Ureteroliths are small mineral masses that can cause serious problems in horses. Tucked in the narrow passageways from the kidney to the bladder, where veterinarians can’t always easily see or feel them on palpation, these uncommon mineral concretions can cause issues ranging from elevated kidney enzyme values and lack of appetite to abnormal urination and colic. They’re also scarce in the veterinary literature, so a University of California, Davis, research group recently performed a retrospective study to shed some light on the condition for equine veterinarians who might encounter it in their patients.

Linda Dillenbeck, BVSc, MANZCVS (equine medicine), was a fellow in Large Animal Ultrasound at UC Davis during the study, which she presented recently at the 2020 American Association of Equine Practitioners Convention, held virtually. She is now a first-year radiology resident at North Carolina State University’s College of Veterinary Medicine.

Ureteroliths are a segment of a larger category: uroliths, which are stones found anywhere within the urinary tract. “Prevalence (of uroliths) in horses is low at 0.04 to 0.7%, with male horses being most affected at 74%,” Dillenbeck explained. “These stones are predominantly composed of calcium carbonate, a normal constituent of equine urine.”

Of all the uroliths, veterinarians most commonly report cystoliths, or bladder stones. They make up to 62% of all urinary stones and can be readily diagnosed with transrectal palpation/ultrasound and cystoscopy (endoscopy of the bladder via the urethra). But given it is more difficult to detect ureteroliths, not a lot is known about them.

Dillenbeck and her co-authors sought to report on diagnosing ureterolithiasis in live horses and describe the clinical features, ultrasonographic findings, and outcomes in horses with ureteral stones. They reviewed 18 years of UC Davis Veterinary Medical Teaching Hospital records (January 2002-December 2019).

“Ureterolithiasis is uncommon in horses, and we had 15 cases in an 18-year time period, which is the largest retrospective study to date,” Dillenbeck said, a count helped over the years by advancing ultrasound technology and techniques.

She and her co-authors noted variable presenting complaints for these cases: colic in four of 15 horses and hematuria (blood in the urine) in only three (as a comparison, this is seen in 45% of horses with cystolithiasis). They noted elevated BUN (blood urea nitrogen)/creatinine, indicators of compromised kidney function, in 12 (80%) of the horses. This compares to 9% of horses with cystoliths.

“And it’s important to point out as well that no ureterolith was detected prior to referral compared to 33% of cystoliths in another study,” she said, underscoring how difficult it can be to catch these stones.

Practitioners used ultrasonography—specifically, transrectal ultrasound of the proximal, mid, and distal ureters—as the primary imaging modality to detect ureteral calculi. Dillenbeck and her co-authors recommend transverse views of the ureters with a microconvex transducer to improve their ability to see the stones. She also told TheHorse.com that ureteral ultrasound is somewhat challenging, most often performed by specialists at referral hospitals due to the structures’ location and small size.

“All horses showed significant renal (kidney) abnormalities with at least one Grade 2 (enlarged) or Grade 3 (small) kidney,” she added.“ This can be compared to the study of horses with cystolithiasis, where 30% of horses had normal-appearing kidneys.”

The veterinarians saw multifocal calculi—concretions in several locations—in 14 (93%) horses, with concurrent nephrolithiasis (kidney stones) being the most common finding in 13 horses (87%). This compares to previous studies of urinary and bladder stones where 9-61% of horses had stones in multiple locations, respectively.

“Some limitations of this study were that we had small case numbers, as this is a sporadic disease,” said Dillenbeck. “There was no long-term follow-up available for all horses, and we were unable to assess the recurrence rate in affected horses. The treatment choices varied widely, and we were thus unable to assess the best treatment options.”

Treatment approaches included surgery, medical, or conservative management and depended on clients’ budget and commitment. In some cases horses were euthanized based on the extent of renal and urinary tract disease. The three horses that underwent surgery experienced significant postoperative complications but were discharged after intensive management.

Take-Home Message

“Renal disease and multifocal urolithiasis are common in horses with ureteral stones,” said Dillenbeck. “Complete transcutaneous and transrectal ultrasonographic examination of all portions of the urinary tract is important in horses with urolithiasis. And ultrasonography was valuable to guide clinical decision-making and interventional procedures in affected horses.”