Enteritis in Horses

“Enteritis” is a common general term for inflammation of the intestine. Conventionally the term is applied to all forms of bowel disease whether they are inflammatory, infectious or infiltrative in origin. A better overall term for bowel pathology is “enteropathy” since it is a more encompassing term which refers to any pathology of the intestine. Although Enteritis refers specifically to an inflammation of the intestine, making it a more specific term than “enteropathy,” the two are sometimes used interchangeably.

As to be expected for a general term like enteritis, there are many different etiologies for its development in horses. Some of these have a predilection for one or more of the various sections of the small intestine: the duodenum, the jejunum and/or the ileum.

Enteritis may affect the horse in two ways:

  1. Acute Enteritis is usually caused by infectious agents resulting in inflammation, and potentially necrosis, of the intestine. The horse usually presents with lethargy, anorexia, and fever; and may also show colic, gastric reflux, and/or diarrhea.
  2. Insidious / Chronic Enteritis causing weight loss. This results when the architecture of the small intestine has been altered significantly, resulting in loss of plasma into the lumen of the gut and impaired absorption (malabsorption) of nutrients and water.

The table below summarizes the main types of enteritis and their causative agents.

Acute EnteritisBacterial Enteritis – FoalsClostridium difficile, Clostridium perfingens, E. coli, Salmonella spp., others.
Acute EnteritisProximal Enteritis
Bacterial Enteritis – Adults
Clostridium difficile other bacteria, idiopathic
Acute EnteritisViral EnteritisCoronavirus Rotavirus
Chronic / Insidious Enteritis
Lawsonia intracellularis
Equine Proliferative Enteropathy
Chronic / Insidious EnteritisGranulomatous EnteritisIdiopathic – likely abnormal immune response, possibly to Mycobacteria spp.
Chronic / Insidious EnteritisLymphocytic-Plasmacytic EnteritisIdiopathic – likely abnormal immune response, possibly a precursor to lymphosarcom
+/- acute colicIdiopathic Focal Eosinophilic Enteritis and Eosinophilic EnteritisIdiopathic – likely abnormal immune response, can be seasonal.

As can be appreciated, enteritis is caused by a variety of quite different disease processes. This article will briefly discuss the important aspects of the etiology, pathophysiology, clinical signs, diagnosis and treatment for each disease. In-depth descriptions of many of these diseases can be found by following the links.

Pathophysiology and Clinical Presentation of Enteritis

Acute Enteritis in Horses

Acute enteritis is generally considered to be caused by infectious agents, although historically the disease was often considered idiopathic because the diagnostic technology was not available to identify the causative agents. The basic presentation of acute enteritis is lethargy and anorexia +/- colic, fever, and gastric reflux.

Diarrhea in adult horses usually indicates that the large bowel is affected since a normally functional large bowel can often compensate for a moderately compromised small bowel. However, infections are seldom restricted to the small or large bowel, although this can occur. For example, anterior enteritis may affect the proximal regions of the small intestine and this may cause a functional obstruction; in this case severe colic and continuous high volume reflux will be present, usually without any evidence of diarrhea. By contrast, Clostridium difficile infection of the large colon causes severe diarrhea but little or no reflux and only mild colic signs.

Bacterial and viral infections of the small intestine cause different clinical syndromes, as outlined below:

Bacterial Enteritis in Foals
  • Causative agent – C.difficile, C.perfringens, E.coli and Salmonella spp. most common.
  • Pathophysiology – Inflammation of the small intestine +/- the large intestine.
  • Important clinical findings – Diarrhea is common, and foal can rapidly develop septicemia, especially if there is failure of passive transfer.
Proximal Enteritis (PE)
  • Age of horse – Older than 1.5 years of age.
  • Causative agent – It is generally accepted that this is usually caused by Clostridium difficile. Other possibilities include Clostridium perfringens, Escherichia spp., Salmonella spp. and others.
  • Pathophysiology – Inflammation of the duodenum, and variable lengths of jejunum.
  • Important clinical findings – Tachycardia usually > 60 beats per minute; copious amounts of gastric reflux, often red-brown in color
  • Possible complications – Endotoxemia, laminitis, septicemia and organ dysfunction.

