Mastitis is considerd the most costly disease of dairy cows. When there is a flare-up in the udder, resulting in a gargetty or bloody milk, not only is there a loss of the milk, but there is the far more costly possibility of losing a part or all of the udder. Furthermore, it often doesn'nt stop with the first cow, but may spread through the herd.
Packer, R. Allen
"Check Mastitis "Bugs","
Iowa Farm Science: Vol. 1
, Article 6.
Available at: https://lib.dr.iastate.edu/farmscience/vol1/iss1/6