CMT Score With SCC Estimation

California Mastitis Test and Milk Quality
Michael McFadden
Extension Dairy Educator
Introduction
Mastitis continues to be one of the most costly problems in many dairy farms. Mastitis can
manifest itself in either clinical or subclinical form. Clinical mastitis is when milk appears
abnormal with the presence of flakes, clots, strings or watery. The mammary gland also may
be warm or hard to the touch and may exhibit increased sensitivity. In severe cases, systemic
signs may be apparent, such as, fever, cow off feed, and in shock.
Subclinical mastitis occurs when both milk and mammary gland appear normal but Somatic
Cell Counts (SCC) are elevated to a level above 200,000 cells/mL. It is estimated that
production losses (see Table 1) due to subclinical mastitis cost the U.S. dairy industry $1
billion/yr. Additionally, subclinical mastitis contributes to culling, death losses, and increased
risk of antibiotic residues in milk.
Somatic cells are basically white blood cells (leukocytes) that migrate to the mammary gland
in response to infection in both clinical and subclinical cases. This cell migration to the
mammary gland is part of the inflammatory response to bacterial infection in the udder. Cows
that do not have mammary infections normally have SCC less than 142,000 cells/mL.
The California Mastitis Test (CMT) is a cow-side test that allows dairy producers to assess the
SCC of each quarter of a cow’s mammary gland.
The CMT Procedure

The test is very simple, can be performed at milking time, gives instant results and is
economical. It is a four-compartment paddle with one compartment used per quarter (see
picture on page 18). One or two squirts of milk per quarter are collected in each paddle
compartment after foremilk is removed. The paddle is tilted to allow most of the milk to run
out leaving about 1 to 2 teaspoons (5 to 10 mL) in each compartment.
CMT reagent is added to each compartment in volume equal to the retained milk. The milk
reagent mixture is swirled in a circular motion with presence of gel or slime being recorded
for each quarter. It is the CMT reagent reacting with the DNA of the leukocytes that produces
the measurable response in the paddle. Reaction score results are shown in Table 2 on page
18.
Identifying quarters with higher CMT scores increase the probability of getting a positive
culture. Quarters with a CMT of “3” are three times more likely to yield a positive culture
than a CMT of 1. Conversely, CMT tests that result in “trace” (200,000 to 400,00 cells/mL) are
quarters that are likely to be infected, but may be difficult to detect. Thus, the accuracy of
CMT or somatic cell counts to predict infection is not perfect.
Studies have suggested that a single CMT or somatic cell count may only detect 60 to 80% of
infected quarters. Multiple tests increase the sensitivity of detecting infections, and may be
most accurate several days after calving. Thus, decisions for treatment or mastitis
management programs should be made with a combination of somatic cell testing, cultures,
and cow and herd history.
Potential Uses for CMT

1. Immediate determination of potential infection status of purchased lactating cows.
Because the sensitivity of the CMT is not 100%, multiple screenings are suggested.
2. Testing fresh cows on the fourth day of lactation is 80% accurate for predicting infection
status. Thus, fresh-cow CMT scores, in conjunction with CMT scores prior to dry off, may help
to evaluate the effectiveness of dry cow therapy and the rate of new infections during the dry
cow period. Quarters from fresh cows with high CMT can be selected for milk culture.
Depending on bacteriology results and cow history, these animals should be treated or
segregated.
3. CMT also could be used to evaluate the success or failure of mastitis treatment during
lactation. A negative CMT score at 3 weeks post-treatment with subsequent confirmatory
negative tests would suggest that treatment was successful. However, continued monitoring,
especially for relapsed clinical cases, should be done.
4. Dry cow CMT scores also can be useful in the administration of dry cow treatments on a
selective basis. However, new infection rates during the dry period, and clinical mastitis rates
in early lactation, should be monitored carefully if selective dry cow therapy is practiced. In
addition, selecting infected cows for therapy with CMT is not foolproof; some infected cows
may have low CMT scores, and likewise some non-infected cows may have high CMT scores.
In summary, if the limitations are considered, CMT testing has potential for use in dairy
farms. It is a quick, economical method of screening cows, and particularly quarters with
elevated SCC, especially over 400,000 cells/mL. This information can be part of a program to
determine infection status of mammary glands on a quarterly basis. Implementing CMT testing
as a standard operating procedure on your farm may help fine-tune a mastitis therapy
program, reduce the risk of antibiotic residues in milk, and increase both quality and quantity
of milk produced.
Samples from four quarters on a paddle. One quarter showed CMT-positive and thus needs additional evaluation.
References
i. Shook G. & A. Seaman, 1983. JDS 39 (12)

ii Jasper, D.E. 1967 Proceedings of National Mastitis Council (adapted)
iii Bishop, H,, et al., 2010 Vet Rec, 166 (11)
iv Middleton, J.R. et al., 2004 J Am Vet Med Assoc 224 (3)
v. Dingwell, R.T.,, et al., 2003 Can Vet J 44 (5).

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California Mastitis Test and Milk Quality

The CMT Procedure

Potential Uses for CMT

i. Shook G. & A. Seaman, 1983. JDS 39 (12)

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