A recent hospital case inspired me to research and share information about equine urinary stones, also termed urolithiasis.
Lower urinary tract (urethral) stones are more of a problem in humans, whereas in horses we typically see upper urinary tract (bladder) stones. Male horses, geldings specifically, make up ¾ of all cases and have been attributed to their longer, less distensible urethra preventing stone clearance. Bladder stones are most commonly seen in adult horses, but can occur at any age. Stones form similarly to how an oyster forms a pearl when urinary crystals precipitate (solidify out of solution) and build upon a nidus (cluster) of epithelial cells which have shed from the bladder wall. Factors which are thought to contribute to stone formation include prolonged urine retention, increased excretion of calcium, uric acid or oxalates in urine.
Horses normally excrete a large amount of calcium carbonate crystals in their urine and have an alkaline (high) pH which favors crystallization but there are also are factors found in horse urine, including mucous and proteins which prevent stone formation. There are two types of urinary stones found in horses. Type I is most common (in 90% of cases) and is composed of calcium carbonate, is fairly easily broken up, yellow to green in color and spiculated on the outside. Type II is made up of calcium carbonate, phosphate, and magnesium, hard to break up, white, and smooth.
Signs of urolithiasis include blood in the urine after exercise (due to the stone bouncing around and traumatizing the bladder wall), blood or urine staining on the hindlimbs, dribbling of urine, difficulty urinating, prolonged periods where a male horse’s penis is dropped down, and/or a stilted hindlimb gait.
Your veterinarian can diagnose urolithiasis by means of palpating the bladder rectally, ultrasound of the bladder, and/or endoscopic (camera) exam of the urethra and bladder. The stone(s) should be surgically removed, and there are various methods to do so. With the horse understanding sedation the bladder can be accessed via laparocystotomy, perineal urethrotomy, or pararectal cystotomy. If small enough the stone can be removed whole but often times it must be fragmented with forceps, a laser, shockwave, or a dremmel tool. The stone itself and its fragments can then be scooped up by deploying a special retrieval bag pictured below. The bladder is then flushed thoroughly with saline to remove all stone fragments.
Most often surgical removal is curative, but stones can recur. In an effort to prevent future stone formation calcium levels should meet but not exceed your horse’s daily requirements. This means no calcium supplements, feeds with added calcium, legumes (alfalfa), or early growth grass. Acidification of the urine can also help prevent stones, however it is often a challenge because the feed additives required (ammonium chloride, ammonium sulfate, ascorbic acid) are often unpalatable to horses and the urinary pH at which stones form is unknown. If you have further questions about equine urolithiasis or think your horse may have a urinary tract problem please contact your veterinarian.