Equine Supraspinous Bursitis / Fistulous Withers – NAVLE Study Guide

Overview and Clinical Importance

Supraspinous bursitis, commonly known as fistulous withers, is a chronic inflammatory and often septic condition affecting the supraspinous bursa located at the withers of horses. This condition represents a clinically significant problem in equine practice, particularly in developing nations where horses serve as working animals. The condition is characterized by painful swelling, formation of draining fistulous tracts, and potential involvement of underlying bone structures.

The clinical importance of fistulous withers extends beyond the individual horse due to its strong association with Brucella abortus infection, a significant zoonotic pathogen. Veterinarians must exercise appropriate biosecurity measures when handling suspected cases to protect themselves and other personnel from potential infection.

Anatomy of the Withers Region

The withers represents the highest point of the horse's back, formed by the elongated dorsal spinous processes of the cranial thoracic vertebrae (T2-T12). These processes are angled caudally and can extend up to 7 inches in length, providing critical attachment points for the major ligaments supporting the neck and back.

Supraspinous Bursa

The supraspinous bursa is a synovial structure located between the funicular portion of the nuchal ligament and the dorsal spinous processes of thoracic vertebrae T2-T5 (most commonly T3-T5). Key anatomical features include:

• Dimensions: Approximately 5 cm wide and 5-11 cm in length
• Capacity: Can hold 30-90 mL of synovial fluid
• Function: Reduces friction between the nuchal ligament and underlying bony prominences
• Normal state: Not palpable unless distended or infected

Nuchal Ligament

The nuchal ligament is a critical elastic structure composed of two parts:

• Funicular portion: A thick, cord-like structure extending from the occiput to the withers, where it becomes the supraspinous ligament
• Lamellar portion: A triangular sheet-like structure connecting the funicular cord to cervical vertebrae C2-C5 (note: contrary to older textbooks, modern research shows attachments typically end at C5, not C7)

The nuchal ligament attaches to the T3 and T5 dorsal spinous processes and continues caudally as the supraspinous ligament, which runs along the summits of all thoracic and lumbar spinous processes to L5.

Etiology and Pathogenesis

Fistulous withers has two recognized forms based on etiology:

Typical/Idiopathic Form

In this form, infection begins within the bursa and subsequently spreads to the nuchal ligament and dorsal spinous processes. The primary infectious agents include:

Atypical/Traumatic Form

This form develops secondary to blunt trauma or penetrating wounds of the withers area. Common causes include:

• Ill-fitting saddles or harnesses causing chronic pressure and friction
• Poorly balanced loads on pack animals
• Direct trauma from falls, fences, or contact with other horses
• Penetrating wounds (bite wounds, foreign bodies)

Secondary Bacterial Pathogens

Once the bursa ruptures or is opened surgically, secondary pyogenic bacteria commonly colonize the area:

• Staphylococcus aureus (most common)
• Streptococcus zooepidemicus and Streptococcus equi
• Escherichia coli
• Pseudomonas aeruginosa
• Proteus species

Pathophysiology

The progression of fistulous withers follows a predictable pattern:

• Initial Inflammation: The supraspinous bursa distends with clear, straw-colored, viscid exudate
• Bursal Wall Thickening: Chronic inflammation leads to considerable thickening of the bursa wall and surrounding tissues
• Rupture or Drainage: The distended sac may rupture spontaneously or be opened surgically
• Secondary Infection: Pyogenic bacteria colonize the open wound, creating true suppuration
• Fistula Formation: Chronic draining tracts (fistulas) develop connecting infected tissue to the skin surface
• Extension: Infection may spread to the nuchal ligament, dorsal spinous processes (causing osteomyelitis), and surrounding soft tissues

Clinical Signs

Acute/Early Stage

• Localized swelling at the dorsal aspect of the withers (may be dorsal, unilateral, or bilateral)
• Heat over affected area
• Pain on palpation
• Fluctuant swelling (fluid-filled bursa)
• General stiffness, especially in neck and shoulder movement
• Reluctance to flex the neck
• Lethargy
• Possible pyrexia

Chronic Stage

• Single or multiple draining fistulous tracts
• Serosanguinous to mucopurulent discharge
• Progressively thick and fibrous swelling
• Skin necrosis over affected area
• Variable lameness (many horses are lame)
• Reluctance to be saddled
• Weight loss in prolonged cases

Diagnosis

Physical Examination

A thorough physical examination should include systematic palpation of the spinal ligaments for masses, fiber disruption, and signs of desmitis. The supraspinous bursa is not normally palpable unless distended or infected. Direct palpation of the affected area typically elicits a pain response.

