Merck Manual

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Pet Owner Version

Eosinophilic Granuloma Complex in Horses


Stephen D. White

, DVM, DACVD, University of California, Davis

Reviewed/Revised Oct 2019 | Modified Oct 2022

Equine eosinophilic granuloma (also called eosinophilic granuloma with collagen degeneration, nodular necrobiosis of collagen, or collagenolytic granuloma) is the most common nodular skin disease in horses. There are many proposed causes; however, insect bite reactions are the most likely. Trauma and other environmental allergies may also be involved. Many collagenolytic granulomas occur during the warmer months of the year and may recur seasonally. Some may persist year round or distinctively occur under tack and saddle areas where pressure and trauma occur.

Lesions vary in size and number from less than ¼ inch (0.5 centimeter) to larger than 2 inches (5 centimeters) and usually occur over the neck, trunk and back. The lesions are round or irregularly shaped masses that are not itchy. Some horses can develop a generalized form with hundreds of pea-sized nodules over most of the body. The overlying skin is usually normal but on occasion may open and ulcerate. More chronic lesions may calcify and become “rock hard;” such lesions are often the most difficult to treat effectively.

Treatment most often involves relatively high doses of corticosteroids given by mouth or injection. Persistent lesions can be injected with more potent forms of corticosteroids. Antibiotics can also be helpful in controlling secondary infection related to foreign body reactions. Occasionally surgical removal is needed for calcified lesions. Fly control, along with allergy testing and desensitization in recurrent cases, are often indicated.

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