Eosinophilic Granuloma in Horses

Eosinophilic granuloma is a common equine dermatosis. Etiology has yet to be fully elucidated and is likely multifactorial. Suspected causes include insect-bite hypersensitivity, atopic dermatitis, and food allergy. One case report of three horses with recurrent lesions after hypodermic needle injections suggested that trauma might play a part in lesion development(1), but subsequent reports have not supported this finding.

Lesions most commonly appear when biting and flying insects are present but can occur at any time of year. In some cases, lesions may first be noted weeks to months after insect exposure. They may also be triggered by trauma (eg, under the saddle or at sites of previous hair clipping).

Eosinophilic granuloma lesions, horse

Image

Courtesy of Dr. Karen Moriello.

Eosinophilic granuloma lesions in horses are multifocal round, elevated, firm, well-circumscribed nodules. Lesions can occur anywhere on the body but commonly occur in the saddle and tack area. Lesions may be singular or multiple and pruritic or painful. Lesions may or may not be exudative (see image of eosinophilic granuloma lesions, horse).

Diagnosis is made on the basis of history, physical examination findings, and biopsy. Histological analysis reveals multifocal areas of collagen fibers surrounded by granulomatous inflammation containing eosinophils. Differential diagnoses include neoplasia, exuberant granulation tissue, fungal granuloma, cysts, habronemiasis, and other nodular inflammatory skin diseases.

Solitary or mineralized lesions are often most easily treated by surgical excision.

The lesions are very responsive to glucocorticoids, though risks of glucocorticoids and development of laminitis should be considered. Horses may be treated with triamcinolone acetonide (3–5 mg/lesion), with no more than a total of 20 mg administered intralesionally at any one time because of the potential to induce laminitis. Horses with multiple lesions may be treated with prednisolone (1 mg/kg, PO, every 24 hours for 2–3 weeks).

In horses with recurrent lesions, intradermal allergy testing, particularly with insect antigens, is recommended. Hyposensitization and insect control can be palliative in some cases.