Equine Multisystemic Eosinophilic Epitheliotrophic Disease

Multisystemic eosinophilic epitheliotrophic disease (MEED) is a rare, chronic wasting disease of horses characterized by development of granulomas (nodules or masses) in numerous organs. These granulomas are composed of large numbers of inflammatory cells, especially eosinophils (a type of white blood cell that responds to allergic and parasitic stimuli).

Although the cause and pathogenesis are unknown, possibilities include an exaggerated Type 2 hypersensitivity response involving helper T-lymphocytes and a hypersensitivity response to nematode parasites. MEED has also been reported in cases with concurrent lymphoma.

Affected horses rarely present with acute disease and veterinary attention is not usually sought until chronic signs are apparent, which can take several months. Younger horses are predominantly affected and there is no sex or breed predisposition or specific geographic incidence, with sporadic cases reported from the United States, United Kingdom, Canada, and Australia.

Presenting signs vary according to the body systems involved, with the skin and gastrointestinal tract most commonly affected. Dull demeanor, inappetance, severe weight loss, diarrhea, and dermatitis are frequently reported. Skin lesions start with dry, scaly cracks and inflammation at the coronary bands and oral mucosa, then develop into widespread crusting and exudation over the face, limbs, and ventral abdomen. Lesions may initially be pruritic (itchy) and progress to hair loss and thickening and cracking of the skin. Rarely, respiratory signs, nasal and ocular discharge, swollen submandibular lymph nodes, chronic cough, and respiratory distress can predominate.

No specific diagnostic tests exist for MEED, making it a diagnosis of exclusion. Differential diagnoses include lymphoma with dermal and systemic involvement, systemic lupus erythematosus (a rare, chronic, multisystemic autoimmune disorder, characterized by the production of autoantibodies to DNA and normal cellular constituents), pemphigus foliaceus (an autoimmune skin disease characterized by autoantibody production and the subsequent development of vesicles and pustules in the superficial layers of the skin.), and granulomatous enteritis (a condition in which different types of white blood cells will infiltrate the intestinal wall, possibly stimulated by an infection or immune/ allergic-type response).

A full clinical examination should be performed, including rectal examination to identify any mesenteric lymph node swelling. Clinical pathology commonly shows a nonspecific inflammatory profile and hyperfibrinogenemia. Peripheral eosinophilia is rarely present but increased eosinophils may be found in tracheal wash, bronchoalveolar lavage, and peritoneal fluids. Hypoproteinemia, due to hypoalbuminemia, occurs when gastrointestinal lesions are present. Cases with significant intestinal infiltrate also will have evidence of partial or total malabsorption, determined by glucose absorption test. Elevations in liver enzymes, particularly gamma glutamyl transferase, are common due to hepatic and biliary involvement. Abdominal ultrasound may show enlarged liver, thickened intestine, enlarged and nodular pancreas, and enlarged mesenteric and intestinal lymph nodes. Thoracic radiographs may show an interstitial pattern and nodules.

Affected tissues, such as skin, oral mucosa, rectal mucosa, liver, and intestine, should be biopsied. Histopathology confirms chronic, fibrosing inflammatory reaction with infiltrates composed of lymphocytes, plasma cells, and eosinophils seen in multiple organs. In some tissues, the eosinophilic infiltrates form granulomas.

Treatment is symptomatic, including systemic broad-spectrum antibiotics, anthelmintics, and corticosteroids. Prolonged therapy may be necessary, and some relapses have been reported. Use of the antineoplastic drug hydroxyurea has had limited success; the thiopurine antimetabolite azathioprine has also been suggested as a potentially effective treatment.

A positive response to treatment shows as improved demeanor, weight gain, reduced pruritus, and resolution of diarrhea. The prognosis for horses with MEED is poor, and affected horses are generally euthanized due to lack of response to treatment. Survival and resolution of clinical signs has been reported, however, so treatment should be attempted.

CONTACT—Rachael Conwell, BVetMed, Cert EM (IntMed), Dipl. ECEIM, MRCVS—rcconwell@gmail.com—EquiMed Referrals Ltd—Tadcaster, North Yorkshire, United Kingdom

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd's, London, brokers, and their Kentucky agents.

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Equine Disease Quarterly

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