Ureterolithiasis in Horses: Stones Blocking Urine Flow

Ureterolithiasis is the formation of a mineral stone in one of the ureters, the narrow tubes that move urine from each kidney to the bladder. In horses, urinary stones are more often found in the bladder or urethra, so a ureteral stone is considered rare. Even so, it can become serious because the ureter is small, and a lodged stone can slow or stop urine flow from the affected kidney.

When urine cannot drain normally, pressure builds up behind the blockage. Over time, that can stretch the ureter and kidney, a change your vet may describe as hydroureter or hydronephrosis. If the obstruction is partial, signs may be subtle for weeks or months. If both kidneys are affected, or if the other kidney is not functioning well, the horse can become azotemic or develop kidney failure.

Most equine urinary stones contain calcium carbonate. Horses naturally produce alkaline urine with abundant calcium carbonate crystals, but most do not form clinically important stones. That means stone formation usually involves more than urine chemistry alone, and your vet may look for contributing factors such as urinary stasis, infection, inflammation, or structural problems in the urinary tract.

There is not one single proven cause of ureterolithiasis in horses. Equine urine is normally alkaline and contains large amounts of calcium carbonate crystals, which helps explain why most equine urinary stones are made largely of calcium carbonate. Still, stone disease remains uncommon, so other factors likely need to be present before a clinically important stone forms.

Possible contributors include urinary stasis, inflammation, infection, dehydration, and anatomic changes that slow urine flow. In some horses, stones may begin in the kidney and then move into the ureter, where they become lodged. Case reports and reviews also suggest that horses may have stones in more than one location at the same time, including the kidney, ureter, bladder, or urethra.

Sex may matter in some forms of equine stone disease, with males overrepresented in some reports of upper urinary tract stones. That does not mean mares are protected. Your vet will focus less on assigning blame and more on identifying factors that may affect treatment choices, recurrence risk, and long-term kidney function.

Diagnosis starts with a careful history and physical exam. Your vet will ask about urine changes, appetite, weight loss, performance, previous urinary problems, medications, and how long the signs have been going on. Because ureteral stones can mimic other causes of colic or urinary discomfort, the workup usually needs several pieces put together.

Common tests include bloodwork to check kidney values and hydration, plus urinalysis to look for blood, crystals, inflammation, and other abnormalities. Ultrasound is especially helpful for spotting kidney enlargement, dilation of the renal pelvis or ureter, and mineralized structures that suggest a stone. In some horses, a transrectal exam may help identify enlargement or abnormal structures in the urinary tract.

Referral-level diagnostics may include endoscopy, radiographs, contrast studies, or advanced imaging. These tests can help your vet confirm where the blockage is, whether one or both kidneys are involved, and whether the affected kidney still appears functional. That information matters because treatment planning for a stable horse with partial obstruction is very different from planning for a horse with worsening kidney failure.

Because ureterolithiasis is rare and not fully understood, prevention is not always straightforward. The most practical goal is supporting overall urinary tract health. Good hydration matters. Horses should always have access to clean water, and many do better when water intake is encouraged during hot weather, travel, heavy work, or winter when some horses drink less.

Your vet may also review diet, salt use, and any history of urinary sludge, bladder stones, or kidney changes. Diet changes should be individualized, because horses have unique calcium handling and not every stone problem responds to the same feeding plan. If your horse has had any urinary stone before, ask your vet whether periodic urinalysis or ultrasound monitoring makes sense.

Prompt attention to blood in the urine, repeated straining, or unexplained weight loss can also help catch urinary disease earlier. Early detection may not prevent every stone, but it can improve the chance of protecting kidney function before severe obstruction develops.