Can a somatic cell count be too low?

durst-7-500As herds continue to reduce somatic cell counts and improve milk quality, is there a danger in SCC getting too low?

Phil Durst
MSU Extension Senior Educator
Dairy & Beef Cattle Health

Dairy producers have really been doing a great job with mastitis prevention and control and as a result, herd somatic cell counts (SCC) have been dropping. The NorthStar Cooperative DHI 2012 Annual Performance Summary lists 60 herds with actual SCC under 100,000. The top two leaders in that list had herd SCCs of 31,000 and 35,000. These levels were unimaginable even just a few years ago.

In fact, just five year previous, the annual average SCC of only 17 herds was below 100,000 and the leading herd was at 56,000. As an Educator with Michigan State University Extension, I encourage producers to constantly improve milk quality and udder health.

So, will levels continue to drop? Is there a lower limit? Is it wise to reduce SCC as low as the leaders? Is there any benefit to reducing SCC this low? These are the questions that need to be asked and answered.

At the 2013 National Mastitis Council meeting, Dr. Larry Fox of the College of Veterinary Medicine at Washington State University attempted to do just that in a paper entitled “Can Milk Somatic Cells Get Too Low?” In fact, a talk by the same title was presented at the Annual Meeting of this same group in 2001. Twelve years later it was appropriate to ask again.

Somatic cells (SCs) are primarily leukocytes that are part of the immune system of mammals designed to fight invading bacteria. Fox said that the purpose of SCs in milk is to provide protection to the mammary gland.

Some of those cells serve in a surveillance role seeking foreign invaders. When they find them, they recruit other cells. The first reaction is generally non-specific phagocytosis, or engulfing the bacteria. This is termed innate immunity. If the infection persists, the cells produce specific proteins so that phagocytosis is targeted at the particular pathogen. We call that acquired immunity.

When an infection occurs, there is an influx of immune cells from the blood into the milk and thus we see an increased SCC indicating to us an infection in the mammary gland. So if the number of SCs is low, is there enough ability to detect the invader and respond quickly or do we give up response time and have a greater incidence of clinical mastitis as a result?

Fox concluded based on a number of studies that the number of cells that are in milk at the time is not the primary factor associated with combating an infection, but rather the speed and number that can be mobilized to the gland. In addition, the ability to mount a specific, acquired immune response is critical. These factors are governed by the genetics of the cow. Therefore, low SCC does not leave the cow more vulnerable.

If low SCC is not a problem, is it an advantage? The answer would be “yes” if the cells that produce milk, the secretory cells, are impacted negatively in any way by higher SCC. Research tells us that that is exactly what happens.

Higher SCC is an indication that cells have been recruited from the bloodstream to the milk. It has been shown that the influx of leukocytes causes damage to milk secretory cells. In addition, phagocytosis and bacterial cell death both irritate milk secretory cells. The sensitivity of the secretory cells varies by development stage and it has been found that increased milk SCC is potentially more detrimental during the weeks following calving.

The inflammation associated with chronic or clinical mastitis can lead to blockage of the cell ducts by secretory cells so that the flow of milk is disrupted and reduced. Akers and Nichols in 2011 reviewed data that indicated there is a loss of 1.5 lbs. of milk per day for every doubling of SCC beginning at 12,500 cells/ml. So milk loss begins at even very low SCC.

Therefore, we see that it is unlikely that there is a negative impact of low SCC on the ability of the cow to respond to infection and that there is benefit to low SCC in avoiding milk production loss.

The key is to manage cow udder health on the farm by lowering the exposure of the teats to bacteria in the environment and from other infected cows. Proper preparation for milking, proper milking procedures and a well-maintained milking system are very important.

More and more producers are showing us that lower SCC is not only achievable, but is advantageous. While there are many factors that impact milk production, it is worth noting that the average production of the 19 NorthStar DHI herds with a SCC equal or less than 75,000 was 26,735 lbs of milk.

Monitor udder health not only by watching SCC, but also by culturing clinical cows to determine the problem pathogens and strive to keep clinical mastitis low. Dr. Pam Ruegg recommends a Key Performance Indicator (KPI) for clinical mastitis of less than 25 new cases per 100 cows per year, and a recurrence rate of less than 30 percent of cows with recurrence of mastitis after 14 days post treatment.

Monitor and compare to your KPI’s. Now, set your sights on a low SCC!

One comment

  1. Rosemary /

    Can you tell me what the allowable scc limit is in Washington State. I understand it is 750,000 in the U.S. generally, but that it is lower in Washington, Idaho and California.
    Thank you.

    Rosemary

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