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Infiltrative Colonic Diseases

Pathologic infiltration of abnormal cells, or abnormal quantities of cells, into the large colon can result in intestinal dysfunction and vague symptoms of weight loss, colic, diarrhea and ill-thrift in horses. These cellular infiltrations can be neoplastic, inflammatory or immune-mediated. Differentiating between them is often a challenge unless a biopsy of the colon is performed. Young horses are as likely to be affected as older horses, and the long-term prognosis is often guarded-to-poor.

Introduction to Infiltrative Colonic Diseases

Infiltrative disease that affect the large colon of the horse include:

These diseases are relatively rare, perhaps with the exception of colonic ulceration, and can occur in both young and old horses. Sand colitis can be more common in certain geographical areas. All of these diseases tend to cause vague clinical signs such as weight loss, ill thrift, and low-grade colic, which mimic clinical signs seen with more commonly occurring diseases such as intestinal parasitism, dental disease and other forms of intestinal disease such as sand colitis, enteroliths and chronic salmonellosis. In addition, weight loss, inappetence and ill thrift are also common signs of gastric ulceration, while too often diarrhea and colic, which are in actuality associated with the hindgut, are often misattributed to this syndrome.

A definitive diagnosis can usually only be made pre-mortem with biopsy of the colon and concurrent histopathological examination, and it is common for the diagnosis to only be made on necropsy.

Pathophysiology & Presentation of Infiltrative Colonic Diseases in Horses

Neoplasia of the Colon

Due to a lack of detailed information on the occurrence of intestinal neoplasia in horses, Taylor et al (2006) performed a comprehensive review of horses that had been diagnosed with intestinal neoplasia over a 15-year period at the University of California, Davis.

Taylor et al (2006) identified 34 horses, with an age range of 2-30 years (mean age 16.6). The Arabian breed was overrepresented in this group and found to be 4.5 times more likely to develop intestinal neoplasia than other breeds. There was no sex predisposition.

The most common presenting complaints included weight loss, colic, anorexia, fever and diarrhea. Common clinical findings included poor body condition, tachycardia, tachypnea, fever and diarrhea.

Alimentary lymphoma was the most common form of neoplasia, followed by adenocarcinoma, and smooth muscle tumors (leiomyoma, leiomyosarcoma).

Alimentary lymphoma and adenocarcinoma more frequently affected the small intestine, but were also found in the large and small colon.  Leiomyosarcoma affected both the small and large intestine.

Metastasis can occur with any intestinal tumor types.

Inflammatory Bowel Diseases

The inflammatory bowel diseases represent a group of diseases caused by abnormal infiltration of inflammatory cells into the wall of the intestine, resulting in malabsorption and intestinal dysfunction. The precise cause of these diseases is not known, but they are thought to be triggered by an excessive immune response to some sort of antigenic stimulus. Following is a summary of the individual diseases that make up the IBD group.

Eosinophilic Colitis (EC)
  • Characterized by eosinophilic infiltration of all layers of the intestinal wall.
  • Left dorsal colon is usually most severely affected.
  • Circumferential mural bands can form that obstruct ingesta flow in the large colon.
  • Young horses are most at risk and tend to present with colic.
  • Weight loss less common.
Multisystemic Eosinophilic Epitheliotropic Disease (MEED)
  • Characterized by the formation of eosinophilic granulomas in the intestinal tract, abdominal organs, respiratory tract and skin.
  • Large intestine more frequently affected.
  • Most typical in young horses.
  • Presents with weight loss and skin lesions.
  • The respiratory tract, when affected, can present with varying grades of severity of disease.
Granulomatous Enteritis (GE)
  • Characterized by the formation of sheets of macrophages or epithelioid cells, and circumscribed granulomas in the intestinal mucosa or submucosa.
  • Predominantly affects young horses.
  • Typically presents with weight loss and anorexia, and occasionally colic or diarrhea.
  • May present with skin lesions.
Lymphocytic-Plasmacytic Enteritis (LPE)
  • Very rare.
  • Can affect horses of any age, breed or sex.
  • Usually present with weight loss, and occasional diarrhea or colic.

