Fistulous withers is an infection of this bursa that generally breaks open and drains chronically. Horses have a bursa, a synovial cushion pad, over the tops of the spines in the area of the withers. This bursa is variable in location and size, but usually runs from about the second to the fifth thoracic vertebrae and may extend to each side to the margin of the scapular cartilage.

Though trauma can be the cause, many cases are thought to be a primary infection and commonly Brucella abortus is implicated. Brucellosis is an important disease of livestock. This article discusses symptoms, diagnosis, treatment and prevention of this serious disease of horses. Poll evil is a related condition of the back of the head of the horse that has a similar set of causes and represents the same therapeutic challenges so will be discussed concurrently.

Trauma to the withers or poll, might occur from flipping over or running under a low stout object. Cattle are a major source of exposure to brucellosis.