Dairy cows face continual exposure to bacteria capable of causing many diseases including mastitis. A key element of mastitis control involves bacteria control. This includes maintaining a clean environment to limit cow’s exposure to environmental pathogens, minimizing cow-to-cow transfer of contagious mastitis organisms at milking time and keeping cows healthy so they can effectively fight off challenges which occur. There are practical limits in minimizing exposure to bacteria and inevitably cows face challenges.
Somatic cell discussions, which inevitably are a part of mastitis discussions, must consider several key points. The following discussion examines the source and function of somatic cells and concentration in milk and relationship to milk production.
Somatic cell origin and function
Cows have a well-developed immune system to help protect against bacterial infections. Specialized somatic cells, produced by the immune system, normally can be mobilized quickly in the event of a bacteria challenge. These cells are transported by the blood stream to the infection site. They can pass through blood vessel walls into infected areas and they have the tools to find and kill invading bacteria and protect the body. While there are several types, as a group they are referred to as white blood cells, leukocytes or somatic cells.
From a dairyman’s perspective, somatic cells in milk can be both friend and foe. Their numbers in milk provide a solid indication of udder health status. Generally somatic cells function with little notice. Occasionally, during a severe mastitis challenge, the number of somatic cells and cellular debris causes noticeable changes in the physical characteristics of milk including clots, flakes etc.
Somatic Cell Counts and Their Use
Electronic cell counters used by milk quality labs allow somatic cell concentrations in raw milk to be determined quickly and accurately. Producer bulk tank milk samples are checked at least monthly, sometimes more frequently, by the milk buyer. These samples are evaluated for bacteria and somatic cell concentrations to determine quality and acceptability of the milk. In many cases results are used to determine producer eligibility for some level of bonus payment based on the milk somatic cell count. This makes somatic cells important in determining the economic value of milk but they are always important in terms of udder health.
Milk somatic cell counts are strongly correlated with udder infection status that, in turn, is related to production losses and milk quality issues. High cell count milk provides less casein for cheese production and the protein generally is of lower quality in terms of curd characteristics, etc. High cell count milk is also known to cause reduced shelf life in fluid milk. For both producers and processors these concerns translate into monetary losses.
SCC-Production and Quality Issues
What is the basis for categorizing somatic cell counts and their relationship to production and quality issues?
Cell count, production and milk quality data collected on huge numbers of cows and farms have been analyzed over the years to help establish the inter-relationships amongst these issues. Generally the raw milk somatic cell count (SCC) is expressed as thousands of cells per milliliter and values for individual cows may range from 50,000 to several million per milliliter depending on their infection status. When testing is done on a bulk tank sample the value is termed the bulk tank SCC or BTSCC.
Researchers have established that a predictable relationship exists between individual cow SCC’s and production losses (2). It is more easily expressed if the raw somatic cell count is mathematically converted to a smaller number or index. The resulting index, the SCC score (SCCS), is determined by converting the raw count to a single digit number, 0-8. A key relationship is that for 2nd calf and older cows, each time the SCC score increases one unit (i.e. from 4 to 5) the SCC doubles, and there is a loss of approximately 400 pounds of milk for that lactation. Additionally, the milk loss starts at low cell counts. Moving from 100,000 to 200,000/ml results in a 400 pound loss, as does a change from 800,000 to 1,600,000 so the production penalty starts early. The following table summarizes this relationship (6).
Production Losses for 2nd Calf and Older Cows Associated with Increasing SCC Scores
|
Lactation Average
SCC Score (SCCS) |
Lactation Average
SCC (1000s/ml) |
Difference in Milk Yield
(lbs/305 days) |
|
0 |
12.5 |
-- |
|
1 |
25 |
-- |
|
2 |
50 |
-- |
|
3* |
100* |
400 |
|
4 |
200 |
800 |
|
5 |
400 |
1200 |
|
6 |
800 |
1600 |
|
7 |
1,200 |
2000 |
|
8 |
2,400 |
2400 |
Production losses are assumed to start at 100,000 cells/ml (SCCS of 3).
The relationship between the raw milk somatic cell count and milk components has been well documented (2, 4). The following table summarizes some key issues (2).
Changes in Milk Associated with Elevated SCC
|
Milk Constituent |
SCC (x1000/ml) |
Reason of Change |
|
<100 |
<250 |
500-1000 |
>1000 |
|
Decrease (in g/100 ml)
· Lactose
· Casein
· Fat |
4.90
2.81
3.74 |
4.74
2.79
3.69 |
4.60
2.65
3.51 |
4.21
2.25
3.13 |
Reduced
synthesis |
|
Increase (in g/100/ml)·
Whey proteins(total)
· Serum albumins
· Immunoglobulins
· Chloride
· Sodium
· Potassium
· pH
|
0.81
0.02
0.12
0.091
0.057
0.173
6.6
|
0.82
0.15
0.14
0.096
0.062
0.180
6.6 |
1.10
0.23
0.26
0.121
0.091
0.135
6.8 |
1.31
0.35
0.51
0.147
0.105
0.157
6.9 |
Leakage from blood |
· Source: Schallibaum, Melchior. National Mastitis Council, Inc. 40th Annual Meeting Proceedings. 2001
Milk casein, milk fat and lactose declines as the cell content increases (3). Such decreases lessen the value of the whole milk, since these components are generally part of the pay formula along with volume produced. The increase in blood components that leak into the milk leads to increased conductivity of milk and off-flavors as well. As a result, high cell content milk is a less valuable raw material for milk processors.
Research has also determined that high SCC milk used for fluid purposes suffers reduced shelf life and more off-flavors than low cell count milk. Even though pasteurization kills bacteria, the presence of enzymes in high cell count milk may damage fat and protein fractions and cause off–flavors (2).
Cow Response to Bacterial Challenges and Stress
The somatic cell response to bacterial challenge can be very rapid in healthy cows and frequently SCC eliminate the challenge and the cow experiences minimal problems. In some cases the challenge lingers producing a prolonged elevation of the SCC and yet the infection persists. This is common with persistent Staph aureus infections in older cows.
Research on SCC and the factors influencing it is extensive. Milk produced in infection free cows typically has a cell count of 200,000/ml or less. There are US herds of all sizes that maintain cell counts less than 100,000/ml, so such levels are attainable.
The primary cause of elevated milk cell counts is udder infection. When udder infections occur, individual cow cell counts can quickly elevate. Udder infections are a direct cause of elevated counts while many other factors appear indirectly to influence counts.
Heat stress, age of cow, lactation number, hoof problems etc. often are linked to elevated cell counts. While the relationships are valid, they are considered to be indirect and most likely act by somehow influencing the rate of new infection. Hot weather for example may cause cows to be under thermal stress, their immune system may be weakened making them more susceptible to new infections. Heat may also cause increased bacteria numbers in areas where cows rest, especially if such areas remain wet or damp. This increases the exposure of cows to more bacteria which, in turn, increases the risk for new infections. The net result is that during the heat of summer there appears to be an increase in the average cell count but it is indirectly, rather than directly, linked to hot weather. This is especially common in areas of the south and southeast where the combination of high temperature and humidity during the summer months places cattle under stress.
Can the cell count get too low?
In part the answer depends on what is considered too low. The key to defending the udder against bacterial challenge is the ability to quickly transfer large numbers of somatic cells to the udder from the circulatory system. Healthy cows tend to have this ability. There is some evidence however that in very low cell count cows, a challenge by bacteria, especially coliforms, may overwhelm the immune system before it can respond and the result can be a very sick animal with clinical mastitis (5).
On Farm Milk Concentration and Somatic Cells
An interesting technology that may become more common in the future is on farm technology to concentrate milk. Various systems are available that allow raw farm milk to be concentrated about 3:1. As water is removed what remains becomes more concentrated including somatic cells. Milk concentrate is presently regulated as whole milk and the legal limit for the SCC is 750,000/ml. Therefore, if the initial somatic cell count of raw bulk milk is too high, the concentrate likely would violate the PMO limit. Raw milk concentration on the farm will require the initial SCC be as low as possible.
References:
1. Adkinson, R.W., R.H. Gough, R. Graham, and A. Yilmaz. 2001. Implications of proposed Changes in Bulk Tank Somatic Cell Count Regulations. J. Dairy Sci. 84: 370-374.
2. Ma Y., C. Ryan, D.M. Barbano, D.M. Galton, M.A. Rudan, and K.J.Boor. 2000. Effects of Somatic Cell Count on Quality and Shelf Life of Pasteurized Fluid Milk. J. Dairy Sci. 83: 264-274.
3. Raubertas, R. F., and G. E. Shook. 1982. Relationship between Lactation Measures of Somatic Cell Concentration and Milk Yield. J. Dairy Sci. 65: 419-425.
4. Schallibaum, M. 2001. Impact of SCC on the Quality of Fluid Milk and Cheese. National Mastitis Council, Inc. 40th Annual Meeting Proceedings. 38-46.
5. Schukken, Y.H., G. Bennet, L. Green, T. van Werven. 2001. Can Somatic Cell Counts Get Too Low? National Mastitis Council, Inc. 40th Annual Meeting Proceedings. 19-28
6. Shook, G. and P. Ruegg. 1999. Geometric Mean Somatic cell Counts: What They Are; What They Do. National Mastitis Council, Inc. 38th Annual Meeting Proceedings. 93-100.
7. U.S. Department of Health and Human Services, Public Health Service, Food and Drug Administration. Grade “A” Pasteurized Milk Ordinance. 1997 Revision.
|