How to Prevent Urolithiasis in Asian Small-Clawed Otters

Urolithiasis is simply the medical term for the development of urinary stones — also called uroliths — anywhere within the urinary tract. These stones can form in the kidneys (nephroliths), the bladder (cystoliths), or the ureters, and they range in size from tiny gritty deposits to large, painful obstructions.

In Asian small-clawed otters (Aonyx cinereus), urolithiasis is one of the most frequently documented health challenges in managed care. Research published in the Journal of Zoo and Wildlife Medicine confirmed that renal calculi were detected in 66.1% of the captive adult population that had been radiographed or necropsied, and cystic calculi were present in 23.2%.

Even more striking, all otters with cystic calculi also had renal calculi — meaning kidney stones almost always come first. A 2025 study examining otter morbidity in U.S. zoos found that chronic renal disease and urolithiasis was among the most common causes of death or euthanasia.

You might wonder: why do ASCOs get kidney stones at such alarming rates compared to other species? The answer is a combination of biology and captive care.

First, there’s a clear geographic and management pattern. North American facilities reported the highest incidence of kidney stones at 62.8%, followed by European institutions at 12.9% and Asian facilities at just 9.4%. This regional disparity is a huge clue: it strongly suggests that risk factors for stone formation in this population are thought to be mainly nutritional.

The Urinary Chemistry Problem

Research into otter metabolites revealed that urinary oxalate levels were quite high when compared with those of dogs and humans without uroliths, and the ratio of urinary oxalate to calcium was close to 1:1 during periods of food consumption. That near-equal ratio creates a perfect storm for crystal formation.

Age is another factor. Research identified age and calcium as risk factors, whereas crude protein and sodium were found to be protective. Older otters, particularly those on calcium-heavy diets, face the highest risk.

If there’s one area where you can make the biggest difference in preventing urolithiasis, it’s diet. The research is remarkably consistent on this point: what you feed your otters matters enormously.

Embrace a Naturalistic Diet

One of the most promising findings in recent research came from a two-year study that introduced a species-appropriate diet. The treatment diet was higher in protein and lower in calcium than standard diets and did not contain any kibble. The results were encouraging: the study demonstrated that a naturalistic crustacean-based diet could effectively control urolith development.

In the wild, Asian small-clawed otters eat primarily crustaceans and mollusks but will also eat fish, insects, amphibians, and reptiles. Replicating this variety in captivity is key.

Foods to Embrace vs. Foods to Avoid

The Kibble Question

Many institutions have historically relied on commercial cat kibble as part of otter diets. However, the research signals caution. The landmark 2022 study that showed no urolith progression specifically used a diet with no kibble at all, suggesting that a whole-food approach may be significantly superior.

Hydration plays a crucial role in flushing minerals through the urinary system before they have a chance to crystallize. Inadequate water intake may contribute to the development of kidney stones, making hydration especially important for this species.

Supplementation can be a powerful tool in your prevention toolkit — but only when done thoughtfully. More isn’t always better, and the wrong supplement at the wrong dose can actually worsen the problem.

Potassium Citrate

Use of potassium citrate has offered some promise in minimizing the incidence of renal calculi in dogs and cats via inhibition of crystallization, formation of complexes with calcium, and acidification of the urine. While its direct efficacy in otters hasn’t been fully proven, it remains a widely discussed option in zoo veterinary circles.

Pyridoxine (Vitamin B6)

Pyridoxine has been successful in treating some cases of calcium oxalate uroliths in humans. Some practitioners have explored its use in otters as well, though species-specific dosing guidelines remain limited.

Vitamin E & Thiamin

If your otter’s diet is heavily fish-based — which it should be for stone prevention — watch out for secondary deficiencies. Since nutrients like vitamin E and thiamin can degrade from fish during storage and freezing, supplementing with these vitamins along with a well-balanced multivitamin is recommended.

Prevention is the gold standard, but early detection is your safety net. Given how common urolithiasis is in this species, a proactive monitoring program isn’t optional — it’s essential.

Here’s a structured approach that aligns with current best practices from zoological institutions: