Supraspinous bursitis, more often known as fistulous withers, is not a common problem in horses — at least in North America. However, it is a serious condition that more often affects horses and donkeys in low and middle-income nations.

Fistulous withers involves inflammation and infection in the supraspinous bursa, a fluid-filled sac located in the area of the horse’s withers. This leads to the formation of open sores or draining tracts (fistulas) that connect to the bursa.

Fistulous withers is commonly associated with the zoonotic disease, brucellosis, which can affect both humans and animals. This infectious disease is caused by various bacteria of the genus Brucella, which can enter a bursa through open wounds or skin abrasions.

This condition can be painful and make the horse lame or reluctant to move. Continue reading to learn more about fistulous withers, including its causes, symptoms, available treatment options, and preventative strategies.

Fistulous Withers in Horses

Fistulous withers is a septic, inflammatory disease of the supraspinous bursa, a fluid-filled sac approximately 5 cm wide and 5 – 11 cm in length.

The supraspinous bursa is situated between the base of the horse’s neck and the start of its back, specifically in the region known as the withers. It can hold 30 – 90 mL of fluid and helps to reduce friction between moving parts in the supraspinatus joint. [1][2]

Types of Fistulous Withers

There are two forms of fistulous withers in horses: [2]

  1. Typical / Idiopathic: In this form, infection starts within the bursa and eventually spreads to the nuchal ligament as well as the dorsal spinous processes.
  2. Atypical / Traumatic: This form develops secondary to blunt trauma or a penetrating wound of the withers area.

Supra-atlantal bursitis, commonly referred to as poll evil, is a similar condition which can also be caused by brucellosis. The symptoms of poll evil are the same as fistulous withers, but they instead occur in the poll region near the horse’s head. [1]

Disease Progression

A fistula is a draining wound from a normally closed structure. In the early stages of fistulous withers, a fistula is not present. However, the fistula develops when the bursal sac becomes infected by bacteria and ruptures or is opened for surgical drainage. [1][3]

The onset of fistulous withers can occur abruptly or gradually. Early symptoms include localized pain, heat, and swelling of the bursa without drainage. The horse may also exhibit lethargy and stiffness. [9]

Local inflammation caused by trauma or infection leads to considerable thickening of the bursa wall. The bursal sac becomes distended and may rupture.

Once the bursa ruptures, thick discharge drains from the fistula, either through a single or multiple drainage sites. This may occur on one side, both sides, or directly over the withers. [3]

The drainage often contains multiple bacteria including Streptococcus spp., Staphylococcus aureus, and Actinomyces bovis. [1]

As the condition progresses, infection can spread to nearby soft tissue such as the nuchal ligament that runs along the neck. It can also affect bony structures such as the dorsal vertebral spines, leading to a condition known as osteomyelitis, or inflammation of the bone. [3]

The latter stages of fistulous withers also involve considerable tissue necrosis (death) surrounding the withers. After the area stops draining, there may be signs of healing, fibrosis, and refistulation. [1][2][9]

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Because brucellosis is commonly associated with fistulous withers, it’s important for owners of horses, donkeys, and mules, to understand the causes and symptoms of this disease.

In 1902, British bacteriologist and surgeon, Sir Percy William Bassett-Smith studied Mediterranean fever, also called Malta fever, in human patients infected with the bacteria Brucella melitensis. Bassett-Smith noted the recurrent nature of disease that could last for decades. [4]