Somatic cell count
Encyclopedia
Somatic cell count is an indicator of the quality of milk. Somatic cells are leukocytes (white blood cell
White blood cell
White blood cells, or leukocytes , are cells of the immune system involved in defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a...

s). The number of somatic cells increases in response to pathogenic bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 like Staphylococcus aureus
Staphylococcus aureus
Staphylococcus aureus is a facultative anaerobic Gram-positive coccal bacterium. It is frequently found as part of the normal skin flora on the skin and nasal passages. It is estimated that 20% of the human population are long-term carriers of S. aureus. S. aureus is the most common species of...

, a cause of mastitis
Mastitis
Mastitis is the inflammation of breast tissue. S. aureus is the most common etiological organism responsible, but S. epidermidis and streptococci are occasionally isolated as well.-Terminology:...

. The SCC is quantified as cells per ml. General agreement rests on the values of less than 100,000 cells/ml for uninfected cows and greater than 300,000 for cows infected with significant pathogen
Pathogen
A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...

s. The California mastitis test
California mastitis test
The California Mastitis Test is a simple cow-side indicator of the somatic cell count of milk. It operates by disrupting the cell membrane of any cells present in the milk sample, allowing the DNA in those cells to react with the test reagent, forming a gel...

 provides a cow-side measure of somatic cell count. The somatic cell count in the milk also increases after calving when colostrum
Colostrum
Colostrum is a form of milk produced by the mammary glands of mammals in late pregnancy. Most species will generate colostrum just prior to giving birth...

 is produced.

Somatic cell count and bacteria plate count

The methods of determining Grade A milk quality are well established, and are based on the somatic cell count and the bacteria plate count. Generally a lower somatic cell count indicates better animal health, while the bacteria plate count indicates improved equipment sanitation.

Somatic cells originate only from inside the animal's udder, while the bacteria are usually from external contaminations, such as insufficient cleaning of the milk transport equipment or insufficient external cleansing of the cow's udder and teats prior to milking. Milking equipment can also be accidentally knocked or kicked off an animal onto the floor, and contaminants on the barn floor can be sucked into the milk line by the system vacuum. A filter sock or filter disk in the pipeline prevents large particulate contaminants from entering the milk bulk tank, but cannot remove bacterial contamination once it has occurred.

For example as defined by the State of Indiana administrative code, grade A milk shall meet the following standards:
  • The bacterial estimate classification shall be "acceptable".
  • The bacteria count using the standard plate count, direct microscopic count, or plate loop count methods shall be not more than one million (1,000,000) bacteria per milliliter.
  • The somatic cell count shall be not more than one million (1,000,000) cells per milliliter.
  • The milk shall not contain drug residues.


Milk not meeting these standards shall be designated as undergrade. Undergrade milk may not be sold for human consumption or processing into products for human consumption.


As established, these measurements are taken daily from the milk bulk tank
Bulk tank
In dairy farming a bulk milk cooling tank is a large storage tank for cooling and holding milk at a cold temperature until it can be picked up by a milk hauler....

 and not from individual cows. This is because testing of individual animals at each milking would be expensive, but it also means that milk from a sick cow is diluted and averaged down by the healthy animals. The huge bulk tanks at very large commercial farms are accommodating of more sick animals in the herd, without the sick animals affecting the overall milk quality rating. However many different state and governmental agencies (including FDA) inspect each load of milk delivered to the processing facility as well as the processing facilities themselves to ensure that all milk processed through those facilities is safe for all consumers.

As discussed in the paper Guidelines for Using the DHI Somatic Cell Count Program:
  • The results of many studies suggest that cows with SCC of less than 200,000 are not likely to be infected with major mastitis pathogens, but cows with SCC above 300,000 are probably infected (Smith, 1996).
  • Herds with bulk tank SCC above 200,000 will have varying degrees of subclinical mastitis
    Mastitis
    Mastitis is the inflammation of breast tissue. S. aureus is the most common etiological organism responsible, but S. epidermidis and streptococci are occasionally isolated as well.-Terminology:...

     present. Data from the National Mastitis Council (1987) show that 6% of the [udder] quarters in a herd could be expected to be infected in a herd with a bulk tank SCC of 200,000.
  • At 500,000 SCC, 16% of the quarters may be infected with a 6% reduction in milk production compared to a SCC of 200,000.



Bacteria in milk can come from sources other than the animal. Over time the milking pipeline
Milking pipeline
A milking pipeline or milk pipeline is a component of a dairy farm animal-milking operation, which is used to transfer milk from the animals to a cooling and storage bulk tank.-Setup:...

 and equipment can become coated with residues such as milkstone which are not removed by standard detergents and require periodic flushing of equipment with high strength corrosives. Automatic washing equipment for the bulk tank
Bulk tank
In dairy farming a bulk milk cooling tank is a large storage tank for cooling and holding milk at a cold temperature until it can be picked up by a milk hauler....

may not effectively clean all interior surfaces, and does not clean the exterior of the bulk tank at all.

Milk processors and co-ops purchasing milk routinely award farmers for having the lowest possible SSC counts via "quality bonuses" added to each milk payment to the dairyman.
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