Equine Urolithiasis Study Guide

Overview and Clinical Importance

Urolithiasis refers to the formation of calculi (stones) within the urinary tract. In horses, this is an uncommon but clinically significant condition with a reported prevalence of approximately 0.11% of equine admissions to veterinary teaching hospitals. Despite this low prevalence, urolithiasis represents approximately 7.8% of diagnoses of urinary tract disease in horses, making it an important differential for horses presenting with signs of urinary dysfunction.

The unique composition of equine urine predisposes horses to calcium carbonate stone formation. Unlike other species, equine urine is naturally alkaline (pH greater than 7.0) and supersaturated with calcium and carbonate ions. The presence of large amounts of calcium carbonate crystals in normal equine urine is a normal finding, contributing to the characteristic turbid, mucoid appearance of horse urine.

Epidemiology and Risk Factors

Signalment

Sex: Males are more commonly affected than females, with geldings being overrepresented. Approximately 69% of all equine uroliths are from males. This predisposition is attributed to the longer, less distensible male urethra, which impedes passage of calculi.

Age: Urolithiasis is more common in older horses. The most common age group for stone submissions is 9 to 15 years old (38%). The average age at diagnosis is approximately 8 years.

Breed: No clear breed predisposition has been established, although Miniature Horses may be at increased risk due to differences in conformation.

Urolith Location Distribution

Pathophysiology

Unique Features of Equine Urine

Equine urine has several unique characteristics that predispose horses to urolithiasis. Understanding these features is essential for NAVLE success:

• Alkaline pH: Normal equine urine pH is greater than 7.0, promoting calcium carbonate crystal formation
• Supersaturation: Urine is naturally supersaturated with calcium and carbonate ions
• High mucoprotein content: Mucus glands in the renal pelvis and ureters produce the characteristic viscous, mucoid urine
• Normal crystal excretion: Calcium carbonate crystals are a normal finding in equine urine sediment

Mechanism of Urolith Formation

Urolith formation begins with mineralization around a nidus (nucleus) under conditions favoring crystal growth. The nidus may consist of necrotic debris, mucoproteins, leukocytes, desquamated epithelial cells, or foreign material such as suture. Once formed, the urolith enlarges through aggregation of calcium carbonate crystals (spherules) or precipitation of crystite crystals on the surface.

Types of Equine Uroliths

Risk Factors for Urolith Formation

• Dietary factors: High calcium diets (alfalfa, legumes) increase urinary calcium excretion
• Dehydration: Increases mineral concentration in urine
• Urine stasis: Allows crystal aggregation and stone growth
• NSAID toxicity: Renal papillary necrosis (especially phenylbutazone) may predispose to nephrolithiasis
• Previous urolithiasis: Horses with prior stones are at increased risk
• Urinary tract infection: While 90% of urine cultures are negative at stone removal, 63% of urolith centers show bacterial growth

Clinical Signs

Cystic Calculi (Bladder Stones)

Cystic calculi present with the classic triad of hematuria, dysuria, and stranguria. The most pathognomonic finding is hematuria that worsens after exercise. This occurs because physical activity causes the stone to bounce against the bladder mucosa, resulting in trauma and bleeding.

Clinical Signs by Urolith Location

Urethral Calculi: May cause complete urinary obstruction. Signs include severe stranguria, inability to urinate, distended bladder on rectal palpation, and signs of colic. Risk of bladder rupture if not addressed promptly. Stones most commonly lodge at the ischial arch in males.

Nephroliths: Often clinically silent. Most common presenting sign is weight loss secondary to chronic renal failure. May be discovered incidentally. Horses with concurrent cystic calculi should always have kidneys evaluated.

Ureteroliths: Rare but serious. Most horses present in renal failure at time of diagnosis. Clinical signs include colic (often severe), hematuria, and signs of uremia. Obstruction typically occurs just proximal to the bladder entrance.

Diagnosis

Physical Examination

Rectal palpation is the cornerstone of diagnosis for cystic calculi. The bladder can be palpated through the pelvic floor, and a firm, oval to round structure within the bladder is highly suggestive of a cystolith. Stones are most easily palpated when the bladder is empty. Administration of N-butylscopolammonium bromide (0.2-0.3 mg/kg IV) can facilitate palpation by relaxing the rectum. The pelvic urethra should also be palpated to identify any urethral stones.

Diagnostic Modalities

Treatment

Surgical removal is the treatment of choice for urolithiasis in horses. Unlike small animals, medical dissolution of stones is not effective in horses due to the composition (calcium carbonate) and difficulty achieving sustained urine acidification. The goal of surgery is complete removal of all calculi to minimize recurrence risk.

Surgical Approaches for Cystic Calculi

Lithotripsy Options

When intact stone removal is not feasible (large stones, Type II calculi), lithotripsy can be used to fragment stones for removal:

• Electrohydraulic lithotripsy: Uses electrical discharge to create shock waves
• Laser lithotripsy: Pulsed dye or Holmium:YAG lasers; vaporizes stone surface
• Pneumatic lithotripsy: Mechanical fragmentation; used with retrieval pouch
• Manual fragmentation: Lithotrite forceps; most commonly used

Treatment of Upper Urinary Tract Calculi

Nephroliths: Often left untreated if asymptomatic. If causing problems, options include nephrotomy or nephrectomy (preferred if no azotemia, to eliminate infection risk and recurrence).

Ureteroliths: Surgical options include ureterolithectomy via paralumbar approach or Dormia basket stone retrieval. Prognosis is guarded as most horses are in renal failure at diagnosis.

Perioperative Medications

Prognosis and Prevention

Prognosis

Cystic Calculi: Good prognosis with surgical removal if no concurrent upper tract disease. However, recurrence is common, reported in 40-47% of cases within 1-32 months.

Nephroliths/Ureteroliths: Poor prognosis when associated with chronic renal failure. In one study, 37% (25 of 68) of horses with urolithiasis were euthanized at admission or during hospitalization.

Uroperitoneum: Prognosis significantly worsened if bladder rupture has occurred secondary to obstruction.

Prevention Strategies

• Dietary Modification: Reduce calcium intake by avoiding high-calcium feeds (alfalfa, legumes, beet pulp). Feed oat hay or grass hay instead of alfalfa.
• Increase Water Intake: Add 50-75 g (1-2 oz) loose salt to daily ration to promote diuresis. Provide free-choice salt and clean, fresh water at all times.
• Urine Acidification: Ammonium sulfate (175 mg/kg PO q12h) or ammonium chloride (50-200 mg/kg PO daily) can acidify urine. However, palatability is poor and long-term compliance is challenging.
• Monitor for NSAID Use: Minimize chronic NSAID use (especially phenylbutazone) to reduce risk of renal papillary necrosis.
• Regular Monitoring: Periodic rectal palpation of bladder every 4-6 months in horses with history of urolithiasis.

Exam Focus: Unlike humans, adding salt to the equine diet does NOT increase urinary calcium excretion. This is a species-specific difference that may be tested! Salt supplementation promotes diuresis and is beneficial for stone prevention in horses.