Somatic Cell Count (SCC ) as a Novel Indicator of Raw Milk Quality in Dairy Cows
The Somatic Cell Count (SCC) is a main indicator of milk quality. The majority of somatic cells are leukocytes (white blood cells) – which become present in increasing numbers in milk usually as an immune response to a mastitis-causing pathogen – and a small number of epithelial cells, which are milk-producing cells shed from inside of the udder when an infection occurs. Somatic cell counts are widely used to predict the mammary health status of quarters and cows , the suitability of milk for human consumption , and monetary losses to producers due to mastitis . Increased cell counts indicate increased inflammation, very likely caused by intramammary infection . Infection and disease are end products of failures in proper application of milk production hygiene.
WHAT ARE SOMATIC CELLS?
Somatic cells are mainly milk-secreting epithelial cells that have been shed from the lining of the gland and white blood cells (leukocytes) that have entered the mammary gland in response to injury or infection (Dairyman’s digest, 2009). The milk somatic cells include 75% leucocytes, i.e. neutrophils, macrophages, lymphocytes, erythrocytes, and 25% epithelial cells. Erythrocytes can be found at concentrations ranging from 0 to 1.51×106 /ml (Paape and Weinland, 1988). Studies identifying cell types in milk have shown that epithelial cells or the cells which produce milkare infrequently found in udder secretions, including the dry gland, at levels ranging from 0 to 7% of the cell population (Lee et al., 1980). The epithelial cells of the glands are normally shed and get renewed, however, during infection the numbers increase. The white blood cells serve as a defense mechanism to fight infection and assist in the repair of damaged tissue. During inflammation (mastitis) the major increase in SCC is due to the influx of neutrophils into the milk to fight infection and have been estimated at over 90% (Miller and Paape, 1985; Harmon, 1994) and the measurement of SCC in milk is known as a somatic cell count. The normal composition of milk somatic cells varies with the type of secretion or lactation cycle . Normally, in milk from a healthy mammary gland, the SCC is lower than 1×105 cells/ml, while bacterial infection can cause it to increase to above 1×106 cells/ml.
FUNCTION OF SOMATIC CELLS
Mastitis is caused by bacterial invasion into the udder. The small numbers of somatic cells that are normally present in milk attempt to resolve this intramammary infection immediately. The cellular presence in milk is one of the important protective mechanisms of the mammary gland and may be considered as a surveillance function in the uninfected gland. Both bacteria and leukocytes in the infected quarters release chemo-attractive products for leukocytes, especially neutrophils. The neutrophil polymorphonuclear (PMN) leukocytes are the second line of defense against mammary gland infection. PMN’s are phagocytic cells which engulf and kill bacteria. However, in bovines, the phagocytic ability of PMN of milk can consume milk fat globules and casein (Opdebeeck, 1982) leading to putrefaction of milk. An inflammatory response is usually initiated when bacteria enter the mammary gland through the teat canal and multiply in the milk. Although bacterial toxins, enzymes and cell-wall components have a direct effect on the function of the mammary epithelium, they it also stimulate the production of numerous mediators of inflammation, mainly neutrophils (Gallin et al., 1992), due to edema, vasodilation and increased vascular permeability. Blood monocytes become macrophages in the tissues and are the major cell type in milk during involution of the udder. During bacterial pathogenesis, macrophages serve to facilitate either innate or acquired immune responses. During lactation, the proportion of macrophages is highest (68%) in the early post-partum period and lowest (21%) in late lactation (Park et al., 1992). Similar to neutrophils, the non-specific functions of macrophages are to phagocytise invading bacteria and destroy them with proteases and reactive oxygen species (ROS) (Mullan et al., 1985). Lymphocytes are the only cells of the immune system that recognize a variety of antigenic structures through membrane receptors, which define their specificity, diversity and memory characters (Boyso et al., 2007). T-lymphocytes and B-lymphocytes are two subsets of lymphocytes that differ in function and protein products and play specific immune functions (Harmon, 2001). The mammary epithelial cells may play a protective role in prevention of infection via ingestion and possible digestion of phagocytosed microbes. The mammary epithelial cells are able to produce a variety of inflammatory mediators such as cytokine, chemokines, host defense peptides and arachidonic acid metabolites.
The SCC is quantified as the number of cells per ml of milk. In general terms:
- An individual cow SCC of 100,000 or less indicates an ‘uninfected’ cow, where there are no significant production losses due to subclinical mastitis.
- A threshold SCC of 200,000 would determine whether a cow is infected with mastitis. Cows with a result of greater than 200,000 are highly likely to be infected on at least one quarter.
