decrease until the end of lactation; however, this can vary due to many factors, including seasonal and management effects. Essentially, a lower SCC indicates better animal health, as somatic cells originate only from inside the animal's udder. SCC monitoring is important because as the number of somatic cells increases, milk yield is likely to decrease, primarily due to the damage to milk-producing tissue in the udder caused by mastitis pathogens and the toxins they produce, particularly when epithelial cells are lost (AHDB, 2018). However, a low SCC is sometimes related to a poor immune response, but in general terms, this is not necessarily true; there may simply be a low level of current infection. The immune response is best measured by how quickly the immune system reacts to the disease challenge, not how many white blood cells are present before infection occurs (AHDB, 2018).
The cell count tends to reflect a response to contagious mastitis pathogens. The Bactoscan count, on the other hand, indicates the level of bacterial contamination from external sources, such as insufficient sanitation of the milking equipment or poor udder and teat preparation before milking, and can indicate a high level of environmental pathogens (AHDB, 2018). According to the European Union (EU), the udder is considered infected when the SCC is above 200,000 cells/mL; when the SCC is above 400,000 cells/mL, the milk is not acceptable for human consumption. However, the acceptable dairy industry values for milk vary in different countries (Li et al., 2014).
In practice, there are two methods to control mastitis in a herd: culling cows, which is a short-term solution that can quickly reduce SCC in the bulk tan,; and controlling mastitis, which is a long-term solution. The second method is based on: monitoring each cow monthly (with SCC records for a mastitis prevention programme); improving sanitation (keeping the udder clean and free of pathogenic bacteria, which cause mastitis); environmental conditions (bedding must be dry at all times, the grass sod in the pasture or dry lot should be free of mud and objects such as sticks that can damage the udder); special care heifer management (calves should be reared in separate pens to avoid nursing, the fly population must be controlled to decrease the spread of mastitis-causing bacteria, springing heifers should be separated from cows); dry cow treatment (the risk of intra-mammary infections is greatest during the early and late dry period when pathogens are not flushed out on a day-to-day basis); and nutrition to prevent the problem (nutrition is involved in maintaining immunity, and inadequate energy or deficiencies could affect animal resistance) (Looper, 2012).
One of the strategies that could be used to improve cows’ immunity status and consequently, decrease the SCC, is the supplementation of Saccharomyces cerevisiae yeast as a source of metabolites and yeast cell walls (rich in mannan oligosaccharides [MOS] and β-glucans). The metabolites derived from yeast fermentation are an excellent substrate with nutrients that modulate the ruminal flora, speeding up the digestion of cellulose and hemicellulose, which provides benefits to bacteria and protozoa, stabilises the ruminal pH and increases the production of VFAs (Dias et al., 2017a,b).
β-glucans are known as immune system modulators or stimulants. They are natural and effective stimulants of the innate immune system. When they come into contact with phagocytic cells, which recognise the β-1,3 and 1,6 bindings (Petravic-Tominac et al., 2010), these cells are stimulated and will produce some cytokines that start a chain reaction, inducing immunomodulation and improving the response capacity of the innate immune system.
MOS, as mentioned above, are also structural components of the yeast cell wall, and are known for their pathogen (with type 1 fimbria) agglutination capacity, such as diverse Salmonella and Escherichia coli strains. MOS offer a binding site for pathogens, preventing the colonisation of the intestinal epithelium; these agglutinated bacteria will be excreted together with the indigestible part of the fibre.
According to Dias et al. (2017a), yeast used as a source of metabolites reduces the concentration of blood haptoglobin, which is an acute-phase glycoprotein produced by the liver during inflammatory processes; in this case, it was caused by the high level of starch in the diet. Around the world, different field trials using yeast have shown a decrease in SCC (Table 1).
Several studies have shown that using yeast as a source of metabolites can increase milk production by +2 kg/cow/day, increase milk quality (fat and protein), decrease SCC and disease incidence, and decrease mycotoxin contamination in milk. The combination of proper rumen nutrition with the strengthening of the animals’ immune system leads to higher daily milk production, in addition to eliminating concerns about residues in the milk, a key factor to conquer an increasingly demanding consumer market.