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]

During the 20th century, brucellosis became a global zoonotic problem in domestic animals and humans. Due to its high prevalence in humans, it became well studied.

Today, brucellosis infection is more prevalent in low and middle-income nations where it can cause significant economic losses. In high-income countries, brucellosis has been mostly eradicated from livestock. However, it still exists in wildlife and dogs and is considered an “exotic” human infection. [4]

Brucellosis in Livestock Species

In livestock, brucellosis can be caused by multiple bacteria: [5]

  • B. abortus mainly affects cattle, but can also affect goats, sheep, and horses
  • B. suis mainly affects pigs
  • B. melitensis mainly affects goats and sheep
  • B. ovis exists only in the U.S., mainly affecting sheep. However, this strain does not cause significant disease problems.

In the principal hosts including cattle, sheep, goats, and pigs, brucellosis can cause spontaneous abortion, weak offspring, reduced milk production, and infertility. [4]

Brucellosis can be transmitted from animals to people through direct contact with infected blood, placentas, fetuses, or uterine secretions. It can also be transmitted to people through consumption of raw animal products (such as milk and dairy products). [5]

If not treated, the bacteria can remain in tissue for a long time, causing relapses, sometimes many years apart. [4]

Brucellosis in cattle receives significant attention due to the economic impact caused by abortions and reproductive issues. Moreover, cattle serve as the primary source of infection for both humans and horses. [6]

Brucellosis can be spread through various means, including wounds, airborne transmission, breeding, and even through the eyes (ocular transmission). Additionally, the use of shared needles for vaccines can spread the bacteria.

The respiratory or gastrointestinal tracts are the primary routes of brucellosis infection, and contact with these fluids represents the most common modes of transmission. [7]

Equine Brucellosis

Horses are considered accidental hosts of brucellosis. Brucellosis is most commonly contracted by horses through contact with cattle, either by ingesting contaminated material, exposure through the respiratory system, or through skin wounds. The disease does not show apparent age, gender, or breed susceptibility, although most reported cases in horses occur in those older than three years. [8][9]

Horses are usually infected with B. abortus, but occasionally contract B. suis as well. Poor management and reduced immunity or health status are usually associated with brucellosis infection in horses. [6][9]

In some countries, Brucella infection is fairly prevalent in horses. However, infection is often latent or subclinical in many of these horses. This means that horses show very mild symptoms or no symptoms at all. [8]

Clinical Signs

In horses who do manifest symptomatic brucellosis, clinical signs are characterized by severe inflammation in the supraspinous bursa (fistulous withers) or the supra-atlantal bursa (poll evil). [6][8]

Equine Brucellosis can also cause inflammation in the ligaments, tendons, and joints. This can lead to pain, decreased performance, and higher susceptibility to secondary infections. [6]

Other symptoms of equine brucellosis may include:

  • Weakness
  • Depression
  • Muscle stiffness
  • Intermittent fever
  • Movement disorders [8]

Unlike with other affected livestock, reproductive problems are rare in horses with brucellosis. [8]

Can Horses Transmit Brucellosis?

Studies show that it is unlikely that horses can transmit Brucella to other animals or people; instead, they act as a reservoir or secondary host for this bacteria. [8][11]

However, it is not impossible for a horse to transmit this disease to a person. Rare cases of transmission have occurred through contact with contaminated body fluids, tissues, or carcasses. Because of this, care should always be taken if you are dealing with an infected horse. [12]

In some states, brucellosis is a reportable disease. This means if a horse has the disease or is suspected of having the disease, veterinarians are required by law to report it to agricultural authorities. [10]

Agricultural authorities may then investigate to monitor equine health in the area or state. They may also coordinate with other states and the national government in order to prevent the spread of the disease. [10]

Other Causes of Fistulous Withers

Although B. abortus is often associated with fistulous withers, other infectious organisms as well as trauma to the withers or the supraspinous bursa may also cause this disease.