- Cows infected with significant pathogens have an SCC of 300,000 or greater.
Normal Milk
Mastitis is defined as an inflammation of the mammary gland. Milk SCC are universally accepted and applied as a measure of inflammation in lactating mammary glands. Normal milk does contain cells, and the concentration of these cells is almost always less than 100,000 cells/ml in milk from uninfected/uninflamed mammary quarters . This is based on twice daily milking at regular intervals. Milk somatic cells are primarily leukocytes (white blood cells) and some epithelial cells shed from the lining of the mammary gland . The leukocytes are derived from blood and consist of macrophages, lymphocytes, and polymorphonuclear cells, primarily neutrophils (PMN) . The macrophages are involved in immune recognition and are the predominant cell type present in milk from uninfected quarters. Lymphocytes are responsible for immune memory. The PMN are a small proportion of the cells in milk from uninfected glands, but become the predominant cell type in milk from infected glands . The PMN are a primary means of defense against an invasion of the mammary gland by microorganisms, and intramammary infection is the major determinant of the concentration of cells in milk . Research has clearly shown that environmental or physiological factors such as lactation number, stage of lactation, estrus, mild exercise, and heat stress have marginal effects on SCC from uninfected (bacteriologically negative) quarters .
Abnormal Milk
Clinical mastitis is, by definition, ìabnormal milkî and no reference to SCC is required . However, clinical mammary quarters will almost always have SCC greater than 200,000 cells/ml. Milk from healthy, uninfected mammary glands has a white to white-yellow appearance and is free of flakes, clots, or other gross alterations in appearance. Such abnormalities are indicators of milk that is unsuitable for human consumption. The vast majority of these abnormalities appear as a result of bacterial infection of the mammary gland. In general, the more severe the infection, the greater the abnormal appearance of the secretion from the infected quarter. When quarter SCC are equal to or exceed 200,000 cells/ml and bacteria are isolated in the absence of clinical changes, then the quarter is defined as being subclinically infected or is designated as having subclinical mastitis. At what SCC should quarter milk be considered abnormal? Mammary quarters from which no microorganisms can be isolated and that have no history of recent infection will almost always have a SCC of less than 100,000 cells/ml . When cell counts in quarter milk are equal to or exceed 200,000 cells/ml, the odds favor that the quarter is infected or is recovering from infection . A cell count of 200,000 cells/ml or greater is a clear indication that an inflammatory response has been elicited (subclinical mastitis), the quarter is likely to be infected, and the milk has reduced manufacturing properties such as reduced shelf life of fluid milk, and reduced yield and quality of cheese . At our current state of knowledge, cell counts of 100,000 to199,999 cells/ml represent a range of counts difficult to attribute to inflammation and/or intramammary infection.
The SCC in the milk increases after calving when colostrum is produced before the cow settles into lactation, and tends to rise towards the end of lactation, most likely due to the concentrating effect of lower amounts of milk being produced. SCCs vary, however, due to many factors, including seasonal and management effects.
Dairy farmers are financially rewarded for low herd SCCs and penalised for high ones, because cell counts reflect the quality of the milk produced and how mastitis can affect its constituent parts, having implications for its keeping abilities, its taste and how well it can be made into other dairy products such as yoghurt or cheese. Milk contracts often define several SCC ‘thresholds’ and any respective bonus for attaining them. Milk with an SCC of more than 400,000 is deemed unfit for human consumption by the European Union.
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 fall, 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.
A particularly low SCC is sometimes regarded as a sign of poor immune response, but in general terms this need not be necessarily true; it may be the case that there issimply a low level of current infection. 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.
Cell counts tend 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 cleaning of the milking equipment or poor udder and teat preparation prior to milking, and can indicate a high level of environmentalpathogens.
FACTORS AFFECTING SOMATIC CELL COUNT
There are plenty of factors that influence milk somatic cell count at individual and herd level apart from intramammary infection. The ability to correctly interpret somatic cell counts depends on an understanding of the factors which may affect the number of somatic cells.
Mammary gland infection level (Mastitis)
The most important factor affecting the somatic cell count of the milk from an individual quarter depends upon the infection status of the quarter (Dohoo and Meek, 1982). Sharma (2003) analyzed 2161 milk samples from lactating cows and demonstrated that SCC ≤100,000 cells/ml could be considered as threshold or negative for the California mastitis test (CMT) . The degree and nature of the cellular response are likely to be proportional to the severity of the infection (Figure 2). The average number of composite (cow) milk SCC increases with an increase in thenumber of quarters infected (Meek et al., 1980) and having a major influx of PMN into the milk (Craven and Williams, 1985; Miller et al., 1990).
