These bacteria can spread rapidly through a herd, so it is better not to introduce them in the first place.
Replacement cows may be purchased to increase the size of the herd, to assemble a new herd, or to maintain cow numbers after culling. One of the most common ways of introducing the cow-associated mastitis bacteria into a herd is in the udders
of cows that are brought in. Bacteria such as Staph. aureus and Strep. agalactiae can spread rapidly through a herd. It is better not to introduce the bacteria in the first place.
You should carefully consider the disease history of cows before buying as replacements or assembling a herd from mixed-age cows or from multiple sources.