Equine Eosinophilic Granuloma

Other Names

Equine eosinophilic granuloma with collagen degeneration; nodular necrobiosis; collagenolytic granuloma; nodular skin disease.

What is Equine Eosinophilic Granuloma?

Equine Eosinophilic Granuloma is a non-infectious inflammatory skin condition characterised by the formation of firm, raised nodules in the dermis and subcutis. The lesions contain a central area of degenerated collagen surrounded by a dense infiltrate of eosinophils — a type of white blood cell associated with allergic and hypersensitivity responses — along with macrophages and other inflammatory cells.

The condition is one of the more common nodular skin diseases in horses and is most frequently seen over the neck, withers, back, and saddle region — areas that correspond to pressure points from tack and equipment. Multiple lesions are common, and in some horses the nodules are so numerous that they are described as “collagenolytic granulomatosis.”

Equine eosinophilic granuloma is considered benign in the sense that it does not cause systemic illness and is not contagious or pre-malignant. However, the lesions can cause discomfort — particularly under a saddle — and can be cosmetically problematic. Understanding the triggers for each individual horse is important for preventing recurrence.

Causes

The precise cause of equine eosinophilic granuloma is not fully established in all cases. The prominent eosinophilic infiltrate strongly suggests a hypersensitivity response, and the following triggers have been implicated:

Insect bites — particularly Culicoides midges and other biting insects; the hypersensitivity response may be directed at insect saliva antigens
Contact hypersensitivity — reactions to wool, synthetic fabrics, leather, or chemicals in tack and rugs
Plant contact — urticating plant material on pasture
Feed or environmental allergens
Unknown triggers in many cases — some horses develop lesions without an identifiable hypersensitivity cause

The collagen degeneration seen histologically is thought to be a secondary event — eosinophil-derived proteins damage collagen fibres, which then act as a further stimulus for granuloma formation.

Symptoms and Signs

The primary presenting sign is the discovery of one or more firm, well-circumscribed nodules in the skin. Characteristic features include:

Firm, round nodules in the dermis or subcutis, typically 0.5 to 5 cm in diameter
Lesions most commonly over the neck, withers, back, and saddle region, though they can occur anywhere on the body
Hair coat is usually intact over the nodule, with normal overlying skin colour
Nodules are generally non-painful unless located directly under a pressure point
Multiple lesions are common; some horses develop dozens of nodules simultaneously
Lesions may fluctuate in size — some regress spontaneously, others persist for months
Calcium deposits may develop within chronic lesions (mineralisation), causing them to feel stone-hard on palpation

The horse is otherwise systemically well. No fever, weight loss, or general malaise is associated with eosinophilic granuloma.

Diagnosis

Fine needle aspiration (FNA) of a nodule provides cytological material for examination. The finding of degenerated collagen surrounded by eosinophils and macrophages is characteristic. Skin biopsy with histopathological examination provides definitive diagnosis, distinguishes eosinophilic granuloma from other nodular skin diseases, and identifies whether calcium deposition is present.

Differential diagnoses for firm nodular skin lesions in horses include melanoma, sarcoid, mast cell tumour, lipoma, cyst (epidermoid or follicular), and lymphoma. Biopsy is essential to distinguish between these, as their management and prognosis differ significantly.

Allergy testing may be considered to identify potential hypersensitivity triggers, particularly if multiple lesions are recurring.

Treatment

Small, non-interfering lesions that are not under tack pressure and are not causing clinical signs may simply be monitored — some resolve spontaneously without intervention.

For lesions causing discomfort, interfering with tack fit, or failing to resolve spontaneously:

Intralesional corticosteroids — injection of triamcinolone acetonide or methylprednisolone acetate directly into the nodule is the most commonly used treatment; many lesions resolve completely following one to three injections
Systemic corticosteroids — oral prednisolone may be used for horses with multiple or rapidly developing lesions that are not amenable to intralesional treatment
Surgical excision — appropriate for larger, isolated, or calcified lesions that do not respond to corticosteroid injection; recurrence is possible at the same or new sites
Management of identified triggers — avoiding the offending fabric, chemical, or insect exposure reduces recurrence rates

Calcified lesions respond less well to corticosteroid injection and more often require surgical removal.

Prevention

Where a hypersensitivity trigger can be identified, avoiding or managing that trigger helps prevent recurrence. General measures include using insect repellents and fly rugs during midge season, selecting tack materials that do not cause skin reactions, and reviewing any recent changes to feed, rugs, or topical products when lesions appear suddenly.

Prognosis

The prognosis for equine eosinophilic granuloma is generally excellent for individual lesions. Most respond well to intralesional corticosteroid treatment, and surgical excision is curative for those that do not. Recurrence is common in horses with an underlying hypersensitivity tendency, and some horses develop lesions seasonally year after year. The condition is benign and does not affect the horse’s general health or lifespan.

When to Call the Vet

Any firm skin nodule in a horse that was not present previously should be assessed by a vet rather than dismissed. While eosinophilic granuloma is the most common cause of firm benign nodules, more serious conditions including sarcoid, mast cell tumour, and melanoma present similarly. Definitive diagnosis by cytology or biopsy guides appropriate management and avoids the risk of incorrectly managing a potentially serious lesion as benign.

Frequently Asked Questions

Are eosinophilic granulomas dangerous?

No. Equine eosinophilic granuloma is a benign condition that does not cause systemic illness and is not malignant. The primary concern is comfort — lesions under tack pressure can cause back pain and saddle fit issues — and recurrence. The condition is entirely manageable and does not shorten the horse’s life.

Will the lesions go away on their own?

Some lesions do resolve spontaneously, particularly small, early lesions. However, larger or long-standing nodules — especially those with calcium deposition — are unlikely to resolve without treatment. Many horses that develop lesions will continue to be prone to recurrence, particularly if the underlying hypersensitivity trigger is not identified and avoided.

Can eosinophilic granuloma be confused with sarcoid?

Yes, clinically they can appear similar — both present as firm skin nodules. Key differences include the typical location (sarcoids favour the face, groin, and lower limbs; eosinophilic granulomas favour the neck and saddle region) and the overlying skin texture (sarcoids often have a roughened, verrucous surface). Biopsy is the only reliable way to distinguish between them definitively.

How many corticosteroid injections are needed to resolve a lesion?

Most non-calcified lesions respond to one to three intralesional corticosteroid injections spaced two to four weeks apart. If a lesion has not significantly reduced after three injections, further injection is unlikely to be beneficial and surgical excision should be considered.

For guidance on equine skin diseases and treatment options, consult the British Equine Veterinary Association (BEVA).