Stage of lactation
SCC increases with progressing lactation (late lactation) regardless of whether the cow is infected or not (Dohoo and Meek, 1982). SCC elevation has been linked with an animal’s innate immune response in preparation for calving and to enhance the mammary gland defense mechanism at this critical calving time (Reichmuth, 1975). During early and late lactation the percentage of neutrophils tends to increase while the percentage of lymphocytes decreases (McDonald and Anderson, 1981). At parturition SCC are usually higher than one million per ml and decreases to 100,000 cells/ml in the 7 to 10 days post-partum (Jensen and Eberhart, 1981) (Table 2). The presence of high cell numbers has also been reported in colostrum and appears due to an excessive desquamation of epithelial cells in a small volume of milk in a gland resuming functional activity after a dormant period (Schalm et al., 1971).
Age/Breed
Various researchers have reported that SCC increases with increasing age (Beckley and Johnson, 1966; Blackburn, 1966) (Table 3). This increase is primarily due to an increased prevalence of infection in older cows and is not due to any large increase due to age per se (Reichmuth, 1975). SCC variation has been noted between breeds of dairy animals. The high-producing cattle breeds such as Brown Swiss (423.31×103 cells/ml) and Black Holstein (310.36× 103 cells/ml) have higher presence of SCC/ml in milk.
Parity/Season/Stress
The level of SCC has been reported to be influenced by parity (Blackburn, 1966; Lindström et al., 1981). There is little change in SCC of uninfected quarters as number of lactations increases (Sheldrake et al., 1983) but SCC increases with advanced parities (Skrzypek et al., 2004). Somatic cell counts are generally lowest during the winter and highest during the summer season (Khate and Yadav, 2010). During summer, the growth and number of environmental bacteria is increased in the bedding material of housed stock due to favorable temperature and humidity (Harmon, 1994). Free radicals are generally produced during stress due to milking techniques, environmental and infectious organisms (teat injury). These radicals are unstable and react quickly with other compounds in order to capture the electron to gain stability (Smith et al., 1985).
Diurnal variation
In general, SCC that is lowest just before milking increases rapidly on stripping, and may persist for up to 4 hours after milking and then gradually declines. This difference in high and low SCC varies from 4 to 70-fold for individual quarters (White and Rattray, 1965). Studies have also shown that two consecutive milkings from the same cow could fluctuate in SCC by 30%. Day to day variation in cell counts has also been investigated and revealed that SCC could fluctuate to more than 40% without any of the circumstances described above.
Milk transportation/Management
Methods of transportation and storage of milk samples have been demonstrated to affect SCC count (Dohoo et al., 1984). Gonzalo et al. (2003) used different milk preservatives e.g. potassium dichromate (100 mg/100 ml), azidiol (24 mg sodium azide/100 ml) and bronopol (50 mg/100 ml) for counting and revealed the highest SCC in samples without preservative (5.72×103 cells/ml), with bronopol (5.67×103 cells/ml), potassium dichromate(5.63×103 cells/ml) and azidiol (5.62×103 cells/ml). There are many management factors that play a most important role in the development of contagious disease like mastitis in dairy animals. Amongst these, unhygienic conditions are more important in increasing the chances of intramammary infection (IMI) and resulting in high SCC. Other management factors pertain to the type of flooring, feeding, teat dipping and milking techniques etc. Teat injuries and leakers commonly develop because of stall and platform design raising the incidence of mastitis and causing higher SCC. Using a post-milking teat dip appears to predispose some very low SCC herds to more clinical mastitis-in particular mastitis caused by E. coli. Recently, hygienic milking has come into practice routinely to prevent the spread of Staph. aureus inflicting contagious mastitis.
METHOD FOR MEASURING SOMATIC CELLS
More recently, automated devices for rapidly determining the SCC of milk samples have become available. On-going development in counting technology has resulted in the routine application of high capacity flow cytometric counters with much improved performance in advanced milk testing laboratories. The two most commonly-used devices are the Coulter Milk Cell Counter, which counts particles as they flow through an electric field, and the Fossomatic which stains cells with a fluorescent dye and then counts the number of fluorescing particles. Both devices are capable of rapid determination of the SCC in large numbers of samples. Details of the procedures used by each device have been published by various workers (Heeschen, 1975; Gonzalo et al., 2003) and will not be discussed further in this paper. The direct microscopic method is inexpensive and most commonly used in India (Sharma, 2003) (Figure 3). However, there is very little information on the specific application of these methods in ewe milk (Gonzalo et al., 1993), which has a higher content of total solids than cow milk. When evaluating macrophages on a stained milk film, many will have a “foamy” cytoplasm that could be analysed using the Fossomatic method (Gonzalo et al., 2003).
