SOMATIC CELL COUNT
AND BULK TANK CULTURES

Regulatory authorities grade herd milk by somatic cell concentration.  On average, 75% of the cells in milk are white blood cells and 25% are epithelial  cells.  The white blood cells increase in response to mastitis.  Milk from uninfected cows will have somatic cell counts less than 200,000 scc/ml.

THE CMT TEST

The CMT test is a cow side test that uses a detergent to lyse the somatic cells.  The result is that nuclear material from the cells connects into chains that causes the formation of a gel which can be visually scored.  Equal amount of milk are mixed with CMT solution, and an indicator dye aids in the interpretation of the reaction.   The degree of gelling correlates with the number of somatic cells present.

A relationship exists between the CMT score and lost milk production per quarter.
CMT Score SCC/ml Lost production per quarter
0 - no gel 100,000 0-5%
T - Trace gel 300,000 8%
1 - Mild gel 900,000 9 - 18%
2 - Moderate gel 2,700,000 19 - 25%
3 - Severe gel >8,100,000 Lots and lots

Dairy producers have the option to obtain electronic SCC on milk from each cow monthly.  Uninfected cows should have SCC below 200,000 SCC/ml.

The presence of mastitis and high SCC has an adverse effect on milk quality.  Progressive cooperatives have encouraged high milk quality on farms by paying dairymen a premium for high quality milk.  The current legal limit for bulk tank SCC (milk in the tank from all the cows) is 750,000 SCC/ml.

Somatic cell are routinely reported as Linear Scores.
Linear Scores are logarithmic transformations of SCC:
(((LN(SCC)/LN(2))-3.6438)

LS SCC/ml Milk loss per day for each
increase in LS
Total milk loss per day
0 0-17,000    
1 18 - 35,000    
2 36-70,000    
3 71-141,000 1.5 1.5
4 142-282,000 1.5 3.0
5 283-565,000 1.5 4.5
6 566-1,139,000 1.5 6.0
7 1,131-2,261,000 1.5 7.5
8 2,262-4,523,000 1.5 9.0
9 over 4,523,000 1.5 10.5

Bulk Tank Cultures
Wilson, NMC Regional meeting proceedings, pg 10, 1999.

Bulk tank cultures are used as a routine monthly monitor, or to screen potential herds for the purchase of negative replacements.  Sensitivity of one bulk tank culture to detect organisms in milk when it is present in at least one cow are as follows:

Strep ag 90.4%
Staph aureus 74.5%

Mycoplasma 50.5%

Bulk milk culture does not always detect the presence of contagious mastitis in a group of cows.  However, positive cultures of bulk milk are highly predictive that the herd has cows infected with Strep ag, Staph aureus, or Mycoplasma.  A good practice is to culture bulk milk 3 times, in which samples are collected a few days apart.  If each of 3 bulk tank milk cultures are negative, the probability that all cows contributing to the tank are negative are:


Strep ag 99.9%
Staph aureus 98.3%
Mycoplasma 87.5%

 

 
 

 

Copyright 1999-2001
New Bolton Center Field Service Department
Students:  Keith Javic - Class of 2003, C. Nikki Conroy - Class of 2003