Staph aureus grows well on skin tissue, especially in cracks and sores as well as the interior of the gland in the actual secretory tissue. Strep agalactiae on the other hand is not considered a skin inhabitant but is found primarily inside the gland in milk ducts and cisterns. For whatever reason these bacteria have not developed capabilities to thrive well in the environment of the cow. Other species have however. R isk management, in terms of the contagious mastitis bacteria, involves recognizing these issues.
Infected cows are the source of the organisms.
Cow to cow spread is either by the milking unit or the hands of milkers. Control is related to stopping these means of transfer and curing or eliminating infected cows. Do not strip milk directly onto the hand. When milk does get on hands rinse them with a sanitizing solution before moving to the next cow. Milkers should frequently dip or coat their hands with the teat sanitizing solutions that are used in the milking facility.