CanWest DHI - Profitable Dairy Herd Management

SCC: Understanding the Basics

What are somatic cells?
White blood cells in milk, together with a small number of epithelial cells from milk secreting tissues, make up what are known as somatic cells. Somatic cells are produced by the cow’s immune system to remove or destroy infection causing bacteria. When infection occurs in the udder, somatic cells are mobilized to go fight
that infection.

What does an elevated SCC tell you?
The single most important reason for an increase in SCC is infection of one or more quarters with mastitis-causing bacteria. However, the level of SCC response and rise is not always consistent based on the type of bacteria or the severity of the infection. Based on research, it is well accepted that cows with a cell count greater then 200,000 should be considered as suspect for having a mastitis infection. As the SCC rise, so does the probability of an infection.

What is a normal SCC and can it be too low?
Given that somatic cells play an important role in immune response and fighting infections, a certain amount of cells is to be expected in the milk and is totally normal and even desirable. A count of zero, or extremely low is probably not ideal and may compromise the cows’ ability to effectively respond to an infection. Counts in the 25-50,000 cells are perfectly normal, but lower counts should not be a cause for concern.

How is SCC actually measured in the lab?
When a milk sample is analyzed for SCC using an automated method such as at any CanWest DHI lab, the measuring principle used is called flow cytometry. This means a very thin string of milk (think thinner than a human hair) is passed under a counting unit. The diameter of the string of milk is such that only one cell can pass under the counting unit at one time. Before passing through the flow cell under the counting unit, the milk is mixed with a fluorescent dye, which dyes the DNA molecules in the somatic cells.

When passing under the counting unit, the sample is exposed to blue light, which excites the dyed cells, making them emit red light. These red light pulses are magnified and counted to give the number of somatic cells per millilitre.

This is a very fast, proven, accurate and reliable way to get your cow’s SCC result. To ensure that the instrument is reading the SCC correctly, the lab continually purchases sets of “SCC standards”, which is milk that has been analyzed for SCC by drying and staining a thin smear of milk, and having the cells counted by hand under a microscope by a trained analyst.

These samples are very expensive because of the work involved in getting an accurate SCC count, but they confirm the automated instrument performance thus are integral to lab operations. The lab runs these and in-house quality standards for SCC testing weekly, daily, and hourly, to ensure that the instruments continue to give accurate SCC counts when testing. In addition to these quality control checks, if the lab has more than one instrument testing for SCC, the lab will measure each instrument’s performance against the other instruments in the lab, to validate that they agree with each other. This is done on a weekly basis. If the instruments do not agree, the lab will investigate the root cause of the problem and have the instruments serviced to correct the problem if necessary.

SCC naturally goes up in summer months - what, if anything, can be done about this?
Only part of this statement is correct! Yes, on average mastitis infections and SCC levels do increase during the hot, humid summer months. HOWEVER, a large number of herds maintain low SCC well below 200,000 cells year-round.

It really comes down to keeping cows cool, dry, clean and minimizing the exposure to environmental bacteria. It does become more difficult to do during hot summer months, but it is very doable.