How Low Is Too Low?
Weighing the Potential Dangers of Low SCC against the Evidence of the Benefits
It is well known that high Somatic Cell Counts (SCC) within the herd is a reflection of high incidences of mastitis, but could a very low SCC have a similar effect?
SCC is a measure of the cow’s immune response to mastitis causing pathogens.
Some researchers believe that low SCC levels could indicate that the immune system is not functioning adequately to fight off infection. If this is the case, what is the point at which SCC becomes dangerously low? Some studies concluded that low SCC increases the risk of mastitis while others found that there was no relationship between the two. Let’s consider the arguments for both cases.
Low SCC is a Risk Factor for Mastitis
If SCC is a reflection of how well the immune system is working, it seems logical that a low SCC before infection means that the cow will not be able to fight off bacteria and is more susceptible to the disease. The generally accepted threshold for healthy, safe milk is 200,000, but this value is only an indicator of the probability that a cow is free from mastitis. Although unlikely, it is still possible for a cow with an SCC under 200,000 to have mastitis.
One study conducted from 1984 to 1996 by researchers from the Netherlands and Cornell University in a single commercial herd of Holstein-Friesian cows found that low herd SCC levels are negatively correlated with the severity of mastitis; the lower a cow’s SCC was before becoming infected, the more severe the infection would be (Suriyasathaprn, Schukken, Nielsen, & Brand, 2000). This study took into account factors of body condition score, milk yield, fat/protein percentage and additional disease presence in the herd.
Another study conducted between 1995 and 1997 of 121 herds in France found that the incidence of clinical mastitis was higher in herds where more than 50% of cows had an SCC under 250,000 compared to herds where only 15% of cows had an SCC under 250,000 (Beaudeau, Fourichon, Seegers, & Bareille, 2002).
These findings may be related to evidence that slightly elevated SCC due to minor pathogens provides some protection against major pathogens that cause clinical mastitis. This minor infection engages the immune system and outcompetes major pathogens for the resources they need to grow and multiply.
Low SCC does not affect the Incidence of Mastitis
Clinical mastitis can occur without any elevation in SCC, but does this mean that low SCC causes mastitis? Some researchers do not believe so. Somatic cells refer to a number of types of cells present in the udder, one of which are the leukocytes involved in immune response.
The argument that there is no correlation between low SCC and mastitis points to the fact that an effective immune response does not necessarily depend on the number of leukocytes initially present in the udder. Instead, a strong immune response to infection depends on the ability of these cells initially in the udder to recognize bacteria and pathogens and the speed that additional immune cells can then be mobilized from the blood into the udder (Reneau & Farnsworth, 2011).
Researchers from the Veterinary Medicine Teaching and Research Centre of the University of California Davis conducted a short-term study in a single, high yielding, low SCC herd and found no correlation between low individual SCC and mastitis (Deluyker, Gay, & Weaver, 1993). Another study showed that for every doubling of the SCC, yield decreases by 1.5 lbs.
While production decrease is one of the most common effects associated with mastitis, it is surprising that this pattern was found to start at an SCC as low as 12, 500 (Fox, 2013). In addition, there was no difference in the findings when mastitis was caused by major or minor pathogens.
As the most costly disease to the dairy industry, further research into this issue is warranted and it will likely remain a heated discussion topic.
Until the question of low SCC is resolved, it is important to remember that although some studies have found a relationship between the two, high SCC levels are always detrimental to yield, milk quality and herd health. Although being aware that a mastitis infection may occur with low SCC is useful, since most researchers conclude that the correlation between low SCC and mastitis is not the norm, the goal should not be to keep the SCC level above a specific point. If it becomes apparent this is a trend for your herd, professional veterinarian advice should be sought to rule out other contributing factors.
Keeping your SCC low is more likely to ensure that your herd stays healthy and is essential for avoiding penalties and obtaining premiums. There is no substitute for good herd management and disease control which will always be reflected by the absence of mastitis in your herd, regardless of SCC.
The bottom line: Keep striving to lower your herd’s SCC.
Beaudeau, F., Fourichon, C., Seegers, H., & Bareille, N. (2002). Risk of clinical mastitis in dairy herds with a high proportion of low individual milk somatic-cell counts. Preventive Veterinary Medicine, 43-54.
Deluyker, H. A., Gay, J. M., & Weaver, L. D. (1993). Interrelationships of Somatic Cell Count, Mastitis, and Milk Yield in a Low Somatic Cell Count Herd. Journal of Dairy Science, 3445-3452.
Fox, L. (2013). Can Milk Somatic Cells Get Too Low? A Question to be Revisited. Pullman: National Mastitis Council Annual Proceedings.
Reneau, J., & Farnsworth, R. (2011, December). Can SCC be Too Low? University of Minnesota Dairy Initiatives Newsletter.
Suriyasathaprn, W., Schukken, Y. H., Nielsen, M., & Brand, A. (2000). Low Somatic Cell Count: a Risk Factor for Subsequent Clinical Mastitis in a Dairy Herd. Journal of Dairy Science, 1248-1255.
About the Author
Anna Schwanke is an undergraduate student at the University of Guelph, Ontario. She is responsible for researching and writing about a wide variety of topics related to dairy cow welfare and management for Dairy Quality Inc. The 10 years she spent living in Australia, as well as her love of travelling, give her a firsthand viewpoint of issues facing the international dairy community. She plans to graduate from the University’s College of Physical & Engineering Science in 2019 and pursue a career in the Life Sciences or Agriculture industry.
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