SCC BASED INTERPRETATIONS
The two different methods may be used to calculate an “average somatic cell count” when multiple samples have been taken. For example, if the past three-month cell counts were 600,000, 400,000 and 500,000, the average would be calculated by arriving at a total and dividing by 3, (1,500,000/3=500,000). This produces an “arithmetic mean” or average. A different method, used in Europe and other locations, is used to calculate an average somatic cell count. It is termed the “geometric mean”. The geometric mean calculation always produces a value somewhat less than the arithmetic mean for the same data set. A single high count in a data set has a greater impact on the arithmetic mean than the geometric mean and one very high value is not as likely to trigger regulatory action using the geometric mean procedure (Ingalls, 2001). Research has established a straight-line relationship between milk loss and the logarithm of the SCC. This value is referred to in Canada as “Linear Score” (LS) and in the US as Somatic Cell Score (SCS). Increase in linear score with the doubling of SCC has been recorded by Ingalls (2001) as shown in Table 5. All lactating cows have a low baseline SCC even if they do not have an intramammary infection (IMI). When an infection is detected by the immune system in a healthy cow, a rapid influx of leukocytes will quickly raise the SCC far beyond the baseline level, usually to over a million cells/ml. In most developed dairy industries various regulatory limits has been applied to milk for human consumption. The European Union Directives (92/46CEE and 94/71 CEE) set a limit of 400,000 cells/ml for SCC in raw buffalo milk, when the milk is used for products made with raw milk. In US, the legal maximum somatic cell count for Grade A farm bulk milk is 750,000 cells/ml, this limit is high compared to many international standards. Much of Europe, New Zealand and Australia has a limit of 400,000 cells/ml and Canada has a limit of 500,000 cells/ml of raw milk. Milk SCC is a diagnostic figure for subclinical mastitis (International Dairy Federation, 1999). Cow milk SCC of >200,000 cells/ml indicates mastitis (International Dairy Federation, 1997; Hillerton, 1999). Recently, a line has been drawn for SCC that a level below 100,000 cells/ml represents a healthy quarter. However, some researchers consider a normal SCC to be up to 500,000 cells/ml. However, it has been proposed that quarters having a cell count of 200000 cells/ml and whole cow milk cell count of 400,000 cells/ml to indicate mastitis (Hillerton, 1999). Therefore, mastitis should be detected in a reliable and timely fashion based on SCC values, otherwise subclinical mastitis could develop into a clinical disease (Hallén Sandgren et al., 2008).
NDRI ‘sets’ somatic cell counts limit in milk
Scientists of the National Dairy Research Institute (NDRI) have set standard limits of somatic cell counts (SCC) in milk of indigenous cattle and Murrah buffalo to monitor the prevalence of sub-clinical mastitis in dairy herds, quality of raw milk and hygienic conditions at farms.
They are trying to set up a reference value of milk cells in other animals across the country so that the milk products can meet international standards. “Setting up of legal limit for SCC in milk will reduce antibiotics residues in milk as well as will help in reducing udder infection and produce grade-A milk. It will also help in growth of both quality and quantity of milk leading to increase in farmers’ income and also improve cow comfort and welfare,” claimed NDRI Director Dr MS Chauhan.
Dr Chauhan said the research would help in enhancing the export quality of milk and milk products. “With 189 million metric tonne of milk production annually, India is leading the world, but our milk export is less than 10 per cent.
All the developed countries such as the US, European Union and others use SCC in milk for quality parameter and paying more premiums for the milk having low somatic cell counts, while in India, milk quality and pricing are still based on the fat percentage in milk. This research will help in enhancing the milk quality,” he said. After a research of more than 10 years, Dr Ajay Dang, principal scientist, lactation and immunophysiology laboratory, NDRI, along with his team members consisting of scientists and students set the SCC limits up to 1 lakh/ML in Murrah buffalo milk and 1-1.5 lakh/ML in ingenious cattle milk, which are gold value in these animals.
“We conducted the research on more than 500 indigenous cattle and Murrah buffalo. We isolated and cultured milk cells from the udder gland in order to develop cell count sensors for these animals. Any type of stress to the animals increases the SCC in milk. We consider SCC above 2 lakh in milk may indicate sub-clinical mastitis, poor milk quality and products,” said Dr Dang.
Compiled & Shared by- Team, LITD (Livestock Institute of Training & Development)
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