Inflammatory Lesions of the Large Colon:

Colonic ulceration
  • Generally not well understood, but currently the subject of ongoing research.
  • Clearly associated with the use of NSAIDs, understood to be causative in development of Right Dorsal Colitis.
  • May also occur in the absence of NSAID use.
  • Possibly under diagnosed due to the difficulty of visualizing the large intestinal mucosa, and due to the vague clinical signs that the disease causes, particularly in the early stages.
  • Chronic ulceration can result in inflammatory cell infiltration and thickening of the intestinal wall from edema and fibrosis. Young performance horses are considered to be at greatest risk and may present with vague symptoms of abdominal discomfort, weight loss and ill-thrift.
Colonic ulceration
  • Ingestion of large quantities of sand, which tend to settle in the ventral colon.
  • Can result in mechanical irritation of the large colon, leading to inflammation and potentially fibrotic changes.
  • Clinical symptoms are usually not recognized until large quantities of sand have accumulated and the horse develops colic, however an early indication of the disease may be the development of diarrhea.

Diagnosis of Infiltrative Colonic Diseases in Horses

Many of these diseases are quite rare (neoplasia and IBD), and all of the diseases are challenging to achieve a diagnosis for without visualizing and biopsy of the large intestinal mucosa.

The initial work up is usually aimed at ruling out more commonly occurring diseases and would include a thorough history, physical exam, complete blood count, biochemistry, urinalysis, fecal analysis, rectal examination, abdominocentesis, and abdominal and rectal ultrasound.

Additional tests that can be performed include a rectal biopsy, biopsy of skin lesions, a carbohydrate absorption test, and fecal analysis for blood and protein.

The SUCCEED Equine Fecal Blood Test (Freedom Health) provides a rapid, stall-side test utilizing equine-specific antibodies to detect the presence of hemoglobin and albumin in a fresh fecal sample. Supporting literature for the product indicates that a positive result for both hemoglobin and albumin (or only albumin) likely represents damage to the mucosal lining of the hindgut. If only hemoglobin is present in the feces, then bleeding from the foregut is more likely.

Treatment for Infiltrative Colonic Diseases in Horses

Intestinal neoplasia is generally difficult to successfully treat, unless a focal lesion can be surgically removed. Advances have been made in chemotherapy options that are beyond the scope of this article. Some types of neoplasia, especially lymphosarcoma, will temporarily respond the corticosteroid treatment – or at least the symptoms of the horse will temporarily improve.

Taylor et al (2006) found that in horses with neoplasia the median time from onset of clinical signs to death or euthanasia was 1.9 months.

With the inflammatory bowel diseases, MEED and LPE respond poorly to treatment and the prognosis is poor. With GE and EC, the lesions can be focal and may be surgically removed, however the long-term prognosis for GE is poor. EC lesions may respond well to corticosteroid treatment over time, and surgery may not be necessary.

Treatment for colonic ulceration involves discontinuation of NSAID use, a decrease of bulk in the diet to rest the large colon, adding corn oil to the diet etc. See the colonic ulceration article for a complete discussion on treatment of colonic ulcers.

Sand-induced colitis can be treated with the administration of paraffin oil, or intermittent doses of psyllium, but sometimes surgical removal of the sand is required for complete resolution of the disease.


The neoplastic and inflammatory bowel diseases cannot be prevented at this stage, mostly due to the fact that the inciting causes are not known.

Colonic ulceration may, to some degree, be prevented by recognizing that young performance horses are more at risk and by minimizing stress and travel. Further, by instigating appropriate dietary practices the occurrence of colonic ulcers may be minimized.

Recommended management includes ensuring horses are fed an appropriate diet that constitutes primarily roughage, such as grass, hay and chaff, that is available throughout the day. This will ensure healthy function of the colon and help promote an appropriate microbiota in the colon. Carbohydrate-rich supplements such as grain should only be added to the diet as necessary to fulfill energy requirements. Rice oil, bran oil or wheat oil and other omega-3 rich oils are a good alternative energy sources to grain, and have been shown to have a mucosal-protectant effect.

Sand colitis can be avoided by preventing access of the horse to sandy pastures. If the horse cannot be removed from a sandy paddock, then a grazing muzzle may be required, and then the horse is fed intermittently in an enclosed space where no sand is accessible, or fed off the ground.


  • Andrews, F.A. et al. Colonic Ulcers: a pain in the hindgut!
  • Edwards, G.B. et al. (2000) Segmental Eosinophilic Colitis: a review of 22 cases. Equine Veterinary Journal Supplement 32:86-93.
  • 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) Multisystemic Eosinophilic Epitheliotropic Disease. In: Current Therapy in Equine Medicine, ed. N.E. Robinson and K.A. Sprayberry. Elsevier pp. 440-441.
  • Taylor, S.D. et al. (2006) Intestinal Neoplasia in Horses. J Vet Intern Med 20: 1429-1436.

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