Similar to Fistulous Withers, where injury has occurred at high spot on horse, withers is usually where it is seen mostly now, but used to be pretty common in horses that wore a harness bridle daily, with the constant rubbing on the poll area.
It creates a sore, which then may get infected, looks as if it is healing, but in reality, has closed over on top layer but the infection is going down, in withers, down body and into leg even, as the infection makes a channel more or less, in the poll area, goes down neck/face area internally.
Fistulous Withers and Poll Evil
Fistulous withers and poll evil are rare, inflammatory conditions of horses that differ essentially only in their location in the respective supraspinous or supra-atlantal bursae. This discussion is of fistulous withers but, except for anatomic details, also applies to poll evil. In the early stage of the disease, a fistula is not present. When the bursal sac ruptures or when it is opened for surgical drainage, and secondary infection with pyogenic bacteria occurs, it usually assumes a true fistulous character.
The condition may be traumatic or infectious in origin. Agglutination titers support an infectious etiology. Brucella abortus and occasionally B suis can be isolated from the fluid aspirated from the unopened bursa, and outbreaks of brucellosis in cattle (see Brucellosis in Large Animals: Brucellosis in Cattle) have followed contact with horses with open bursitis. A Brucella titer should always be evaluated in these cases; if significant, the owners should be made aware of the public health significance.
The inflammation leads to considerable thickening of the bursa wall. The bursal sacs are distended and may rupture when the sac has little covering support. In more chronic, advanced cases, the ligament and the dorsal vertebral spines are affected, and occasionally these structures necrose.
In the early stage, the supraspinous bursa distends with a clear, straw-colored, viscid exudate. The swelling may be dorsal, unilateral, or bilateral, depending on the arrangement of the bursal sacs between the tissue layers. It is an exudative process from the beginning, but no true suppuration or secondary infection occurs until the bursa ruptures or is opened.
Treatment and Prevention
The earlier treatment is instituted, the better the prognosis. The most successful treatment is complete dissection and removal of the infected bursa. The expense of the protracted treatment required in chronic cases often exceeds the value of the animal, and the public health aspects (in cases in which Brucella spp are involved) should be carefully considered. Brucella vaccines have not proved helpful. Sodium iodide therapy is of limited value. It is reasonable to keep horses separate from Brucella-infected cattle, and cattle separate from horses with discharging fistulous withers.
Last full review/revision March 2012 by C. Wayne McIlwraith, BVSc, PhD, DSc, FRCVS, DACVS