Nephrolithiasis in Horses: Kidney Stones in Horses

Nephrolithiasis means kidney stones are present within one or both kidneys. These stones are mineral concretions that form somewhere in the urinary tract and, when they are located in the kidney, they are called nephroliths. In horses, urinary stones are uncommon overall, but when they do occur, the mineral is usually calcium carbonate, which fits with the horse's naturally alkaline urine and calcium-rich urinary sediment.

A horse with a kidney stone may look normal for quite a while. That is one reason this condition can be frustrating for pet parents. Some horses do not show clear signs until the stone interferes with urine flow, damages kidney tissue, or contributes to chronic kidney disease. When signs do appear, they may be vague at first, such as weight loss, lower energy, poor performance, or reduced appetite.

Kidney stones are different from bladder stones and urethral stones, even though all are forms of urolithiasis. Location matters because it changes the symptoms, the testing your vet may recommend, and the treatment options available. A stone in the kidney may be monitored in some horses, while a stone causing obstruction or severe loss of kidney function may require referral-level care.

The outlook varies widely. Horses with a small, incidental stone and preserved kidney function may do reasonably well with monitoring, while horses with bilateral disease, obstruction, or advanced renal damage can have a more guarded prognosis. Your vet helps match the plan to your horse's comfort, kidney function, and your goals.

Kidney stones form when minerals precipitate and build into a solid mass within the urinary tract. In horses, the chemistry of the urine already favors calcium carbonate crystal formation because equine urine is typically alkaline and normally contains abundant calcium carbonate crystals. Even so, stones are still rare, which suggests that most horses have protective factors such as high urine flow and urinary mucus that help prevent stone formation.

In many horses, there is not one single clear cause. Contributing factors may include changes in urine flow, local tissue damage, chronic inflammation, infection in some cases, or abnormalities that allow crystals to collect and enlarge over time. Some reports also suggest that renal crest necrosis, especially in horses exposed to dehydration plus NSAID use, may create a setting where mineralized debris becomes a nidus for stone formation.

Dehydration is an important practical concern because concentrated urine can make crystal precipitation more likely. Long periods of reduced water intake, heavy sweating, illness, transport stress, or winter water avoidance may all matter. Your vet may also look for underlying kidney disease, ureteral obstruction, or structural abnormalities that change how urine drains from the kidney.

Sex may play a role in some equine stone patterns. Reports have described a higher prevalence of nephrolithiasis in stallions, although kidney stones can occur in any horse. For many pet parents, the key takeaway is that prevention focuses less on one magic diet and more on hydration, careful medication use, and early evaluation of urinary or kidney-related signs.

Diagnosis starts with a careful history and physical exam. Your vet may ask about weight loss, appetite, urine changes, water intake, exercise tolerance, past colic-like episodes, and any recent use of NSAIDs or other medications. Because signs can be vague, the history often helps guide how aggressively to investigate.

Most horses need bloodwork and urinalysis early in the process. Blood chemistry can help assess kidney function by checking values such as creatinine and BUN, while electrolytes and calcium may provide more clues. Urinalysis can show blood, crystals, inflammatory changes, and urine concentration. These tests do not prove a kidney stone by themselves, but they help your vet understand how much the kidneys may be affected.

Imaging is usually the next step. Transabdominal and transrectal ultrasound are commonly used to look for hyperechoic mineralized structures, dilation of the renal pelvis, asymmetry between kidneys, or evidence of obstruction. In some horses, rectal palpation may detect an enlarged or abnormal kidney. Depending on the case, your vet may also recommend cystoscopy, radiographs, or referral imaging.

Referral hospitals may use more advanced tools when the diagnosis is unclear or surgery is being considered. These can include endoscopy of the urinary tract, sampling urine from each kidney, or computed tomography in select horses. The goal is not only to confirm that a stone is present, but also to determine which kidney is involved, whether both kidneys are affected, whether urine flow is obstructed, and how much functional kidney tissue remains.

Prevention centers on reducing the conditions that make stone formation more likely, even though not every case can be prevented. The most practical step is to support consistent hydration. Horses should always have access to clean, palatable water, and some need extra encouragement during cold weather, travel, heavy work, or illness. Your vet may also suggest ways to increase water intake if your horse tends to drink poorly.

Medication review matters too. NSAIDs are valuable drugs in equine medicine, but they should be used thoughtfully and under your vet's guidance, especially in horses that are dehydrated, systemically ill, or already have kidney concerns. Avoiding unnecessary kidney stress can lower the chance of renal injury that might later contribute to stone formation.

Routine monitoring is helpful for horses with a history of urinary disease, unexplained weight loss, recurrent hematuria, or previous kidney abnormalities. Follow-up bloodwork, urinalysis, and ultrasound may catch changes earlier, before a horse becomes obviously sick. Early detection is especially important because horses often show only subtle signs until disease is advanced.

Diet changes are not one-size-fits-all in horses with kidney stones. Because the underlying causes vary, your vet should guide any feed or supplement adjustments. In general, a balanced ration, good water access, and prompt attention to urinary or kidney-related signs give your horse the best chance of avoiding more serious complications.