Also known as Anterior Enteritis, or Duodenitis-Proximal Jejunitis.
Viral EnteritisFoals
  • Causative agent – Rotavirus is a very common cause but, coronavirus can also be involved.
  • Pathophysiology – Enterocyte damage at the tips of the intestinal villi, destruction of brush border. Decreased lactase production. May result in malabsorption and maldigestion of milk
  • Important clinical findings – Neonates may suffer from diarrhea, dehydration and electrolyte derangements.
  • Causative agent – Coronavirus
  • Pathophysiology – Enterocyte damage, usually superficial but can cause mucosal necrosis.
  • Important clinical findings – Often associated with herd outbreaks of disease – lethargy, anorexia and fever +/- diarrhea and mild colic.
  • Possible complications – Septicemia, and/or encephalopathy secondary to hyperammonemia (Pusterla, 2013, Fielding, 2015).

Chronic/Insidious Enteritis in Horses

The diseases that cause insidious enteritis are often slow in developing, and clinical symptoms are not detected until significant alterations in the architecture of the intestine have developed; these cause malabsorption of nutrients and often plasma leakage into the gut lumen. Chronic weight loss accompanied by hypoalbuminemia is therefore the most common presenting complaint. Chronic/insidious enteritis can be caused by at least two distinct syndromes: Equine Proliferative Enteropathy and the idiopathic Inflammatory/Infiltrative Bowel Diseases.

Equine Proliferative Enteropathy (EPE)
  • Age of horse – Foals, 4-7 months old, most commonly affected.
  • Causative agent Lawsonia intracellularis, a gram-negative obligate intracellular bacteria
    • Affects the ileum and jejunum
    • Proliferation of enterocytes and thickening of the intestinal wall, leading to malabsorption of nutrients and hypoproteinemia (Slovis, 2014).
  • Important clinical findings – Weight loss, ventral edema, depression, anorexia, fever, diarrhea and colic. Profound hypoalbuminemaia is acardinal feature of this disease (it is a protein losing enteropthy)
  • Acute, fatal onset of EPE has been reported in rare cases (Page, 2012).
The Inflammatory Bowel Diseases Complex (IBD)IBD describes a group of diseases caused by abnormal infiltration of the intestinal wall by inflammatory cells; the basic pathophysiology of most of these is poorly understood. The specific diseases differ by the type of inflammatory cell that predominates in the infiltrate. IBDs can affect both the large and small intestine. The following IBDs primarily affect the small intestine.

Granulomatous Enteritis (GE):
  • Cell types – Infiltration of the mucosa and submucosa by macrophages and epithelioid cells. Formation of distinct circumscribed granulomas.
  • Age of horse – Usually young horses (0-4 years of age).
  • Important clinical findings – Weight loss +/- diarrhea, and a distinct dermatitis
Lymphocytic-Plasmacytic Enterocolitis (LPE):
  • Cell types – Infiltration by lymphocytes and plasmacytes.
  • Age of horse – Can affect horses of any age.
  • Important clinical findings – Weight loss +/- colic, diarrhea.
  • Rare, and difficult to diagnose.
Eosinophilic Enteritis (EE) and Idiopathic Focal Eosinophilic Enteritis (IFEE)
  • Cell types – Eosinophil-rich infiltration of small or large intestine.
  • Age of horse – Usually affects young horses (0-5 years).
  • Important clinical findings – EE – diffuse infiltration, causing weight loss +/- colic
  • IFEE – focal inflammation, usually secondary to EE. Circumferential mural bands can form that obstruct ingesta flow, causing colic.

Diagnosis of Enteritis in Horses

The Inflammatory Bowel Diseases are rare, and often difficult to diagnose, but should be considered in young horses with weight loss, particularly when the more common diseases have been ruled out.

The infectious diseases, bacterial enteritis in foals, Anterior Enteritis and Equine Proliferative Enteropathy, are more common, and should be considered earlier in the clinical work up. Viral enteritis should be considered a possibility, particularly in foals and in herd outbreaks of disease.

IFEE is usually diagnosed following exploratory laparotomy for acute colic when focal or more generalized areas of bowel thickening are found. Ultrasonographic examination may assist the diagnosis and elimination of other strangulating or non-strangulating causes of bowel obstruction.

Important Diagnostic Aspects of Acute Enteritis


Gastric reflux is a significant clinical finding that indicates functional or physical obstruction of the small intestine. Stomach problems alone rarely result in sustained large volume gastric reflux. A volume of gastric reflux greater than 3 liters is usually considered a significant finding.