In regions where Brucella infection is uncommon among cattle, cases of horses with fistulous withers caused by B. abortus are rare. When purulent discharge is cultured from affected horses, it is common to find the presence of multiple bacteria. [1][9]

One type of trauma to the withers region that can result in fistulous withers is from poorly-fitted tack. This is more commonly seen in working donkeys in low and middle-income countries. [3]

Some natural variations in conformation can increase a horse’s risk of fistulous withers. Horses with low or high withers and incorrectly-fitting tack may also be at risk for trauma to the region and fistulous withers. [1]

Other possible causes of fistulous withers include infection with the following:

  • Actinomyces bovis – a bacteria that typically affects cattle, affecting their mouth and nasal passages
  • Streptococcus spp bacteria
  • Onchocerca cervicalis (neck threadworms), a parasite that lives in the horse’s nuchal ligament [13]

The presence of Onchocerca cervicalis in relation to fistulous withers in horses is not fully understood, as it may be an incidental finding due to the high prevalence of these parasites in horses. However, one study suggested that neck threadworms serve as a predisposing factor in the development of fistulous withers. [13]

Diagnosis & Treatment

Diagnosis of fistulous withers is made based on the animal’s history, physical examination, bacterial culture results, as well as radiographic findings in some instances. [2]

Horses that are positive to B. abortus are much more likely to have radiographs that show osteomyelitis in the underlying dorsal spinous processes. [9]

Horses can be tested for presence of Brucella bacteria in blood, semen, or milk. However, a negative culture does not necessarily rule out presence of Brucella, as it can be difficult to culture. [10][11]

An antibody titer can also be done to detect the antibody in the blood.


Timely treatment of fistulous withers is crucial to a good prognosis. Studies show that the most successful treatment is the complete surgical removal of the infected bursae, nuchal ligament, and associated necrotic tissues. [3]

Surgical Debridement

Surgery for fistulous withers can be performed using either local anesthesia in a standing horse or general anesthesia. The surgical procedure involves debridement, which entails the removal of infected material from the bursa and, in some cases, from the dorsal vertebral spinous processes. Additionally, drainage is added under rhomboideus muscles to facilitate proper healing. [3][13]

The choice of anesthesia and the extent of debridement may depend on the severity and extent of the infection. While surgery is the best treatment option, studies report that the success rate is approximately 50%, with a reoccurrence rate of 25 – 30%. [13]

Some equine hospitals may not perform surgical debridement in horses positive for brucellosis because of the risk of transmission to veterinary surgeons. [13]

Other Therapies

In cases where surgery is not feasible or prior to considering that approach, antibiotics are often employed as an alternative treatment for fistulous withers. However, achieving complete eradication of the infection through medication alone is challenging.

The veterinarian will typically obtain a culture of the drained fluid and prescribe antibiotics based on the specific bacteria identified. [3][11]

Cleaning draining tracts with antiseptic solutions and applying dimethyl sulphoxide (DMSO) may also be helpful. [1] Other therapies for fistulous withers may involve treating drainage wounds with a topical ointment.


The treatment of horses with chronic fistulous withers can be difficult and expensive. It may require significant surgery, long-term antibiotics, and a lengthy recovery process. [3][10]

Unfortunately, recurrent fistulation is common in horses with fistulous withers infection. Possible causes for recurrence include persistent infection with O. cervicalis, inadequate surgical removal of all infected tissue, or inadequate drainage of the area following surgery. [13]


As previously noted, fistulous withers is not a common disease in many countries. However, to help prevent this disease, there are several things you can do, including:

  • Keeping horses and cattle separate
  • Vaccinating heifer calves between 4 – 12 months of age (or follow your state’s vaccination guidelines) [5]
  • Using properly-fitting tack and good padding
  • Support your equine’s immune function with a healthy diet including balanced vitamins and minerals as well as adequate protein

If you do notice signs of fistulous withers or brucellosis in your equine, get your veterinarian involved right away. Early treatment may give them the best prognosis for recovery.

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  2. Hajighahramani, S. and Nazhvani Dehghan, S. Fistulous withers and eosinophilia in a horse. Comp Clinical Path. 2009.
  3. Adams, S.B. Bursitis in Large Animals. Merck Vet Manual. 2022.
  4. Moreno, E. and Barquero-Calvo, E. The Role of Neutrophils in Brucellosis. Microbiol Mol Biol Rev. 2020.
  5. Glenn, M.K. and Lynn, T.V. Brucellosis. AVMA. 2008.
  6. Fideles Resende, C. et al. Glanders and brucellosis in equids from the Amazon region, Brazil. Acta Tropica. 2022. View Summary
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  12. Diseases with horse to human transmission. UC Davis Veterinary Medicine.
  13. Hawkins, J.F. and Fessler, J.F. Treatment of supraspinous bursitis by use of debridement in standing horses: 10 cases (1968–1999). J Am Vet Med Assoc. 2009. View Summary
  14. Hernandez-Mora, G. et al. Brucellosis in mammals of Costa Rica: An epidemiological survey. PLoS One. 2017. View Summary
  15. Ehizibolo, D.O. et al. Serologic prevalence of brucellosis in horse stables in two northern States of Nigeria. J Equine Sci. 2011. View Summary