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.
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.
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 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 or require urethral endoscopy for visualization. Nephroliths require ultrasonography for detection.
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:
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.
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.