Reflux can resemble normal gastric contents, or have a red-brown color, with a foul smell and a more alkaline pH. Samples can be submitted for bacterial and viral analysis (although the viruses do not tend to cause significant gastric reflux). Analysis may include culture and PCR diagnostic profiles for gastrointestinal disease.

The differential diagnosis for gastric reflux includes:

  • Proximal Enteritis
  • Impaction of the small intestine – Coastal Bermuda hay, foreign body
  • Strangulating intestinal lesions – torsion, epiploic foramen entrapment, pedunculated lipoma etc.

Removing the fluid from the stomach is an essential emergency process and tends to give immediate pain relief to a horse with anterior enteritis (at least temporarily); where there is a strangulating intestinal obstruction the pain relief will be less obvious.


Abdominocentesis findings are more abnormal with strangulating lesions, with higher white cell counts and protein concentrations, and elevated lactate concentrations than would normally be found with infectious enteritis.


Abdominal ultrasound may detect loops of small intestine that are distended and fluid-filled (hypoechoic). The duodenum is located in the mid right abdomen, and the jejunum in the left ventral flank. The ileum is usually less accessible for imaging unless there is gross distention. With Anterior Enteritis, the distended loops tend to maintain some peristaltic movement, unlike with a strangulating lesions where the loops tend to be dilated and a-motile. Thickening and edema of the large bowel wall is a feature of severe infections whilst dense thickening is more consistent with an infiltrative origin.


Sometimes an exploratory laparotomy is the only way to differentiate between the diseases, and since strangulating lesions are a surgical emergency, many cases of anterior enteritis are taken to surgery. The small intestine, when affected by anterior enteritis, often has a “tiger stripe” pattern in the serosa, due to circumferential rings and linear “stripes” of inflammation. Remarkably, it often affects relatively short lengths of the duodenum and proximal jejunum but on occasion more extensive areas are involved.

Important Diagnostic Aspects of Chronic/Insidious Enteritis

The differential diagnosis list for weight loss is extensive, and so a logical diagnostic approach is required, ruling out the common diseases first, keeping in mind the age of the horse. Basic problems such as insufficient nutrition, worm infestations and dental disease should always be ruled out first. Neoplastic disease of the bowel (such as intestinal lymphoma, adenocarcinoma) is rare but important since there is usually little effective treatment that can be undertaken.


Young horses and foals presented with weight loss are most likely to be affected by parasites, gastric ulcers, foreign body ingestion and infections such as Rhodococcus equi and Lawsonia intracellularis.


Older horses (> 5 years) presented with chronic weight loss are less likely to have the insidious forms of enteritis causing weight loss (EPE, GE, EE), as these diseases mostly affect younger horses, or are extremely rare, like LPE. Other IBDs, that affect predominantly the large intestine, such as MEED (Multisystemic Eosinophilic Epitheliotropic Disease), can affect older horses, and should be considered once the more common diseases have been ruled out. Neoplastic disease always has to be considered even though it is rare.


  • Weight loss with edema is usually caused by hypoproteinemia, however differentials for this presentation include diseases that cause vasculitis (i.e. Purpura hemorrhagica), or obstruction of lymphatic vessels and nodes (i.e. lymphooma).
  • Hematology, urinalysis and bacterial cultures provide strong diagnostic support for detection of infections, internal abscesses, organ dysfunction, anemia and protein loss, which will guide the subsequent work-up. Anemia is a consistent finding with Granulomatous Enteritis, but is less common with the other diseases. Hypoalbuminemia is also consistently encountered with Granulomatous Enteritis, and Proliferative Enteropathy, but may also be due to diseases such as colonic ulceration (including Right Dorsal Colitis).
  • Gastric ulcers are a very common differential for weight loss in horses, and therefore gastroscopy should form part of the early work-up.
  • The possibility of respiratory disorders (Rhodococcus equi, pleuropneumonia, pneumonia etc) or cardiac disorders (rare, pericarditis etc) causing weight loss should not be ignored.
  • Fecal analysis can be very helpful and should include analysis for worm eggs and larvae, sand, the presence of albumin or blood (SUCCEED Equine Fecal Blood Test), bacterial culture and PCR analysis for R.equi, L. intracellularis, Clostridium spp., Salmonella spp., rotavirus and coronavirus.
  • Rectal biopsy. Granulomatous Enteritis can sometimes be diagnosed by rectal biopsy. Diagnosis of LPE and EE usually require biopsy of the small intestine by exploratory laparotomy. As a rule this is an unreliable diagnostic aid.
  • Ultrasonography. Thickening of the wall of the small intestine in young horses is highly suspicious of Lawsonia intracellularis infections. Ultrasound can also be useful in adult horses for diagnosing weight loss diseases such as Right Dorsal Colitis, intestinal lymphoma, abdominal abscesses and pleuropneumonia. Changes associated with IBD lesions are usually not distinctive on ultrasound.
  • Carbohydrate (CHO) Absorption Tests can help to determine if the weight loss is being caused by malabsorption of nutrients, and tend to point toward infiltrative bowel diseases such as EPE, the IBDs and lymphosarcoma. This test of course only provides information about the proximal small intestine.
  • Exploratory laparotomy with intestinal biopsy is the gold-standard test for weight loss when the diagnosis is otherwise unable to be made.

