Somatic Cell Count - milk quality indicator

The Somatic Cell Count

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 is simply 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 environmental pathogens.

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