Urolithiasis and obstructive urolithiasis are far less common in horses than in ruminants. The most common stone type in horses is calcium carbonate and is associated more with older rather than young, growing animals. Males are more often affected, with geldings overrepresented. There does not appear to be a breed predilection.
Etiology and Pathogenesis of Urolithiasis in Horses
The general principles of urolith formation are common to all species. An initial organic matrix forms and is bound to by inorganic minerals, establishing the stable structure. Horses normally have high amounts of mucoprotein present in their urine, which may serve as a binding substrate for minerals. Equine urine is also typically alkaline and has a high mineral content, providing a suitable environment for urolith formation.
Calcium carbonate crystals may be found in normal equine urine, and therefore calcium carbonate is the most common urolith type. It can occur in two forms, one in which the surface is rough and the urolith may be crumbled relatively easily and a second that is smooth on the surface and quite resistant to crushing or fragmentation. This second form is most similar to calcium carbonate in ruminants, although both types in horses are chemically identical. Struvite, calcium phosphate, and sabulous uroliths may also occur.
Uroliths in horses form most often in the urinary bladder, where they may remain, or may drop into the urethra and be passed or may obstruct. Less commonly, uroliths form in the renal pelvis and may break off to obstruct the ureters. Renal papillary necrosis associated with NSAID administration may also result in nephrolithiasis.
Clinical Findings and Diagnosis of Urolithiasis in Horses
Horses with cystic calculi or urethral calculi may present with dysuria, pollakiuria, and hematuria. Incontinence may result in urine scalding of the perineum in females or of the hindlimbs in males. Males may also extend their penis and dribble urine. General signs of colic Overview of Colic in Horses Depiction of a horse’s GI tract as viewed from the right side. In its strictest definition, the term “colic” means abdominal pain. Throughout the years, it has become a broad term for a variety... read more are also often present, including sweating and restlessness.
On physical examination, it is often possible to palpate a cystolith within the urinary bladder. In cases of obstruction, the urinary bladder will be distended on palpation. A urolith may be located at the neck of the urinary bladder or at the level of the ischial arch on ultrasonography. Retrograde passage of a urinary catheter will help to determine urethral patency and the approximate location of an obstruction, if present.
Depending on location of an obstructive urethrolith, it may be visualized with ultrasonography Ultrasonography in Animals Ultrasonography is the second most commonly used imaging format in veterinary practice. It uses ultrasonic sound waves in the frequency range of 1.5–15 megahertz (MHz) to create images of body... read more or require urethral endoscopy for visualization. Nephroliths require ultrasonography for detection.
Treatment of Urolithiasis in Horses
Generally, surgery is required in cases of urolithiasis in horses. Occasionally, in mares, an individual with a small hand may be able to retrieve a urolith transurethrally. Laser lithotripsy may also be performed via endoscopy to fragment a stone for removal without surgery. A variety of surgical options are available for relief of urinary obstruction and removal of uroliths in horses:
cystotomy via midline or paramedial laparotomy
cystotomy via laparoscopy
perineal urethrotomy or urethrostomy
Laser or shock wave lithotripsy may be required as an adjunct to any of these procedures.
Culture of the urine and urolith analysis is indicated with any retrieved urolith material to guide additional treatment and prevention measures.
For pharmacological considerations, see Pharmacotherapeutics in Urolithiasis in Animals in Systemic Pharmacotherapeutics of the Urinary System , see Controlling Urine pH in Animals The ideal urine pH is 7.0–7.5 in dogs and 6.3–6.6 in cats. If the urine pH remains low after diet modification, potassium citrate can be administered in food to increase the pH. Because it complexes... read more , and see Cystine-Binding Agents Used to Treat Urinary Disease in Animals Cystinuria, with subsequent cystine urolith formation, results from a breed-related inherited disorder of renal tubular transport in dogs. Cystine stones are dissolved by means of dietary modification... read more .
Control and Prevention of Urolithiasis in Horses
Mineral consumption from all feed and water sources should be balanced in light of nutritional requirements and the mineral components of uroliths. Urinary acidification may be achieved using ammonium chloride (50–200, mg/kg, PO, daily) or ammonium sulfate (200–300 mg/kg, PO, daily). Because calcium-containing uroliths predominate in horses and acidification increases calcium excretion and availability in the urine, high calcium-containing feeds, including alfalfa and other legumes, should be restricted. The longterm use of acidifying salts should be considered with caution, particularly in athletic horses, because osteoporosis is a potential adverse development.
For More Information
Also see pet health content regarding uroliths in horses Urinary Stones (Uroliths, Calculi) Not every disease is caused by infection with bacteria, viruses, or other outside agents. There are a variety of noninfectious disorders that can impair the urinary system. All of these diseases... read more .