Importantly, any of these diseases in their most severe forms can result in intestinal necrosis, and so endotoxemia, peritonitis and sepsis are possible, but this is a rare outcome in all cases.

Treatment for Enteritis in Horses

Treatment for enteritis varies depending on the causative agent. The infectious forms of enteritis respond better to treatment than do the non-infectious IBD syndromes.

Bacterial Enteritis in Foals

  • Intensive monitoring and treatment, often including broad-spectrum antibiotics, hyperimmune plasma, intravenous fluids, parental nutrition, gastric ulcer prevention etc.

Proximal Enteritis

  • Broad-spectrum antibiotics and intravenous fluids.
  • Anti-inflammatory medications (NSAIDs) and anti-endotoxin medications (e.g. polymixin B)
  • Frequent decompression of stomach to relieve accumulated reflux.
  • Surgery was once considered a successful treatment option, but recent research has demonstrated that the outcome is usually better with medical management.

Viral Enteritis

  • Similar supportive therapy to AE, but no antibiotics (unless there are indications of bowel compromise).

Equine Proliferative Enteropathy

  • Antibiotics such as oxytetracycline and doxycycline, administered for 2-3 weeks is usually effective.
  • Hypoalbuminemia – intravenous plasma provides the best immediate support +/- synthetic plasma expanders such as hetastarch, to maintain the colloid osmotic pressure and decrease tissue edema.
  • +/- IV fluids, NSAIDs and polymixin B.

Granulomatous Enteritis and Lymphocytic-Plasmacytic Enteritis

  • Poorly responsive to treatment and a poor prognosis.
  • Corticosteroids may alleviate clinical signs, but have not been shown to provide effective long-term treatment.

Idiopathic Focal Eosinophilic Enteritis and Eosinophilic Enteritis

  • Surgical – resection of circumferential mural bands can be effective but is seldom performed now – intestinal decompression usually gives a good outcome on its own in many cases.
  • Medical – corticosteroids and time can also result in improvement of clinical symptoms, with long term resolution of disease. The horse may need analgesia for intermittent bouts of colic.


  • Arroyo, L. G. et al. (2006) Potential role of Clostridium difficile as a cause of duodenitis-proximal jejunitis in horses. J Medical Microbiology, 55:605-608.
  • Fielding, C.L. et al. (2015) Disease Associated with Equine Coronavirus Infection and High Case Fatality Rate. J Vet Intern Med 29:307–310.
  • Page, A.E. et al. (2012) Acute Deterioration and Death with Necrotizing Enteritis Associated with Lawsonia intracellularis in 4 Weanling Horses. J Veterinary Internal Medicine, 26: 1476–1480.
  • Pusterla, N. et al. (2013) Emerging outbreaks associated with equine coronavirus in adult horses. Vet Microbiol. 22;162:228-31.
  • Schumacher, J. (2009) Infiltrative Bowel Diseases. In: Current Therapy in Equine Medicine 6, ed. N.E. Robinson. Saunders, St Louis, Missouri p. 441.
  • Slovis, N.A. (2014) Lawsonia intracellularis Proliferative Enteropathy in Foals. AAEP Focus on the First Year of Life Proceedings. http://www.aaep.org

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