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Trouble Shooting Troublesome Somatic Cell Counts
by Richard L. Wallace

Take Home Messages

  • Begin monitoring milk quality practices by recording bulk tank data, DHIA somatic cell count (SCC) information, and clinical mastitis treatments.
  • Elevated bulk tank SCC indicate herd level infection status with mastitis pathogens.
  • Cows with chronically high SCC (greater than 300,000 for several months) should have an aseptic milk sample obtained from infected quarters.


There are three basic principles of mastitis control: eliminate existing infections, prevent new intramammary infections (IMI) and monitor infection status. Treating clinical cases, dry cow therapy and culling chronic offenders are primary methods to elimiate existing infections. Prevention practices including proper maintenance and use of milking equipment, and employment of proper milking procedure help prevent new IMI during the milking process. Maintaining a clean environment helps prevent new infections between milking times. Segregating infected cows, screening herd replacements and improving cow resistance factors are other preventive measures. Good record keeping, regular evaluations of udder health status, periodic review of the mastitis control program and goal setting for udder health status are the essential monitoring steps. The monitoring process is commonly the weakest link in most mastitis control programs. Yet, the success of any control program rests on the ability and desire to monitor progress. This paper will focus on methods to troubleshoot somatic cell count (SCC) problems.

Monitoring Milk Quality

Monitoring milk quality is essential for production of a safe food product that meets consumer acceptance standards. Specific traits such as milk composition, flavors and odors, bacterial content, SCC, and drug residues are important to maintain consumer demand for dairy products.

Monitoring Somatic Cells

Monitoring somatic cells can illustrate the ability to produce quality milk. Somatic cells in milk above 50,000 per ml are an indication of injury or inflammation. High SCC milk has higher proteolytic and lipolytic activity. Plasmin, a proteolytic enzyme from serum can reduce casein levels in secreted milk, thereby reducing cheese yields. Lipases from white blood cells can react with milk triglycerides causing release of free fatty acids, which leads to off flavors.

Several different measures of somatic cells are available both at the farm level and at the level of the milk processing laboratory. The California Mastitis Test (CMT) utilizes a reagent that causes somatic cells to rupture then forms a gel with the cellular DNA. The thickness of the gel indicates the amount of somatic cells present. A score of "zero" (no gel formation) signifies a SCC range from 0 to 100,000/ml. A score of "trace" (slight gel formation) indicates a SCC range from 200,000 to 500,000/ml. Scores of 1, 2, or 3 (distinct gel formation) are all representations of high SCC or infected quarters. The Wisconsin Mastitis Test (WMT) and Direct Microscopic Somatic Cell Count (DMSCC) are rarely used anymore. Most milk processing laboratories and DHIA centers use automated, electronic cell counters (ECC). The use of computerized records and ECC have greatly enhanced the ability to monitor SCC and milk quality.

Monitoring Udder Health / Subclinical Mastitis

Although subclinical mastitis cannot be seen by the producer, it represents the largest single loss (65-70%) of the total mastitis cost. Evaluation of SCC and microbiologic culturing of milk are the primary monitoring methods to evaluate udder health. The availability of accurate records will determine the depth and breadth of the monitoring program. Less than 50% of dairy producers are enrolled in DHIA testing and the SCC program. Monitoring subclinical mastitis in these herds will be more difficult, but is not impossible. Some dairy cooperatives offer individual cow SCC data and milk culturing on an as needed basis. The use of a CMT paddle can be very beneficial in herds without individual cow SCC data. This method is much more time consuming, but can yield a tremendous amount of information on quarter infection status.

Measures at the Bulk Tank

The most basic level of measurement of subclinical mastitis and udder health is the bulk tank somatic cell count (BTSCC). This value is reported to the dairy producer several times each month. Often milk processors can make BTSCC available from each milk pick up. This data is essential for initial evaluation of herd udder health. Table 2 demonstrates the association between SCC of bulk tank milk and prevalence of mastitis pathogens in a herd. The legal limit for BTSCC in milk sold for Grade A purposes is 750,000/ml. Herds with BTSCC above 600,000 have approximately 80% of their cows infected and are losing between 12 and 15 % in milk production. These herds have increased incidence of clinical cases and are discarding milk due to antibiotic residues. Even at low BTSCC levels (200,000 to 299,000/ml), one third of the herd is most likely infected. Graphing these values is simple and herd level trends can be evaluated. Tank counts can be recorded once a month. These graphs can be saved from year to year to evaluate seasonal incidence of mastitis infections.

