Knowledgebase of Frequently Asked Questions - Cell counts
When a sample of milk is submitted for a cell count, body cells from the cow are counted by a laboratory machine.
Whilst a very small number of these body cells (or "somatic" cells, as the word "somatic" means "body") that are counted can be due to the shedding of some cells from udder tissue into the milk, the vast majority are actually white blood cells - both are a normal part of milk.
White blood cells are part of the body's natural defence mechanism, and are sent to the udder by the cow's immune system in response to an infection - this will usually substantially elevate the cell count of the infected quarter.
An Individual Cow Cell Count (ICCC) on a herd test result is actually the cell count measured from a composite sample of all four quarters of the cow.
Normal uninfected cows will usually have an Individual Cow Cell Count, sometimes also called a Somatic Cell Count, below about 150,000 cells/ml.
If a cow has an Individual Cow Cell Count (ICCC) above 250,000 cells/ml, she is likely to have a mastitis infection in one or more quarters of the udder.
The Bulk Milk Cell Count (BMCC) is the concentration of cells in the vat, which is the sum total concentration of all cows contributing to that vat.
This means that BMCC can be used a method of estimating the level of sub-clinical mastitis in a herd.
Have you ever culled cows, or removed high cell count cows from the vat, or both, only to see little or no change to the Bulk Milk Cell Count?
This is usually a surprise, but is actually quite common, and can best be explained by understanding the three factors that are most likely to have influenced the outcome.
Firstly, the worst cows in terms of having the highest Individual Cow Cell Count (ICCC) may well not be the worst cows in terms of contribution to the overall Bulk Milk Cell Count (BMCC).
A cow’s contribution to the BMCC is a function of both her current ICCC and also the volume of milk she is contributing to the vat.
This means that cows with a high cell count but with lower production can actually be contributing significantly less to the vat than a cow with only a moderately high cell count, but is a very high producer.
The pattern of this contribution may also have an effect - there may be a larger number of cows with a moderately high ICCC rather than a small number of cows with a very high ICCC creating a problem.
Secondly, a cow’s ICCC is not a static number – a cow’s cell count can change from day to day, and even from hour to hour.
A cow that was tested on one day may have a similar ICCC again the next day, or she could have a quite different ICCC the next day simply as a result of both natural biological variation and the cow’s immune system responding to the varying levels of challenge in the udder.
If enough cows change their ICCC, then the overall BMCC will change as a result.
The third likely factor is that mastitis infections in a herd are a dynamic and fluid situation, thereby constantly changing.
Over any period of time, previously uninfected cows are becoming infected, and some infected cows are being cured, either by treatment or by self-cure.
It is the rate at which these changes are occurring which can cause significant changes in which cows are contributing most to the BMCC at any given time, and if this rate of dynamic change is high, then culling or withholding cows from the vat may seem to have little or no effect, simply because other cows have become infected over that time and have become significant contributors to the BMCC.