Mastitis is inflammation of the mammary gland(s). While a common problem in many livestock species it is rare in the horse. The probable reason is the much smaller and better guarded udder, less pendulous teats (mamma), and smaller teat canals and openings (ostea). This makes trauma, a frequent predisposing cause, less common and ascending infection less common. Interestingly neither age, pregnancy, or past parturient status greatly effects the incidence. This article discusses normal mammary anatomy, normal and abnormal mammary secretions, clinical signs, diagnosis, and treatment of mastitis in mares.

The mares udder is divided into two halves, each with its own mamma. Careful examination of the each mamma reveals two, rarely three, ostea at the tip. Each ostea leads back to a separate lobe with its own separate lactiferous (milk producing) tissues, making the horse similar to the cow in that there are 4 separate mammary glands.