Many incidences of mastitis are directly indicated only by a high individual cow Somatic Cell Count through milk recording schemes or via California Mastitis Testing; the cow herself displaying no obvious clinical symptoms of the illness and no visible changes to the composition of her milk. These cases of mastitis are termed sub-clinical, and can be up to 40 times more common than clinical cases of the illness.
Subclinical infection is more likely to be caused by contagious pathogens. The presence of a causative pathogen is done via a bacteriological culture and indirect indications of subclinical mastitis can be given via electro-conductivity testing of milk, which is performed in an automated form in many modern parlours and robotic milking systems.
A subclinical cow, while appearing unaffected by the illness, may experience a reduction in yield potential due to the high SCC, and certainly represents a possible source of infection for other cows, who can become subclinical sufferers themselves, or may go on to show clinical signs of the illness, due to differences in immune status between cows.