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Eosinophilic Granuloma Complex in Horses

By Stephen D. White, DVM, DACVD, Professor and Chief of Service, Dermatology, Veterinary Medical Teaching Hospital; Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis

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In horses, the disease also has been termed equine eosinophilic granuloma with collagen degeneration, nodular necrobiosis of collagen, and collagenolytic granuloma. The lesions are nodular, nonulcerative, and nonpruritic. They often are found in the saddle, central truncal, and lateral cervical areas and may have a gray-white central core. Older lesions may become mineralized. Both insect bites and trauma have been suggested as causes, although the occasional onset during winter in cold climates and in noncontact saddle or tack areas suggests multifactorial causes. Histology reveals multifocal areas of collagen fibers surrounded by granulomatous inflammation containing eosinophils. Thus, histologically, this lesion is similar to eosinophilic granuloma of cats and dogs.

Solitary lesions may be treated with surgical excision or sublesional corticosteroid injections. Mineralized lesions often require excision. Triamcinolone acetonide (3–5 mg/lesion) or methylprednisolone acetate (5–10 mg/lesion) is effective. No more than a total of 20 mg triamcinolone acetonide should be administered sublesionally at any one time because of the potential to induce laminitis. Horses with multiple lesions may be treated with oral prednisone or prednisolone at 1.1 mg/kg/day, for 2–3 wk. In horses with recurrent lesions, intradermal allergy testing, particularly with insect antigens, is recommended. Hyposensitization and insect control can be palliative in some cases.