Equine Enterolithiasis Study Guide

Overview and Clinical Importance

Enterolithiasis refers to the formation and presence of mineral concretions (enteroliths) within the gastrointestinal tract, specifically the large colon of horses. These intestinal stones are composed primarily of struvite (magnesium ammonium phosphate) and form around a central nidus such as a pebble, piece of metal, baling twine, or other ingested foreign material. Enterolithiasis represents a significant cause of surgical colic, particularly in endemic regions such as California, Florida, and the southwestern United States.

The condition is clinically important because enteroliths can cause partial or complete intestinal obstruction, leading to colic of varying severity. At endemic referral hospitals like UC Davis, enterolithiasis accounts for approximately 15% of colic admissions and 28% of surgical colic cases. Early recognition and surgical intervention before intestinal rupture are critical for favorable outcomes.

Etiology and Pathophysiology

Formation Mechanism

Enteroliths form through a process of concentric mineral deposition around a central nidus over months to years. The process is analogous to pearl formation in oysters. The nidus can be any small ingested foreign object including pebbles, sand grains, metal fragments, rubber pieces, hair, or baling twine.

Mineral Composition

Approximately 90% of enterolith mass consists of struvite (magnesium ammonium phosphate, MgNH4PO4·6H2O) and vivianite (hydrous iron phosphate). The remaining 10% includes sulfur, sodium, potassium, calcium, titanium, aluminum, and nickel in varying proportions. Chemical analysis of surgically removed enteroliths typically reveals calcium (15-30%), calcium oxalate (10-20%), ammonium (approximately 10%), phosphates (20-40%), and magnesium (10-15%).

Contributing Factors

Three primary factors contribute to enterolith formation:

• Elevated colonic pH: Alkaline conditions (pH greater than 7.0) favor struvite crystallization. Alfalfa-based diets increase colonic pH due to high protein and mineral content.
• High mineral concentrations: Excess dietary magnesium, calcium, and phosphorus provide substrate for mineral precipitation.
• Decreased colonic motility: Reduced gut transit time allows prolonged contact between minerals and the developing stone. Limited turnout and exercise decrease normal intestinal motility.

Anatomical Considerations

Understanding equine large colon anatomy is essential for comprehending enterolith pathophysiology and surgical approach. The ascending colon forms a double-horseshoe configuration consisting of the ventral colon (lower horseshoe) and dorsal colon (upper horseshoe) connected at the pelvic flexure.

Anatomical Regions and Clinical Significance

Risk Factors

Breed Predisposition

Certain breeds demonstrate significantly higher incidence of enterolithiasis, though the exact mechanism remains unclear. Possible explanations include anatomical variations in gastrointestinal tract conformation or genetic differences in mineral metabolism. Arabian horses and Arabian crosses have the highest rates of enterolithiasis requiring surgical intervention.

Geographic and Environmental Factors

Enterolithiasis prevalence varies dramatically by geographic region. California has the highest prevalence where enterolithiasis is the leading cause of surgical colic. Other endemic areas include Florida, Arizona, Texas, and Indiana. The condition occurs sporadically in other regions of the United States, Europe, and the Middle East. Geographic associations are linked to:

• Soil mineral content affecting feed composition
• Regional feeding practices (high alfalfa hay use)
• Water mineral content (hard water with high magnesium)
• Arid climates with limited pasture access

Dietary Risk Factors

Alfalfa hay is the most significant dietary risk factor for enterolith formation. A study of 900 horses at UC Davis found that two-thirds of horses with enteroliths were fed diets with greater than 50% alfalfa hay, compared to only 60% alfalfa in unaffected horses. Alfalfa contributes to enterolith formation through:

• High protein content increasing colonic pH (alkalinization)
• Elevated magnesium, calcium, and phosphorus concentrations
• California-grown alfalfa may contain particularly high magnesium from serpentine soils
• Wheat bran and rice bran (high phosphorus and magnesium) also increase risk

Management Risk Factors

• Limited pasture access: Stall confinement decreases intestinal motility and increases ingestion of stall-based hay
• Reduced exercise: Decreased physical activity slows gut transit time
• Age: Most commonly diagnosed in horses greater than 10 years old; rare in horses less than 4 years
• Sex: Females appear slightly predisposed, possibly due to prostaglandin effects on gut motility

Clinical Signs and Presentation

Clinical presentation varies significantly based on enterolith size, number, and location within the gastrointestinal tract. Many horses carry enteroliths asymptomatically for years until obstruction occurs.

Chronic and Subclinical Presentation

Horses with a single large enterolith in the large colon often present with a history of chronic, intermittent, mild to moderate colic. Nonspecific signs may precede overt colic and include:

• Changes in attitude or behavior
• Anorexia or decreased appetite
• Lethargy and decreased performance
• Unexplained weight loss
• Loose manure or intermittent diarrhea
• Girthiness or sensitivity to palpation
• Reluctance to exercise

Acute Obstruction Signs

When enteroliths migrate to narrower portions of the intestine (transverse colon or small colon) and cause complete obstruction, signs become acute and severe:

• Moderate to severe abdominal pain (pawing, lying down, rolling, looking at flank, kicking at abdomen)
• Tachycardia (elevated heart rate greater than 60 bpm)
• Tachypnea (increased respiratory rate)
• Abdominal distension due to gas accumulation
• Decreased or absent fecal production
• Decreased intestinal borborygmi on auscultation
• Sweating and signs of distress

Diagnosis

Definitive diagnosis of enterolithiasis often requires exploratory surgery. However, several diagnostic modalities can support a presumptive diagnosis.

