Fistulous withers is a chronic inflammatory skin disease characterised by pus-filled wounds and swelling on the horse's withers. The swelling is called a fistula and the withers are the highest part of the horse's back.
Fistulous withers is the term also used to describe any severe withers skin condition.
The organism Brucella Abortus, normally found in cattle, is the main cause of fistulous withers. The organism enters the horse's body through an orifice i.e. the mouth, nose or eyes, or through broken skin.