Bit & Bridle

Equine Fistulous Withers

Other Names

  • Withers Abscess
  • Fistula of the Withers
  • Suppurative Bursitis of the Withers
  • Poll Evil (when the same condition affects the poll/nuchal bursa at the base of the mane)

What is Fistulous Withers in Horses?

Fistulous withers is a chronic, draining infection of the supraspinous bursa — a fluid-filled sac situated between the skin and the spinous processes of the upper thoracic vertebrae at the withers. The condition is characterised by the development of one or more draining tracts (fistulas) from the bursa through the skin, discharging pus. In severe or longstanding cases, the underlying bone and cartilage of the vertebral spinous processes may become infected (osteomyelitis), significantly complicating treatment.

Fistulous withers is related to “poll evil” — the same pathological process occurring at the nuchal bursa at the poll (top of the head). Both conditions have historically been associated with Brucella abortus infection, though other bacteria are also important causes. The disease tends to be chronic and difficult to resolve, and significant scarring may result even after successful treatment.

Causes

Fistulous withers can develop from several initiating causes:

  • Poorly fitting saddle or equipment: An ill-fitting saddle, especially one with a narrow tree that presses on the withers, causes trauma to the bursa. Repeated bruising and pressure can lead to secondary infection.
  • Brucella abortus infection: Historically the most important bacterial cause; the same organism responsible for brucellosis (contagious abortion) in cattle. Horses are typically infected by contact with infected cattle or aborted foetal material. Brucella-associated fistulous withers tends to be more severe and resistant to treatment. Note: this is also a zoonotic infection (can infect humans).
  • Actinomyces bovis: Another bacterial pathogen implicated in fistulous withers, producing a characteristic “sulphur granule” discharge.
  • Other bacteria: Secondary infection with Staphylococcus, Streptococcus, Pseudomonas, and other environmental organisms can occur following trauma or as a primary infection.
  • Foreign body: Penetrating wounds from wire, wood splinters, or other debris can seed the bursa with bacteria.
  • Parasitic migration: Onchocerca cervicalis microfilariae may be implicated in some cases.

Symptoms and Signs

  • Swelling at the withers: Warm, fluctuant (fluid-filled) swelling over the withers, which may be painful on palpation.
  • Draining tract(s): One or more openings in the skin through which pus or serosanguineous (blood-tinged) fluid discharges. The discharge may be thin and watery or thick and purulent, sometimes containing granules in Actinomyces cases.
  • Pain and stiffness: Reluctance to move, stiffness through the back, or pain when pressure is applied to the withers or saddle region.
  • Gait changes: In severe cases, the horse may have an altered gait due to pain.
  • Generalised malaise: Occasional fever, reduced appetite, or weight loss, particularly if the infection is deep or chronic.
  • Extensive scarring: Chronic longstanding cases develop significant fibrous tissue around the withers, distorting the normal anatomy.

Diagnosis

  • Clinical examination: The characteristic location and appearance — draining tracts at the withers — are often sufficient for a working diagnosis.
  • Culture and sensitivity: Samples of the discharge should be taken for bacteriological culture to identify the causative organism and guide antibiotic selection.
  • Brucella serology: Blood testing for Brucella antibodies (complement fixation test or ELISA) is important given the zoonotic implications and to guide treatment. A positive Brucella titre does not definitively confirm active infection but warrants careful management.
  • Radiography: X-rays of the withers to assess for osteomyelitis (bone infection) of the spinous processes — a finding that significantly worsens the prognosis and indicates a need for more aggressive surgical intervention.
  • Ultrasound: Useful to define the extent of bursal distension and the presence of draining tracts and pockets.
  • Fistulogram: Radiographic contrast study to trace the extent of draining tracts through the tissues.

Treatment

Treatment of fistulous withers is challenging and often prolonged. Early cases without bone involvement have a significantly better prognosis:

  • Antibiotics: Systemic antibiotics based on culture results. Prolonged courses are often required (weeks to months). For Brucella-associated cases, tetracyclines (e.g., doxycycline) are the usual choice, though cure rates are variable.
  • Surgical drainage: Opening and flushing the bursa to allow adequate drainage. In straightforward cases, this may be performed under standing sedation.
  • Debridement: Thorough removal of infected and necrotic tissue from the bursa and surrounding area. Cases with bone involvement require more extensive debridement of infected vertebral spinous processes — this is performed under general anaesthesia.
  • Bursectomy: Surgical removal of the entire bursa sac may be required in chronic, refractory cases.
  • Wound management: Open wound management, daily flushing of drainage tracts, and bandaging where possible.
  • Remove the cause: Correct saddle fit — never return to using an ill-fitting saddle. Consult a qualified saddle fitter.

Prevention

  • Saddle fit: The most important preventive measure. Have saddles checked by a qualified saddle fitter, particularly after changes in the horse’s topline, weight, or workload. The saddle tree must clear the withers with at least two fingers’ width of clearance.
  • Equipment inspection: Check rugs, breast collars, and other tack regularly for pressure points at the withers.
  • Promptly treat injuries: Wounds at the withers region should be treated promptly and monitored carefully for signs of deeper infection.
  • Biosecurity with cattle: If horses are kept alongside cattle, ensure the herd is monitored for brucellosis. Be particularly careful around abortions in cattle.
  • Zoonotic precaution: Wear gloves when handling discharging tracts, particularly until Brucella status is known.

Prognosis

The prognosis for fistulous withers depends on the cause and the extent of tissue involvement:

  • Early, superficial cases without bone involvement: Good prognosis with appropriate surgical and medical management.
  • Cases with osteomyelitis: Guarded to poor. Bone infection is very difficult to resolve and may require multiple surgeries. Even with aggressive treatment, permanent deformity and weakness of the withers may result.
  • Brucella-associated cases: Generally more refractory and carry a less favourable prognosis.

When to Call the Vet

Contact your vet if you notice:

  • Any swelling, heat, or discharge at the withers
  • A draining wound at the withers that persists for more than a few days
  • Your horse showing unexplained back pain, stiffness, or reluctance to be saddled
  • An open wound at the withers following an injury — do not ignore it, even if it appears small

Frequently Asked Questions

Can fistulous withers be caused by a saddle?

Yes — poorly fitting saddle equipment is one of the most common causes. A saddle that sits too low on the withers, has a narrow tree, or has uneven panel pressure creates repeated trauma to the supraspinous bursa, predisposing it to infection. This is why proper saddle fit from a qualified saddle fitter is one of the most important investments a horse owner can make. See a saddler for a check-up at least once per year, or whenever the horse’s condition changes significantly.

Is fistulous withers contagious to humans?

If the causative organism is Brucella abortus, yes — brucellosis is a zoonotic disease that can infect humans through contact with infected discharges. Human brucellosis can cause prolonged, debilitating illness. Gloves should always be worn when handling draining tracts, and veterinary testing for Brucella should be performed as standard in any fistulous withers case. If you are pregnant or immunocompromised, extra caution is warranted.

How long does it take fistulous withers to heal?

Recovery time varies considerably. Simple cases without bone involvement may resolve over 4–8 weeks with appropriate treatment. Chronic cases with osteomyelitis may require months of treatment and repeated procedures, and some may never fully resolve. The longer the condition has been present before treatment begins, the more difficult resolution becomes — early veterinary intervention is key.

Can the horse be ridden during treatment?

In most cases, no — or at least not in tack that contacts the withers. Continued pressure from a saddle will delay healing and can worsen the infection. Your vet will advise on when and whether it is appropriate to return to work. Horses with significant bone involvement may require months of box rest.