Somatic Cell Count, an indicator of milk quality
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.
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.
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.
Somatic Cell Count (SCC) measurements taken from the bulk tank obviously describe the average herd count. This information cannot define the spread or degree of SCC variation within the herd; only individual testing of cows will determine whether there are, for instance, a significant number of cows with slightly higher-than-average counts or, conversely, a small number of cows with very large counts.
Generally, a threshold of 200,000 cells/ml of milk is used to decide whether a cow is infected. Some cows with few clinical symptoms of mastitis can have a SCC of 400,000 or higher, thus the importance of individual cow monitoring of cell counts is important to determine how many cows in the herd are affected and to what extent.
In smaller herds, the total effect of one or more cows with very high cell counts can be enormous. In the larger herd, milk from a sick cow is diluted and averaged down by the healthy animals. The bulk tanks on very large dairy units can accommodate mastitic cows in the herd without those cows affecting the overall milk quality rating, but for small to average-sized herds, knowing which cows are the 'culprits' enables action to be taken and decisions to be made over how to approach and deal with the problem. Milk recording schemes can calculate the contribution of a single high cell count cow to the herd's overall SCC, in percentage terms, demonstrating the potential improvements in total SSC if that cow's milk was to be not included in the bulk tank.
Where one or more cows with chronic symptoms of mastitis are present, they should be culled as they are a reservoir of infection for unaffected cows and are unlikely to be curable. Subclinically affected cows with high cell counts can be targeted for treatment, and their counts monitored over time to see if they recover or become chronic cases themselves.
In an ideal world, controlling cell counts in the dairy herd by following recommend practices would be simple, but unfortunately due to the innate variability of conditions on dairy farms in the UK and the multifactorial nature of mastitis incidence, the problem of high Somatic Cell Counts can be particularly perplexing for many herds that meet best practice recommendations but fail to significantly lower cell counts.
For these reasons, and because of the differing business aims of different dairy farmers and the differing contractual stipulations of their milk buyers, no single SCC target for all dairy herds may be appropriate or in many cases achievable. However, due to an increasing awareness of cow welfare and pressure to improve milk quality, it is obvious that most dairy farmers have to attempt to improve levels of both clinical and subclinical mastitis in their herds, and to benefit from the bonuses they can receive for producing milk with lower SCCs.
The generally-quoted aims for mastitis control and milk quality on UK dairy farms are:
- A mastitis incidence rate of no more than 30 cases per 100 cows per year.
- A mastitis persistence rate of no more than 20% of the herd affected per year.
- A mastitis re-occurrence rate of less than 10% of the total number of cases.
- A herd-average Somatic Cell Count below 150,000 cells/ml.
- An average Bactoscan result of below 5,000.
In general terms, rule of thumb is for every 100,000 cells/ml increase in the herd bulk SCC, there is an 8-10% increase in the proportion of cows infected in the herd. Persistently high individual cow SCC results can indicate chronic mastitis problems.
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