Indeed, subclinical mastitis is not visible to the naked eye. Therefore, you will need some kind of diagnostic test to determine which cows or mammary gland quarters have subclinical mastitis. The most widely used test is probably somatic cell count (SCC). Somatic cells are immune cells that will transfer from the bloodstream to the milk in response to an infection. In a normal mammary gland, the concentration of those immune cells is usually quite low (around 50,000 cells/ml of milk). When a “bad” bacteria enters the gland, these immune cells will send a signal to attract more immune cells from blood to the mammary gland to help fight the infection. In a few hours, SCC can rise to very high levels in milk (e.g. >1 million cells/ml), and this will often be sufficient to eliminate the infection.
Some bacterial species, however, are excellent at creating some inflammation, and therefore an immune response (i.e. an increase in SCC), but not enough inflammation to mount a large enough immune response to be eliminated. In general, a threshold of >200,000 cells/ml of composite milk (i.e. pooled milk from the 4 mammary glands) has been suggested to identify cows experiencing subclinical mastitis.
If you do not have regular SCC testing on your farm, another avenue is to collect milk samples and send them for bacteriological analyses to verify if an unwanted bacteria is present. But testing the whole herd regularly with this approach would be a lot more expensive. Your bulk milk SCC can also tell you about the level of subclinical mastitis in your herd, but it will not help you identify the problem cows.
If you do have subclinical mastitis problems on your farm, know that some of the unwanted bacteria can be acquired from the environment of the cow between milkings, while others are transmitted from cow to cow during milking (contagious). Your veterinarian can help you find out which bacteria are causing the problem on your farm, and help you target the best practices to control them.
-Dr. Simon Dufour, Université de Montréal
Director, Mastitis Network