Inflammatory Bowl Disease (IBD)

Introduction to Inflammatory Bowel Disease in Horses

The term Inflammatory Bowel Disease in horses describes a group of intestinal diseases that are caused by infiltration of leukocytes into the wall of the intestine, resulting in intestinal dysfunction. Each disease in the IBD group involves a different combination of white cells, or involves a different part of the intestinal tract.

The IBD diseases include the following:

• Granulomatous Enteritis (GE)
• Lymphocytic-Plasmocytic Enterocolitis (LPE)
• and the eosinophilic infiltrative diseases:
• Multisystemic Eosinophilic Epitheliotropic Disease (MEED)
• Eosinophilic Colitis (EC)
• Eosinophilic Enteritis (EE)
• Idiopathic Focal Eosinophilic Enteritis (IFEE)

Etiology and Pathophysiology of IBD in Horses

IBD commonly affects younger horses, although older horses can also be affected, particularly with LPE and MEED.

Horses with GE, LPE and MEED present primarily with weight loss. These diseases do not respond well to therapy and have a poor long-term prognosis.

Horses with EC, EE and IFEE primarily present with symptoms of colic. The abdominal pain is caused by obstruction of ingesta flow due to the formation of focal inflammatory lesions that result in narrowing of the intestinal lumen. These eosinophilic diseases have a better prognosis as they may respond to surgery and/or corticosteroid therapy.

The precise pathophysiology behind the Inflammatory Bowel Diseases is not known. Veterinary immunologists believe these diseases are a result of complex interplay between the immune system, intestinal microbiota, dietary constituents, and/or environmental triggers of inflammation. Genetics may also play a role (Perkins et al. 2016).

Helminths were long believed to be a trigger for the eosinophilic diseases however extensive research has failed to find a significant link between the two. One hypothesis is that the eosinophilic diseases are caused by a delayed hypersensitivity response, which means that helminth involvement cannot be completely ruled out as a cause as, by the time the disease has been detected, the helminths may no longer be present in significant numbers.

There is some evidence that the eosinophilic diseases show a seasonal pattern of occurrence, and this gives some support to the theory that they occur as a result of an allergic response.

Protozoa have been found in the intestines of affected horses in some studies. The significance of these findings is not known.

Granulomatous enteritis has recently been linked to the presence of Mycobacteria species in the intestines using PCR analysis.

Lymphocytic-plasmacytic enterocolitis has been considered as a possible precursor to the development of intestinal lymphosarcoma.

Clinical Signs and Diagnostic Findings

Below is a summary of each of the diseases, describing the common presenting signs and the most common associated diagnostic findings. Detailed descriptions of each of these diseases can be found by following the links.

Granulomatous Enteritis (GE):

GE affects mostly young horses (0-4 years) and causes predominantly weight loss and anorexia, with one-third of the cases also showing signs of colic and diarrhea. Some affected horses develop skin lesions of the head, limbs and coronary bands. Anemia and hypoalbuminemia are consistently found on blood work. Carbohydrate (CHO)-absorption tests are usually abnormal. Rectal biopsies can be useful. Histopathology shows sheets of macrophages or epithelioid cells, and circumscribed granulomas in the mucosa or submucosa. Surgical resection of affected intestine may be possible, although generally the disease has a guarded to poor prognosis.

Lymphocytic-Plasmocytic Enterocolitis (LPE):

LPE is very rare and can affect horses of any age, breed or sex. Weight loss is the most consistent clinical sign, sometimes with diarrhea and colic. Blood work is generally not distinctive. Anemia and hypoalbuminemia are NOT consistently found. CHO-absorption tests are often abnormal. Rectal biopsies are generally not diagnostic, as the finding of lymphocytic-plasmacytic infiltrates in rectal mucosa is non-specific, and can occur with other intestinal disease and in normal horses. The long-term prognosis is poor. Potentially, LPE is a pre-lymphomatous disease (e.g. a precursor to lymphosarcoma).

Multisystemic Eosinophilic Epitheliotropic Disease (MEED):

MEED occurs in young horses, and Standardbreds may be predisposed. Weight loss is the most common presenting signs. Dermatitis is also a common feature of MEED, affecting the face, limbs and ventral abdomen, with ulceration of the coronary bands. The disease is systemic and can involve the liver, pancreas and lungs. The horse may therefore also present with respiratory symptoms. There are no specific blood work changes. Anemia is rare. CHO-absorption tests are often normal as MEED affects the large colon more than the small intestine. Rectal biopsies can be diagnostic. Histopathology shows distinctive eosinophilic granulomas. The prognosis for MEED is poor.

Eosinophilic Enteritis (EE) and Idiopathic Focal Eosinophilic Enteritis (IFEE):

EE and IFEE also affect young horses, and cause colic. These diseases result from eosinophilic infiltration of the small intestine. EE lesions are usually diffuse and can cause low-grade colic or weight loss. IFEE lesions are focal, due to an exacerbated eosinophilic inflammatory response, which can result in the formation of a fibrous circumferential mural band. These focal lesions cause narrowing of the intestinal lumen and blockage to the flow of ingesta. Histopathology of EE shows infiltration of predominantly eosinophils and lymphocytes, while histopathology of IFEE shows predominantly eosinophil and macrophage infiltration. Rectal biopsies are not diagnostic as the lesions are confined to the small intestine. CHO-absorption tests may be abnormal if diffuse EE is present. Exploratory laparotomy is the best diagnostic tool, and focal lesions can be excised. EE and IFEE lesions may also respond to corticosteroids, or improve spontaneously with time.

Eosinophilic Colitis (EC):

EC affects predominantly young horses (0-5 years) and usually presents as colic. The left dorsal colon is predisposed to eosinophilic infiltration of the intestinal wall. This can result in the formation of circumferential mural bands that obstruct ingesta flow and result in impaction of the large colon. Impaction of the pelvic flexure and left ventral colon may be palpable on rectal examination, but this is a non-specific finding. Rectal biopsies are usually not diagnostic. CHO-absorption tests may be abnormal. Exploratory laparotomy is the best diagnostic tool, and can be used to resolve the impaction. The lesions also may respond to corticosteroids therapy.

The table below highlights the differences between each of the IBD syndromes in clinical signs, diagnostic findings and prognosis.