| Urolithiasis is a less common condition in horses than in small ruminants or steers. The disease can affect immature horses but is seen most frequently in adults. There is no breed predilection. Urolithiasis is seen more frequently in males than females, which has been attributed to anatomic differences between the male and female urethra. |
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Equine uroliths have a diameter of 0.5-21 cm, weigh as much as 6.5 kg, and are found most often within the bladder. Most equine uroliths are composed of calcium carbonate, in various hydrated forms, with struvite uroliths occasionally noted. Calcium carbonate uroliths have 2 separate clinical forms. The first form is a concretion of salts and mucoproteins that varies in consistency from friable to firm. These uroliths are usually yellow and oval
or irregularly shaped; they frequently have a rough or spiculated surface and are generally soft enough to be fragmented during surgery. The second calcium carbonate form is a firm concretion that is hard and resistant to fragmentation and is typically smooth and white. There appears to be no difference in chemical composition between these forms. |
| Etiology: |
| The mechanism of urolith formation in horses is unknown, although the alkaline pH and high mineral content of normal equine urine may favor crystal formation and precipitation. Normal equine urine also contains large amounts of mucoproteins, which may serve as a cementing substance to adhere crystals. Consumption of feed and water high in mineral content may increase urinary solute concentrations and thereby promote crystallization and precipitation. Multiple nephroliths may
develop in horses with renal papillary necrosis (associated with NSAID administration while dehydrated) and mineralization of the papillae. |
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| Clinical Findings: |
| Clinical signs depend on the urolith location. Most uroliths are located in the bladder and cause dysuria, pollakiuria, and hematuria. Hematuria is most evident after exercise and toward the end of a voided urine stream. Affected horses frequently stretch out to urinate and may maintain this posture for variable periods before and after micturition. Additional signs may include scalding of the perineum in females or of the medial aspect of the hindlimbs in males. Geldings and
stallions may protrude the penis flaccidly for prolonged periods while intermittently dribbling urine. Affected horses may occasionally exhibit recurrent bouts of colic or an altered hindlimb gait. Urethral obstruction may also develop as the result of a trapped urolith and is typically accompanied by restlessness, sweating, varying degrees of colic, and frequent attempts to urinate. The bladder is distended on rectal examination. In most fatal cases, a single large urolith,
occasionally accompanied by smaller ones, is found in the bladder; less frequently, the urolith may be found lodged at the bladder neck or the ischial arch. Nephroliths are occasionally found via ultrasonography in horses with cystic calculi; owners of such horses should be informed that obstruction may recur. |
| Bilateral nephroliths are not uncommon in adult horses that have been used for performance. Intermittent chronic obstruction of ureters will eventually cause renal failure, resulting in weight loss and anorexia. |
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| Diagnosis: |
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Tentative diagnosis of urolithiasis is usually based on the history and clinical signs and confirmed most easily by rectal palpation of a firm, ovoid, intravesicular mass at or near the neck of the bladder. In most cases, urolith palpation is not difficult because clinical signs are rarely evident until the stone is several centimeters in diameter. Transrectal ultrasonography with a 7.5 mHz linear probe allows visualization of the stone. If
ultrasound examination cannot be performed, the distended bladder should be catheterized to facilitate palpation and eliminate the possibility of urethroliths, urethral stricture, or smegma impaction of the urethral sinus. Urinalysis frequently reveals RBC, neutrophils, calcium carbonate crystals, and proteinuria. Cystoscopy, ultrasonography, and radiology are not essential for detection of urocystoliths in horses but may provide additional diagnostic or prognostic information.
Ultrasonography is necessary to identify nephroliths. |
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| Treatment: |
| Several surgical procedures have been described for urocystolith removal. The surgical options include midline or paramedian laparotomy and cystotomy, pararectal cystotomy, subischial urethrostomy, urethral sphincterotomy, and laser lithotripsy. The selection of a procedure is dictated by the size, location, and number of uroliths; the sex and physiologic status of the horse; and the availability of surgical facilities. |
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