Eosinophilic Granuloma Complex: Introduction - The Merck Veterinary Manual

Eosinophilic Granuloma Complex: Introduction

The etiology of this group of diseases that affects cats, dogs, and horses has focused on an underlying hypersensitivity reaction. This is particularly true in cats and horses. Insect, environmental, and dietary hypersensitivities have been documented in cats, while insect hypersensitivity has been seen in some equine cases and in a smaller number of canine cases. Genetic predisposition and bacterial infections have also been seen in cats. In all species, idiopathic cases exist.

Clinical Findings and Diagnosis:

In cats, 3 disease entities have been grouped in the complex.

Eosinophilic Ulcer:

This well-circumscribed, erythematous, ulcerative lesion, usually not painful or pruritic, is usually found on the upper lip. Although reported to occur, progression to squamous cell carcinoma is extremely rare. Histology shows an ulcerative dermatitis, with a cellular infiltrate of neutrophils, plasma cells, and mononuclear cells predominating. Mild to moderate fibroplasia is common. Tissue or peripheral eosinophilia is uncommon.

Eosinophilic Plaque:

This well-circumscribed, erythematous, raised lesion is most commonly found in the medial thigh and abdominal regions; it is extremely pruritic. Regional lymphadenopathy can be seen. Histology shows a diffuse eosinophilic dermatitis, with marked inter- and intracellular edema and vesicles containing eosinophils in the epidermis and dermis. Mast cells may also be present. Peripheral eosinophilia is common.

Eosinophilic Granuloma:

These typically raised, well-circumscribed, yellowish to pink lesions may be found anywhere on the body but are most common on the head, face, bridge of the nose, pinnae, pads of the feet, perineal region, lips, chin, oral cavity, and caudal thighs. The caudal thigh location is usually distinctly linear. Linear lesions have been seen on other body locations, but more commonly these are papular, nodular or diffusely swollen, and firm. Histologically, a granulomatous inflammatory response surrounds degenerative collagen. Tissue and peripheral eosinophilia are marked when the lesions are in the mouth but vary when lesions are on the skin.

In dogs, the lesions reported as eosinophilic granulomas histologically resemble the eosinophilic granuloma of cats, with marked collagen degeneration surrounded by a granulomatous and eosinophilic infiltrate. These lesions may be seen as ulcerated or vegetative masses in the oral cavity or, less commonly, as plaques, nodules, or papules on the lips and other areas of the body. Any breed may be affected, but Siberian Huskies may be at greater risk.

In horses, the disease 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 etiologies, 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 degeneration surrounded by granulomatous inflammation containing eosinophils. Thus, histologically, this lesion is similar to eosinophilic granuloma of cats and dogs.

Treatment:

In cats, hypersensitivity disorders (allergy to fleas, food, or inhalants) should be investigated by allergy testing (intradermal or in vitro) and dietary elimination trials. Hyposensitization, insect control, and dietary management should be instituted when appropriate. Antibiotic therapy (amoxicillin-clavulanate, cefadroxil, or fluoroquinolones) should always be tried empirically, especially in refractory cases. If no underlying cause can be determined and the condition is refractory, corticosteroids, such as methylprednisolone acetate (4 mg/kg, IM, once every 2 wk for 2-3 injections), oral prednisolone (2-4 mg/kg/day), or oral triamcinolone (0.8 mg/kg/day), can be tried. Oral corticosteroids should be tapered to alternate days (or to every third day in the case of triamcinolone), and dosages reduced when used for longterm management. Long-acting injectable methylprednisolone acetate should not be used more often than every 6-12 wk due to the potential for inducing hyperadrenocorticism. Aurothioglucose (gold salts) at 1 mg/kg, IM, weekly for 6-14 wk, may also be effective. If responses are seen, the dosage and frequency can be reduced or discontinued. Chlorambucil at 0.2 mg/kg, 3 times/wk, has also been used in refractory cases and requires more extensive blood monitoring due to its bone marrow suppressive properties; 6-12 wk may be needed before a response is seen and, like gold salts, the dosage and frequency should be reduced if response is seen. Recently, cyclosporine (5 mg/kg/day) has been used in refractory cases. This requires monthly laboratory monitoring for metabolic (eg, renal) changes. Other methods of treatment include radiotherapy, cryosurgery, laser, surgical excisions, interferon, and levamisole. Progestational drugs, such as megestrol acetate or medroxyprogesterone acetate, have also been effective; however, they are not recommended because of their potential side effects.

In dogs, antibiotics should also be tried initially. Many lesions seem much more responsive to corticosteroids, and therapy is usually oral prednisone or prednisolone (0.5-2 mg/kg/day initially, tapering the dosage over 20-30 days). Lesions recur in some dogs, in which case low-dose, every-other-day corticosteroid therapy is indicated.

In horses, solitary lesions may be treated with systemic antibiotics, surgical excision, or sublesional corticosteroid injections. Mineralized lesions require excision. Triamcinolone acetonide (3-5 mg/lesion) or methylprednisolone acetate (5-10 mg/lesion) is effective. No more than 20 mg triamcinolone acetonide should be administered sublesionally because of the potential to induce laminitis. Horses with multiple lesions may be treated with oral prednisone or prednisolone at 1.1 mg/kg, sid, 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.