Mastitis, or inflammation of the mammary gland, is predominantly caused by bacterial pathogens and occasionally mycotic or algal microbes. Pathologic changes to milk-secreting epithelial cells from inflammation decreases function, ie, milk production. Depending on the pathogen, deceased milk production may continue into further lactations, which reduces productivity and, for beef and other meat-producing animals, potential weight gain for suckling offspring. Although most infections result in subclinical local inflammation or mild clinical cases, more severe cases can lead to agalactia or even profound systemic involvement, including death.
Mastitis has been reported in almost all domestic mammals and has a worldwide geographic distribution. Climatic conditions, seasonal variation, bedding, housing density of livestock populations, and husbandry practices, such as milking protocols for dairy animals, affect the incidence and etiology. However, mastitis is of greatest frequency and economic importance in species that primarily function as producers of milk for dairy products, particularly dairy cattle and goats. (Also see Udder Diseases.) Mastitis in sows is discussed elsewhere.