Physical Examination

Initial assessment reveals clinical signs consistent with colic severity. Cardiovascular parameters (heart rate, capillary refill time, mucous membrane color) reflect perfusion status and potential endotoxemia if intestinal compromise has occurred.

Rectal Examination

Rectal palpation rarely identifies enteroliths directly because most are located cranial to the pelvic inlet in the transverse colon or right dorsal colon. However, rectal examination may reveal distension of the large colon and cecum, tight mesenteric bands, and occasionally an enterolith in the small (descending) colon. Positioning the horse with the head uphill may improve chances of palpating a stone.

Abdominal Radiography

Radiography is the best screening test for enterolithiasis in endemic areas. Enteroliths appear as spherical, homogeneously radiopaque (radio-dense) structures within the intestinal lumen due to their mineral composition.

Factors affecting radiographic detection include horse body size (easier in smaller horses), intestinal content (fasting 24 hours improves visualization), enterolith location (large colon easier than small colon), and radiographic equipment capability (requires high-output generators typically found at referral hospitals).

Peritoneal Fluid Analysis (Abdominocentesis)

Peritoneal fluid is typically within normal limits early in the disease course. Progressive changes indicating intestinal compromise include elevated protein concentration, increased nucleated cell count, and serosanguineous appearance. A slight elevation in peritoneal fluid protein may indicate impending need for surgical intervention.

Abdominal Ultrasound

Ultrasonography has limited utility for enterolith detection in adult horses but can be useful for ruling out other conditions and assessing intestinal wall thickness and motility.

Treatment

Surgical removal is the only effective treatment for horses with colic caused by enterolith obstruction. Medical management alone is not curative and delays in surgical intervention risk intestinal rupture and death.

Preoperative Stabilization

• Intravenous fluid therapy: Lactated Ringer's solution or 0.9% NaCl to address dehydration and maintain circulating volume
• Nasogastric intubation: Decompress stomach and rule out gastric reflux
• Analgesia: Flunixin meglumine (1.1 mg/kg IV) or other NSAIDs for pain control and anti-endotoxic effects
• Broad-spectrum antibiotics: Prophylactic antimicrobial therapy (e.g., potassium penicillin 22,000 IU/kg IV q6h plus gentamicin 6.6 mg/kg IV q24h)

Surgical Approach

Surgery is performed under general inhalation anesthesia with the horse in dorsal recumbency. The standard approach is ventral midline celiotomy (15-40 cm incision centered on or cranial to the umbilicus).

Surgical Procedure

• Systematically explore abdomen and palpate entire large and small colon
• Exteriorize left portion of large colon onto sterile colon tray
• Perform pelvic flexure enterotomy to evacuate colonic contents
• Gently manipulate enterolith to enterotomy site for removal
• If enterolith in right dorsal colon is too large, a second enterotomy may be required
• For transverse colon enteroliths: retrograde flushing via rectal enema may facilitate movement back to dorsal colon
• For small colon enteroliths: antimesenteric enterotomy directly over stone or manipulation to more accessible location
• Close enterotomy sites and lavage abdomen before routine closure

Perioperative and Postoperative Medications

Prognosis and Outcomes

Postoperative Complications

• Incisional complications: SSI (8.3%), hernia (6.9%), especially with enteroliths greater than 15 cm
• Postoperative ileus
• Diarrhea
• Peritonitis (rare if no perforation)
• Recurrence if dietary changes not implemented

Horses are typically kept out of work for 3 months following surgery to allow complete incisional healing.

Prevention Strategies

Prevention is critical for horses with a history of enterolithiasis and those at risk. Recurrence is highly likely if recommended dietary and management changes are not followed.

Dietary Modifications

• Reduce or eliminate alfalfa hay: Diet should consist of at least 50% grass hay or pasture
• Avoid wheat and rice bran: High phosphorus and magnesium content promotes stone formation
• Apple cider vinegar supplementation: 1 cup daily to acidify colonic contents and decrease pH
• Psyllium supplementation: Occasional doses to increase bulk movement through intestines
• Ensure adequate hydration: Provide free access to fresh, clean water and salt blocks

Management Changes

• Maximize pasture turnout: Continuous grazing encourages regular movement of feed material through intestines
• Regular exercise: Promotes normal intestinal motility
• Avoid feeding from ground in sandy environments: Use elevated feeders or mats
• Gradual diet changes: Sudden changes may cause stones to shift and create obstruction

Memory Aids and Board Tips

ENTEROLITH Mnemonic

E - Endemic regions (California, Florida, Southwest)

N - Nidus required (foreign body center)

T - Ten years or older (typical age)

E - Elevated pH promotes crystallization

R - Right dorsal colon formation site

O - Obstruction at transverse or small colon

L - Lucerne (alfalfa) as major risk factor

I - Intermittent colic is typical history

T - Treatment is SURGICAL

H - Have flat sides? Hunt for more stones!

Arabian + Alfalfa + Arizona = At Risk

This simple alliterative reminder captures the three key risk factors: breed (Arabian), diet (Alfalfa), and geography (Arizona/arid Southwest). Any combination increases suspicion for enterolithiasis.