Equine Chronic Diarrhea and Granulomatous Enteritis – NAVLE Study Guide

Overview and Clinical Importance

Chronic diarrhea in horses, defined as diarrhea persisting longer than one month, represents a significant diagnostic and therapeutic challenge. Unlike acute diarrhea, chronic cases often involve inflammatory or infiltrative bowel diseases rather than infectious etiologies. Granulomatous enteritis (GE) is one of several idiopathic inflammatory bowel diseases (IBD) affecting horses, characterized by macrophage and epithelioid cell infiltration of the intestinal wall.

Understanding the differential diagnosis, diagnostic approach, and management of chronic diarrhea is essential for NAVLE success, as these conditions require systematic clinical reasoning and integration of multiple diagnostic modalities.

Definition and Classification of Chronic Diarrhea

Chronic diarrhea is defined as loose or watery feces persisting for greater than 1 month. The causes differ substantially from acute diarrhea, which is typically infectious. Chronic diarrhea results from inflammatory, infiltrative, or neoplastic conditions, or from disruption of normal intestinal physiology.

Differential Diagnosis of Chronic Diarrhea

Granulomatous Enteritis (GE)

Etiology and Pathophysiology

Granulomatous enteritis is characterized by infiltration of macrophages and epithelioid cells into the mucosa and submucosa of predominantly the small intestine. The infiltrating cells form diffuse sheets or circumscribed granulomas, giving the disease its name.

Proposed Etiologies

• Mycobacterial involvement: Mycobacterium avium subspecies hominissuis identified in cases from Finland; M. bovis and M. genavense sporadically isolated
• Aluminum toxicity: Aluminum found in intestinal cells of affected horses (similar to Crohn's disease in humans)
• Immune dysfunction: Abnormal T-cell activation leading to transmural inflammation
• Genetic predisposition: Familial predisposition suspected in Standardbreds

Signalment and Breed Predisposition

• Age: Young horses, typically less than 4-5 years old
• Breeds: Standardbreds and Thoroughbreds overrepresented
• Sex: No sex predilection reported

Clinical Signs

The hallmark presentation is chronic weight loss despite a normal or increased appetite. The disease is typically insidious in onset.

Laboratory Findings

Serum Biochemistry

• Hypoalbuminemia: Very common finding due to protein-losing enteropathy and malabsorption
• Hypoproteinemia: Albumin decreases faster than globulins
• Hyperglobulinemia: May be present reflecting chronic inflammation
• Hypocalcemia: Secondary to hypoalbuminemia

Hematology

• Anemia: Consistent finding (anemia of chronic disease; occasionally Coombs-positive)
• WBC: Usually normal; peripheral eosinophilia is inconsistent
• Hyperfibrinogenemia: May be elevated indicating chronic inflammation

Diagnostic Approach to Chronic Diarrhea

A systematic, stepwise approach is essential for diagnosing chronic diarrhea. Begin with minimally invasive tests and progress to more invasive procedures as needed.

Step 1: History and Physical Examination

• Duration and progression of clinical signs
• NSAID administration history (phenylbutazone, flunixin meglumine)
• Deworming history and pasture management
• Geographic location (sandy soils, endemic areas for PHF)
• Rectal palpation for thickened intestine, mesenteric lymphadenopathy

Step 2: Basic Diagnostics

Step 3: Advanced Diagnostics

Abdominal Ultrasonography

Transabdominal ultrasound can identify small intestinal wall thickening (greater than 3-4 mm), which suggests inflammatory or infiltrative disease. For RDC, evaluate the right dorsal colon at the 11th-13th intercostal spaces; wall thickness greater than 4-5 mm is abnormal.

Oral Glucose Absorption Test (OGAT)

The OGAT assesses small intestinal absorptive capacity. Administer 1 g/kg dextrose via nasogastric tube after 12-hour fast. Measure blood glucose at baseline and every 30 minutes for 2-3 hours.

Exam Focus: The OGAT only evaluates small intestinal function. It will be normal in horses with large intestinal disease (RDC, sand enteropathy) and should not be used to rule out those conditions.

Rectal and Duodenal Biopsy

Rectal biopsy is minimally invasive and useful for approximately 50% of GE and MEED cases. It is less helpful for LPE and IFEE. Duodenal biopsy via gastroduodenoscopy allows evaluation of proximal small intestine but provides only superficial mucosal samples.

Exploratory Celiotomy/Laparoscopy

Full-thickness intestinal biopsy is the gold standard for definitive diagnosis of IBD. However, horses with hypoproteinemia are at increased risk for wound dehiscence, and the cost is significant.

Other Inflammatory Bowel Diseases

Lymphocytic-Plasmacytic Enteritis (LPE)

LPE is characterized by infiltration of lymphocytes and plasma cells into the lamina propria. It affects horses of any age with no breed predilection. Median age at presentation is 12 years. Clinical signs include weight loss (100%), diarrhea (50%), and lethargy (50%). Prognosis is poor; corticosteroid treatment is often unsuccessful. LPE may be a precursor to intestinal lymphosarcoma.

Multisystemic Eosinophilic Epitheliotropic Disease (MEED)

MEED is a multisystemic disease with eosinophilic infiltration of GI tract, skin, liver, pancreas, and occasionally lungs. Key distinguishing feature: concurrent dermatologic lesions (ulcerative, exfoliative dermatitis). Histopathology shows eosinophilic granulomas with vasculitis and fibrinoid necrosis. Peripheral eosinophilia is present in only 14% of cases. Prognosis is poor.

Idiopathic Focal Eosinophilic Enteritis (IFEE)

IFEE is characterized by focal eosinophilic intestinal lesions causing partial bowel obstruction. Unlike MEED, disease is restricted to the intestine. Presents with colic rather than weight loss. Prognosis is better than other IBDs; surgical resection can be curative.

Comparison of Equine IBD Types

Treatment

Treatment of Granulomatous Enteritis

No specific treatment has been proven definitively effective. Management is aimed at reducing intestinal inflammation and providing supportive care.

Treatment of Other Chronic Diarrhea Causes

Right Dorsal Colitis (RDC)

• Discontinue all NSAIDs immediately
• Dietary modification: pelleted complete feed, small frequent meals, restrict roughage
• Psyllium mucilloid supplementation
• Corn oil (1 cup daily) for SCFA production
• Sucralfate for mucosal protection
• Misoprostol 2-5 mcg/kg PO q6-12h (may cause colic/diarrhea)

Larval Cyathostominosis

• Moxidectin or fenbendazole (larvicidal dose: 10 mg/kg x 5 days)
• Corticosteroids for severe inflammation
• Colloid support (hetastarch, plasma) if albumin less than 2 g/dL
• Recovery may take months

Prognosis