Obstructive urolithiasis is relatively uncommon in horses and affects primarily older animals. Calcium carbonate stones are the most common type affecting horses. Clinical signs include dysuria, pollakiuria, and hematuria. Diagnosis is based on physical examination, ultrasonography, or urethral endoscopy. Surgery is the usual treatment.
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 animals than with young, growing animals. Males are more often affected, with geldings overrepresented. No breed predilection appears to exist.
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 can serve as a binding substrate for minerals. Equine urine is also typically alkaline and has a high mineral content, providing a suitable environment for calcium carbonate urolith formation.
Calcium carbonate crystals can be found in normal equine urine, and therefore calcium carbonate is the most common urolith type in horses.
Calcium carbonate uroliths can occur in two forms: one in which the surface is rough and the urolith can be crumbled relatively easily and a second that also contains phosphate ions, is smooth on the surface, and is quite resistant to crushing or fragmentation. Struvite, calcium phosphate, and sabulous uroliths can also occur.
Uroliths in horses form most often in the urinary bladder, where they may remain; they may also drop into the urethra and either be passed or obstruct. Less commonly, uroliths form in the renal pelvis and can break off to obstruct the ureters. Renal papillary necrosis associated with NSAID administration may be a predisposing factor for nephrolithiasis and ureterolithiasis (see nephrolithiasis image).
Courtesy of Dr. Sameeh M. Abutarbush.
Clinical Findings and Diagnosis of Urolithiasis in Horses
Horses with cystic calculi or urethral calculi can develop dysuria, pollakiuria, and hematuria. Incontinence can 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 clinical signs of colic are also often present, including sweating and restlessness. Hematuria after exercise is a common clinical sign in horses with cystic calculi.
On physical examination, it is common to rectally palpate a cystolith within the urinary bladder or by palpation of the perineal urethra when obstruction occurs. In cases of obstruction, the urinary bladder will be distended on palpation. An obstructing urolith may be located at the neck of the urinary bladder, at the level of the ischial arch, or in the perineal area.
Ultrasonography is the most valuable ancillary procedure for confirming a urolith. Retrograde passage of a urinary catheter will help to determine urethral patency and the approximate location of an obstruction, if present. Although uncommon, cystic calculi adhered to the bladder wall in the trigone area can obstruct the ureteral openings, causing hydronephrosis and renal failure.
Depending on the location of an obstructive urethrolith, it can be visualized with ultrasonography or endoscopy (see endoscopic image, urolithiasis). Nephroliths require ultrasonography for detection. Ureteroliths can be detected by ultrasonography or by rectal palpation of the stone in the ureter. Ureteral obstruction most often occurs just proximal to the ureter entrance into the bladder.
Courtesy of Dr. Sameeh M. Abutarbush.
Treatment of Urolithiasis in Horses
Several procedures are available for removing urinary calculi in horses. Generally, surgery is required in cases of urolithiasis in horses. Occasionally, a person with a small hand might be able to retrieve a urolith in a mare 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:
Cystic calculi
dilatation of the mare's urethra and manual or specimen retrieval pouch removal of intact or lithotripsy-fragmented stone(s)
perineal urethrotomy and manual dilatation of the urethra in male horses, followed by pouch removal of intact or lithotripsy-fragmented stones
laparoscopic and/or cystoscopic specimen retrieval pouch removal of intact or lithotripsy-fragmented stones
cystotomy via midline or paramedian laparotomy
perirectal cystotomy
Urethral calculi
perineal urethrotomy
urethral sphincterotomy
Ureteroliths
ureteroscopy and lithotripsy fragmentation
Nephroliths
laparotomy and nephrotomy
ureteropyeloscopic basket retrieval
When removing cystic calculi, ultrasonographic examination of the ureters and kidneys should always be performed to determine if other urinary calculi are present.
Pearls & Pitfalls
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Culture of the urine and, ideally, urolith analysis are indicated with any retrieved urolith material to guide additional treatment and prevention measures.
For pharmacological considerations, see the topics Pharmacotherapeutics in Urolithiasis, Controlling Urine pH, and Cysteine-Binding Agents Used to Treat Urinary Disease.
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. Substantial urinary acidification is difficult to accomplish in horses fed hay. It can be achieved short-term in some horses with ammonium sulfate (175 mg/kg, PO, every 12 hours) (1). Very few horses will voluntarily eat ammonium chloride or sulfate. 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 long-term use of acidifying salts should also be considered with caution, particularly in athletic horses, because osteoporosis is a potential adverse development.
Key Points
Horses with obstructing urethral or cystic calculi are in distress and require immediate intervention.
Horses with cystic calculi often are examined for hematuria after exercise.
Several methods for removing calculi are available.
Horses with ureteral stones often have bilateral partially obstructive disease and may be in renal failure at the time of stone detection.
Horses with nephroliths are often clinically normal.
For More Information
Fubini SL, Delco M. Surgery of the equine urinary tract. Vet Clin North Am Equine Pract. 2022;38(1):141-153.
Ternisien T, Dunn M, Vachon C, Manguin E, Bonilla AG, Jean D. Minimally invasive removal of obstructive ureteral stones by intracorporeal lithotripsy in horses: 3 patients. Can Vet J. 2023;64(1):25-30.
Sassot LN, Ragle C, Pentzke-Lemus L, Jones ARE, Farnsworth K. Progressive urethral dilation in male horses undergoing perineal urethrotomy for cystolith removal: 22 cases. J Equine Vet Sci. 2023;131:104955.
Moss AL, Acutt EV, Ullal T, Applegate TJ, Hackett ES. Ureteropyeloscopic removal of a nephrolith from a 19 year old Hanoverian gelding. Vet Surg. 2022;51 Suppl 1:O53-O59.
Also see pet owner content regarding uroliths in horses.
References
Remillard RL, Modransky PD, Welker FH, Thatcher CD. Dietary management of cystic calculi in a horse. Equine Vet Sci. 1992;12:359-363. doi:10.1016/S0737-0806(06)81362-6