Monthly bulk tank culturing has proven useful in monitoring udder health, particularly with regard to contagious pathogens (Staphylococcus aureus, Streptococcus agalactia and Mycoplasma bovis). Sanitation problems should be suspected if bulk tank milk contains multiple environmental pathogens (Coliforms and fecal Streptococci). The sensitivity of a single bulk tank culture for contagious organisms is fairly low, especially when the herd prevalence of contagious mastitis is low as well. In other words, often one bulk tank sample will be culture negative for S. aureus or S. agalactia even though a herd may have cows infected with one or both of these organisms. The specificity of bulk tank cultures for contagious organisms is high (94%). So it is rare that a bulk tank culture will be positive when in reality no cows in the herd have contagious mastitis.

Multiple sampling will improve the sensitivity of bulk tank culturing, particularly with intermittently shedding organisms like S. aureus. Most dairy herds have every-other-day milk pick up. Serial testing can be performed by having the producer aseptically collect an agitated bulk tank sample in a sterile container. This procedure can be repeated every other day when the bulk tank contains four milkings. The samples can be frozen immediately after they are obtained and delivered to the testing facility once each month. Screening for contagious pathogens can be performed on the samples and management practices can be modified as needed.

Herd Level Evaluation

Individual cow data has more significance when grouped and evaluated on a herd basis. Initially, the proportion of cows harboring mastitis pathogens needs to be determined (defined as prevalence). Once control measures are implemented, the monitoring process determines the number of new infections occurring (defined as incidence) and old infections resolving or eliminated. Vital to this analysis is regular milk culturing and monthly SCC data.

Somatic cell count data from DHIA is conveniently grouped by parity and stage of lactation. Somatic cell score (SCS) is the logarithmic transformation of the SCC. Each doubling of the SCC increases the SCS by one score. The relationship between lost milk production and SCS becomes linear and easier to evaluate. Beginning at an SCS of 3.0 (mid-point SCC of 100,000/ml) mature cows will lose 1.5 lb of milk production per day. As the SCS increases by one log score, the loss increases by 1.5 lb/day, so that an SCS of 6.0 would be associated with a milk loss of 6.0 lb/day. Values for lost milk production in first calf heifers is half that of mature cows. At least 90% of first calf heifers should have SCS of 4.0 or less. Eighty percent of mature cows should be in the low SCS range (4.0 or less). A herd goal is 85% with low SCS. This value can be graphed over time just like the BTSCC discussed previously.

Whole herd, quarter milk samples may be necessary for research purposes but are impractical for field monitoring of subclinical mastitis. Using individual cow SCS to determine which cows to culture helps streamline the process. Keep in mind that one high score may not be indicative of a chronic infection. Cows with multiple scores above 4.0 are most likely to be infected. The monthly DHIA somatic cell data is generated from a composite sample. Average SCC when one quarter is infected is 500,000/ml. When 2 or 3 quarters are infected the average SCC can reach 700,000 or 1,500,000/ml, respectively. Once cows to sample are selected, the CMT can be used to determine which quarter(s) to culture. If SCC data is unavailable then the CMT paddle can be used on the entire herd.

Individual quarter samples are preferred to composite milk samples. Even under the best conditions many composite samples become contaminated. Contaminated milk samples are impossible to interpret and a waste of resources. A small (3-5 ml), sterile quarter sample is preferable to a voluminous contaminated one. Cultures should be immediately chilled and if microbiologic procedures are to be delayed, the samples should be frozen. Culturing all cows and heifers at 2 to 3 weeks postpartum may be helpful. This will assess the efficacy of the dry cow program and offer assurance that reservoirs of contagious organisms are not introduced to the herd.

Table 1. Interpretation of bulk tank analysis for bacteria and SCC in raw milk.

Test Procedure Excellent Good Cause for Concern
SPC # 1,000 # 10,000 $ 20,000
PI # 10,000 # 20,000 $ 50,000
LPC # 100 # 300 $ 500
SCC # 100,000 # 200,000 $ 400,000
Contagious pathogens None None Present

Source: Shearer, J. et. al. 1992

Table 2. Association between BTSCC and percent of cows infected with mastitis pathogens.

SCC Percent of Cows Infected
0 - 99,000 6 %
100,000 - 199,000 17 %
200,000 - 299,000 34 %
300,000 - 399,000 45 %
400,000 - 499,000 51 %
500,000 - 599,000 67 %
Over 600,000 79 %

Source: Eberhardt, R. et. al. 1979

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