Multisystemic Eosinophilic Epitheliotropic Disease (MEED) is a rare infiltrative eosinophilic disease of horses that affects multiple body systems, including the gastrointestinal tract, abdominal organs, skin and occasionally lungs. The common clinical presentation of MEED is in young horses with weight loss and skin lesions. The precise cause of MEED is unknown; it may be an abnormal allergic response to dietary, or inhaled or parasitic antigens, but research is ongoing. The long-term prognosis for MEED is often poor, as it does not respond well to therapy.
Etiology and Pathophysiology of MEED in Horses
Abnormal infiltration of eosinophils into the tissues and the formation of granulomatous eosinophilic lesions in multiple organs are characteristic of MEED. The body systems commonly affected include the gastrointestinal tract, pancreas, liver, respiratory tract and the skin. The kidneys and abdominal lymph nodes may also be involved.
The precise cause of MEED in horses is still unknown, however it is believed that abnormal allergic, or immune, responses play a role.
Type 1 hypersensitivity is the “classic” allergy syndrome that causes diseases like hay fever, asthma, food allergy, and in extreme cases, anaphylaxis. The organs most affected by Type I allergy are the gastrointestinal tract, the skin, and the respiratory tract – which are the same organs affected in a horse suffering from MEED. Eosinophils play a small role in the early “acute” phase of Type I hypersensitivity, but a much larger role on the long-term chronic sensitization of an animal to Type I hypersensitivity (mcb.berkeley.edu). It is not known whether dietary, inhaled or parasitic antigens are particular triggers for MEED.
Migrating nematodes can traverse many of the same organs as affected in MEED, and the nematodes are known to have endogenous factors that attract eosinophils, however there is very little evidence available to confirm this relationship with the occurrence of MEED (Schumacher 2009).
There also exists a theory that lymphoproliferative diseases such as lymphosarcoma, or T-cell lymphoid hyperplasia, may be involved in triggering MEED (Schumacher 2009).
Clinical Presentation of Multisystemic Eosinophilic Epitheliotropic Disease in Horses
Weight loss in young horses is the most common clinical presentation. MEED has been reported in older horses, and in several different breeds, however the Standardbred breed appears to be overrepresented. The horses frequently present with a concurrent exudative dermatitis of the face, limbs, ventral abdomen, and ulceration of the coronary bands and oral mucosa. The skin lesions may be pruritic.
Fever has been reported in some cases.
The horse may also present with respiratory symptoms of varying grades of severity and chronicity. Tachypnea, increased respiratory effort and intermittent epistaxis have been reported (Horan 2013).
Diagnosing MEED in Horses
Histopathology, performed by an experienced pathologist, can be diagnostic for MEED. Skin biopsies and rectal biopsies are relatively easy to collect. Rectal tissue should be fixed in Formalin, not Bouin’s solution, to best preserve the eosinophils. If available, tissue biopsy and histopathology can be performed on affected internal organs such as the liver, lung and pancreas.
The finding of eosinophilic granulomas, often in conjunction with vasculitis and fibrinoid necrosis of intramural vessels, is considered diagnostic for MEED. Eosinophilic infiltrates can be found in the rectal mucosa and submucosa of normal horses, but not in the form of granulomas (Schumacher 2009).
Bloodwork. The white blood cell count is usually normal, peripheral eosinophilia is rare, and the horses are rarely anemic – unlike with granulomatous enteritis, another infiltrative disease with a similar presentation. If the liver or pancreas are also affected there may be some evidence in the blood work, such as an elevated gamma glutamyl transferase (GGT) (Schumacher 2009).
On ultrasound there may be some non-specific changes to the liver parenchyma.
The small intestine is more frequently affected than the large intestine; therefore these horses will often have a normal response to a carbohydrate (CHO) absorption test (Schumacher 2009).
Treatment for MEED
To date, no treatment methodology has achieved reliable long-term success. Corticosteroids have been reported to give temporary relief to clinical signs, as has administration of Hydroxyurea. Medications used to treat similar human diseases have been tried, but, to date, with no significant success. One case report describes a successful outcome with a combination of Dexamethasone, Trimethoprim-Sulphonamide and an antihistamine (McCue 2003). Anthelminthics have frequently been administered, but with no change in outcome.
The prognosis for horses affected by MEED is poor.
Beaty, R. (n.d.) Lecture20(6) [PDF]. Retrieved from http://mcb.berkeley.edu/courses/mcb150/Lecture20/Lecture20(6).pdf.
Horan, E.M. et al. (2013) Pulmonary and hepatic eosinophilic granulomas and epistaxis in a horse suggestive of Multisystemic Eosinophilic Epitheliotropic Disease. Equine Veterinary Education 25:607-613.
McCue, M. et al. (2003) Dexamethasone for treatment of Multisystemic Eosinophilic Epitheliotropic Disease in a horse. Journal of the American Veterinary Medical Association 223:1320-3.
Schumacher, J. (2009) Multisystemic Eosinophilic Epitheliotropic Disease. In: Current Therapy in Equine Medicine, ed. N.E. Robinson and K.A. Sprayberry. Elsevier pp. 440-441.