Mastitis in dairy cows is caused by bacteria, commonly Streptococcus, Staphyloccocus, and coliform species, according to the Merck Veterinary Manual. Contaminated milking equipment and unwashed hands spread the bacteria directly from cow to cow during milking. Cows are also exposed to bacteria from soiled bedding, dirty ponds, insects and medical treatments.
Staphylococcus aureus is the most common cause of mastitis in the United States, with Streptococcus agalactiae and Mycoplasma as the two other major pathogenic species, according to the University of Illinois. Other species, such as Klebsiella and E. coli, are rarer, as are yeast-caused mastitis infections. While heifers have some of these bacteria in their systems before calving, the increase in udder size and physiological changes during lactation make the udder more vulnerable to bacterial colonization.
Without treatment and careful sanitation practices, bacteria persist in the udder after infection, contaminating any equipment or surfaces that the cow's udder touches, according to the Merck Veterinary Manual. Cows are most commonly infected during lactation rather than during their dry period, although infection can occur at any time. Mycoplasma is a unique mastitis agent, as it is spread via respiratory aerosol droplets from cow to cow as well as through contaminated surfaces, and infects the udder either through the bloodstream or through direct contact with the udder.