Hidden Infection

Somatic Cell Count (SCC) measurements taken from the bulk tank obviously describe the average herd count. This information cannot define the spread or degree of SCC variation within the herd; only individual testing of cows will determine whether there are, for instance, a significant number of cows with slightly higher-than-average counts or, conversely, a small number of cows with very large counts.

Generally, a threshold of 200,000 cells/ml of milk is used to decide whether a cow is infected. Some cows with few clinical symptoms of mastitis can have a SCC of 400,000 or higher, thus the importance of individual cow monitoring of cell counts is important to determine how many cows in the herd are affected and to what extent.

In smaller herds, the total effect of one or more cows with very high cell counts can be enormous. In the larger herd, milk from a sick cow is diluted and averaged down by the healthy animals. The bulk tanks on very large dairy units can accommodate mastitic cows in the herd without those cows affecting the overall milk quality rating, but for small to average-sized herds, knowing which cows are the 'culprits' enables action to be taken and decisions to be made over how to approach and deal with the problem. Milk recording schemes can calculate the contribution of a single high cell count cow to the herd's overall SCC, in percentage terms, demonstrating the potential improvements in total SSC if that cow's milk was to be not included in the bulk tank.

Where one or more cows with chronic symptoms of mastitis are present, they should be culled as they are a reservoir of infection for unaffected cows and are unlikely to be curable. Subclinically affected cows with high cell counts can be targeted for treatment, and their counts monitored over time to see if they recover or become chronic cases themselves.