Mastitis is an inflammation of the mammary gland or udder. Mastitis in dairy cows is caused by udder infections, usually resulting from bacteria introduced either during the milking process or from environmental contact. Examples include contamination from milking equipment, milking personnel, manure contamination or dirty stalls.

Antibiotics can be administered to treat mastitis in dairy cows. As we answered in our response to Shelley’s question, antibiotics are only given to a cow when she is sick. When a cow receives antibiotics, she must be clearly identified and her milk properly discarded for a mandatory withdrawal period until the medication has cleared the cow’s system. This process is to ensure that no antibiotics end up in your milk. 

The Canadian Bovine Mastitis and Milk Quality Research Network website has detailed information on mastitis.