Eosinophilic Colitis in Horses

Eosinophilic Colitis is a rare condition that causes colic in young horses. Eosinophilic Colitis, Eosinophilic Enterocolitis and Idiopathic Focal Eosinophilic Enteritis are related diseases that affect various parts of the intestinal tract. With treatment, this spectrum of disease has a reasonably good long-term prognosis, unlike the systemic eosinophilic disease, MEED (Multisystemic Eosinophilic Epitheliotrophic Disease). The inciting cause of all of these eosinophilic diseases is not fully known; an abnormal immune response to antigenic stimulus is considered most likely.

The spectrum of intestinal eosinophilic infiltrative diseases are quite rare. They can affect horses of any age and many breed types, but tend to be more common in young horses, with a reported predilection in the Standardbred breed (Schumacher 2009).

The nomenclature used with eosinophilic intestinal disease indicates the anatomical location of the lesion.

Eosinophilic Colitis Lesions of the large intestines.
Eosinophilic Enterocolitis Lesions of both the large and small intestines.
Eosinophilic Enteritis Lesions of the small intestines.
Idiopathic Focal Eosinophilic Enteritis (IFEE) Circumferential mural lesions in the small intestines (usually seen in combination with diffuse disease, like eosinophilic enteritis).
Multisystemic Eosinophilic Epitheliotrophic Disease (MEED) Lesions usually affect the gastrointestinal tract, skin, liver, pancreas and lung. These lesions are histologically different to the other eosinophilic diseases.

This article discusses specifically Eosinophilic Colitis (EC).

Etiology and Pathophysiology of Eosinophilic Colitis in Horses

The cause of Eosinophilic Colitis is still an enigma. Equine immunologists believe that the pathophysiology is due to complex interplay between host genetics, the immune system, the intestinal microbiota, dietary antigens, and environmental constituents (Perkins 2016).

Helminths have traditionally been suspected to play a role, however there is very little evidence to confirm this. Ciliated protozoa have occasionally been found on histopathology, but their role is unknown. Some studies have shown the disease to have a geographic and seasonal distribution. At this time further research is needed.

Histopathologic examination shows eosinophilic infiltration of all layers of the intestinal wall. Macrophages and lymphocytes are also commonly present and there are varying degrees of mucosal and submucosal edema and mucosal necrosis (Mäkinen 2008). The lesions can be generalized or focal. The left dorsal colon is predisposed to more severe eosinophilic lesions, and circumferential mural bands can develop that can cause partial bowel obstruction and colic (Edwards 2000).

Clinical Presentation of Eosinophilic Colitis

Colic, of varying degrees of severity, is the most common presentation. Weight loss, unlike many large intestinal diseases, is not a common symptom (Schumacher 2009). The horses affected are often young (0-5 years), although the disease has been reported in older horses

Diagnosing Eosinophilic Colitis in Horses

Making a diagnosis of EC is not easy without histopathology, which requires an exploratory laparotomy or postmortem.

  • Blood work shows no specific changes – hypoalbuminemia and hypoproteinemia are not commonly found.
  • Abdominocentesis, especially in severe cases, can be turbid in color with elevated protein levels and increased neutrophil counts (Edwards 2000).
  • Rectal exam may detect a soft impaction of the pelvic flexure and left ventral large colon (Schumacher 2009).
  • Abdominal ultrasound may show thickening of the colon wall, or edema. These are both non-specific findings in a horse with colic.
  • Rectal biopsy is also not, in many cases, diagnostically helpful, as eosinophilic infiltrates can be present in normal rectal mucosa. Compare this to MEED, where rectal biopsy can be diagnostic if eosinophilic granulomas and vasculitis are present.
  • Glucose or Xylose absorption tests may be abnormal; depending on how much large bowel is affected.

Treatment for Eosinophilic Colitis

If the intestinal lesions are causing persistent or severe colic symptoms, then exploratory laparotomy may be indicated in order to relieve impaction and possibly resect necrotic lesions or circumferential mural bands.

Postoperative treatment with corticosteroids is indicated to treat diffusely affected bowel, however careful consideration is required to find a good balance between the therapeutic effect, and detriments to wound healing, and increasing the risk of incisional infection.

Corticosteroids may be required for 1-6 months before the lesions improve or resolve, and, in the meantime, horses may need to be fed small frequent meals of a pelleted complete feed and administered analgesics as needed for intestinal pain. Recurrence of disease after discontinuation of corticosteroids is rare (Schumacher 2009).

Anthelminthics with larvicidal activity are often administered, however, there is very little evidence that helminthes are involved in the pathophysiologic process (Edwards 2000).

Eosinophilic Colitis should be considered as a rare differential in horses suffering from intermittent colic.

References:

Edwards, G.B. et al. (2000) Segmental Eosinophilic Colitis: a review of 22 cases. Equine Veterinary Journal Supplement 32:86-93.

Mäkinen, P.E. et al. (2008) Characterization of the inflammatory reaction in equine idiopathic focal eosinophilic enteritis and diffuse eosinophilic enteritis. Equine Veterinary Journal 40:386-92.

Perkins, G.A. (2016) Chronic Inflammatory Bowel Disease. In: Equine Clinical Immunology, ed. M.J. Felippe. Wiley Publishers, Ames, Iowa pp. 113-119.

Schumacher, J. (2009) Infiltrative Bowel Diseases. In: Current Therapy in Equine Medicine 6, ed. N.E. Robinson. Saunders, St Louis, Missouri p. 441.