Is there an ideal somatic cell count?

Can somatic cell counts get too low?
That is the title of a recent article in Dairy Herd Management magazine. In the article, Amber Yutzy, from Pennsylvania State University, suggests cows with counts below 50,000 cells per milliliter might respond more slowly to infection.
Is this true? No.
The preponderance of evidence suggests low SCC cows and herds have lower rates of clinical mastitis. To be fair, Yutzy is quoted at the end of the article saying, “The benefits of having a low SCC far outweigh the risks.” But, she also states, “Ideally an individual cow cell count should be between 100,000 and 150,000,” which is clearly not true.
There is no benefit to the cow by having a count between 100,000 and 150,000, and there are clear benefits to having a lower count. Normal, healthy cows typically have counts around 20,000 to 50,000. A cow with a count of 100,000-150,000 most likely has one infected quarter that is causing the composite sample to increase. Remember, we do not typically measure SCC by quarter. So, how is it possible that a cow with one infected quarter with, say, a quarter level SCC of 750,000 and three quarters with SCC levels of 25,000, for example, is less likely to get infected or to get clinical mastitis? In reality, we do not have healthy cows with four uninfected quarters each having a SCC of 100,000 to 150,000.
The idea of decreased susceptibility due to increased counts assumes there are more cells present in the udder, and the increased cell numbers somehow make the udder respond quicker to infection. However, in reality, those cells are continually being released into the milk in response to an infection and being flushed out of the udder two or three times per day via milking. Cows with elevated counts have already been infected, and cells are continually being released into the udder in response to that infection. So, the idea of infecting a cow to prevent her from becoming infected makes no sense. It is like cutting off one’s arm to prevent it from being cut off.  
Some have claimed an association between low herd somatic cell counts and a greater proportion of coliform infections. However, if true, this is likely explained by the observation that the cow’s SCC drops rapidly following a coliform infection, yet often takes weeks to months with infections from gram-positive environmental organisms and may not drop at all when infected by contagious pathogens. Thus, very high SCC herds would be expected to have more gram-positive and chronic infections, and low SCC herds would have a greater proportion of gram-negative infections. This does not mean low SCC causes more coliform infections.  
According to a review paper of this subject by Rainard (Journal of Dairy Science, 101:8, 2018), “It seems unlikely that the low concentrations of leukocytes found in milk from a healthy mammary gland play a significant role in its defense.” In other words, the whole concept that resident somatic cells of the udder actually defend the udder from infection is not true. If fact, a feature of a healthy mammary gland is its capacity to mount a swift and massive inflammatory reaction, bringing hordes of a particular type of white blood cell, the neutrophil, to milk that can reach concentrations of more than one hundred million cells per milliliter, according to the article.   
So, what is a healthy herd SCC? The typical Holstein cow at one month in lactation has a baseline SCC of about 20,000. The herd SCC is determined by how much shedding and how much milk infected quarters produce. For example, if you have a few cows with very high quarter level SCC, and they produce a lot of milk in those quarters, the herd SCC may rise significantly just from their contribution. In simple terms, the herd SCC is a combination of the contribution of healthy cows with SCCs 20,000-50,000 and infected, unhealthy cows with much higher counts. Thus, even a herd SCC of 100,000 is not really healthy. Ideally, herd counts would be the same as the count of a normal cow, or 20,000-50,000. It is just that it is so darn difficult to keep at least some cows from getting infected, even in really clean herds, so we do not see many herds with herd SCCs below 50,000.
My own 42 years of experience working with top dairy producers confirms the available evidence in that herds with very low herd SCCs have very little clinical mastitis. In fact, in those herds, clinical mastitis is an unusual occurrence, and producers may be unconcerned about the occasional case.  Maintaining great udder health in those herds seems to be much easier than maintaining decent udder health in herds with SCCs in the range of 150,000 to 200,000. This seems to be one of those dairy secrets that only certain producers seem to know.   
The idea that a herd’s somatic cell count can be too low seems to surface in the press every few years. It is not true. One cannot be too healthy nor can an udder. The rewards of excellent udder health are remarkable, and we should continue to pursue that goal.
Bennett is one of four dairy veterinarians at Northern Valley Dairy Production Medicine Center in Plainview, Minnesota. He also consults on dairy farms in other states. He and his wife, Pam, have four children. Jim can be reached at [email protected] with comments or questions.