Diagnostic Imaging

Radiography

Radiographs of the withers are essential to evaluate bone involvement:

• Soft tissue swelling dorsal to thoracic spinous processes
• Osteitis or osteomyelitis of dorsal spinous processes (bony lysis)
• Mineralization within soft tissue swelling (associated with Onchocerca cervicalis)
• Foreign body identification
• Caution: Do not misinterpret normal granular radiopaque appearance of incompletely ossified spinous process summits as pathology

Ultrasonography

Ultrasound provides a non-invasive method to:

• Visualize bursal distension and effusion
• Map fistulous tracts prior to surgery
• Assess extent of soft tissue involvement
• Guide needle aspirations for sample collection
• Evaluate nuchal ligament integrity

Contrast Radiography (Fistulography)

Injection of contrast media into draining tracts can delineate the extent of fistula formation and connections between cavities.

Laboratory Diagnostics

Exam Focus: ALL suspected cases of fistulous withers should be tested for Brucella abortus because of its ZOONOTIC POTENTIAL. If the initial test is seronegative in acute cases, repeat testing should be performed 14 days later. While awaiting results, isolate the animal and handle using infectious disease protocols.

Differential Diagnosis

• Saddle sores/withers sores: Superficial skin lesions from ill-fitting tack; do not involve bursa or deep structures
• Supraspinous ligament desmitis: Ligament injury without bursal involvement
• Kissing spines (overriding dorsal spinous processes): Contact between adjacent spinous processes causing pain
• Spinous process fractures: Usually traumatic; focal pain and swelling
• Neoplasia: Rare; sarcoids, melanomas, or other tumors in withers region
• Abscess from other causes: Foreign body, injection site reaction

Treatment

Early treatment provides the best prognosis. The most successful treatment combines surgical debridement with targeted antimicrobial therapy.

Surgical Management

Surgery is the primary treatment modality for established cases:

Pre-operative Considerations

• Injection of diluted vital dye (methylene blue or Evans blue) facilitates intraoperative recognition of infected tissue and fistulous tracts
• Ultrasound mapping of draining tracts pre-operatively
• Culture and sensitivity prior to surgery when possible

Surgical Procedure

• Can be performed standing under local anesthesia or under general anesthesia
• Complete dissection and removal of infected bursae, affected nuchal ligament tissue, and all necrotic tissues
• Remove all discolored (dye-stained) membranes
• Establish ventral drainage (critical for success)
• Curettage of affected dorsal spinous processes if osteomyelitis present
• Multiple surgeries may be required

Medical Management

Prognosis

The prognosis for fistulous withers is guarded to poor, with a high rate of recurrence.

Prevention

• Separate horses from cattle: Especially in regions with endemic bovine brucellosis; avoid co-grazing
• Proper tack fitting: Ensure 2-3 inches clearance between saddle pommel and withers; use appropriate saddle pads
• Avoid previously contaminated pastures: Do not graze horses in areas where infected cattle have been for at least 3 months after cattle removal
• Prompt wound care: Immediate attention to any withers injuries
• Regular tack inspection: Check for areas of pressure or rubbing
• Parasite control: Regular ivermectin treatment to control Onchocerca microfilariae

Public Health and Zoonotic Considerations

Brucellosis is a significant zoonotic disease. Human infection can occur through:

• Direct contact with infected bursal fluid or discharge
• Contact with contaminated tissues during surgery
• Mucous membrane exposure
• Skin wounds or abrasions

Required Actions

• Notify public health officials of suspected cases
• Follow state/federal regulations regarding transportation and treatment
• Use appropriate personal protective equipment (gloves, eye protection, gowns)
• Isolate positive animals
• Dispose of contaminated materials properly

"FISTULA" Mnemonic for Fistulous Withers

F - From cattle (Brucella transmission)

I - Ill-fitting tack (traumatic cause)

S - Supraspinous bursa location (T3-T5)

T - Test for Brucella (always!)

U - Undulant fever (zoonotic to humans)

L - Long-term treatment required

A - Antibiotics alone insufficient (surgery needed)

Poll Evil vs. Fistulous Withers

Same disease process, different location: Poll evil affects the atlantal (cranial nuchal) bursa at the poll, while fistulous withers affects the supraspinous bursa at the withers. Both can be caused by Brucella and have similar treatment approaches.