CONDITION
Calcium Oxalate Urolithiasis
Calcium oxalate urolithiasis describes the formation of stones made of calcium oxalate crystals within the urinary tract—most often in the bladder, though they can also form in the kidneys or urethra. These stones develop when certain minerals become concentrated in the urine and precipitate out to form solid material. Once formed, they tend to remain and may grow over time, though they do not typically dissolve with medical management alone. Owners often notice signs related to difficulty or discomfort when urinating: frequent small amounts, straining, blood in the urine, or accidents in unusual places. In some cases, stones may be discovered incidentally during imaging for another reason, before any signs appear. Male dogs and cats can develop a blockage if a stone lodges in the narrow urethra, which in male dogs is most often just behind the small bone within the penis. A complete blockage presents as repeated attempts to urinate that produce nothing or only drops, often with crying out, restlessness and a tense, painful abdomen. Once urine cannot leave the bladder, waste products and potassium build up in the blood. Within roughly a day or two this can cause vomiting, weakness and disturbances of heart rhythm, and without relief of the blockage it can be fatal within about three days, sometimes sooner. Straining from an irritated bladder and straining from a blockage can look the same from the outside, and an overfull bladder cannot be reliably identified at home. This page explores the signals that may point toward stones, what happens at the metabolic and physical level to cause them to form, how they are identified through imaging and urine testing, and the range of approaches—from surgical removal to dietary adjustment—used to manage the condition and reduce the chance of recurrence.
Why this matters now
Calcium oxalate stones tend to form in middle-aged to older animals, with the typical age of first diagnosis around seven to eight years in both dogs and cats. Certain breeds carry higher risk: in dogs, small breeds such as Miniature Schnauzers, Yorkshire Terriers, Shih Tzus, and Bichon Frises are predisposed, while in cats, Persian, Himalayan, and Ragdoll breeds are overrepresented. Male animals are affected more often than females, and neutering appears to increase risk in both species.
Once a stone has formed, it may remain stable for months or years, or it may grow gradually as more mineral accumulates around it. Some animals show signs early, while others carry stones silently until imaging reveals them incidentally. The picture changes if a stone moves into the urethra of a male dog or cat: a blockage can develop within minutes to hours, and the effects on the kidneys and heart rhythm build over the following day or two. Recurrence after removal is common in dogs—approximately half experience recurrence within two to three years—but appears much less common in cats, where around seven per cent had a recurrence submitted for analysis over roughly five years of follow-up in one large case series, a figure that likely underestimates true recurrence since not every new stone is submitted for testing. The tendency to form stones often persists even after the first episode has been managed.
Signals & patterns
Early signals
Frequent small urinations
An owner may notice their pet asking to go out more often, or visiting the litter tray repeatedly, passing only small amounts each time. This pattern reflects irritation of the bladder lining by the stone's surface.
Blood in the urine
Urine may appear pink-tinged, red, or contain visible clots, most often noticed at the end of urination. The stone's rough surface can erode the delicate bladder wall as it shifts position.
Straining or discomfort when urinating
The animal may adopt a prolonged posture, vocalise, or appear tense during urination. This can signal either bladder discomfort or partial obstruction if a stone has moved into the urethra.
Urinating in unusual places
A previously housetrained dog may urinate indoors, or a cat may avoid the litter tray and choose smooth surfaces such as tile or bathtubs. This change often reflects urgency or an association of the tray with discomfort.
Later signals
Repeated unproductive straining
An animal may adopt the posture to urinate but produce nothing, or only a few drops. In male dogs and cats, this pattern can indicate a stone lodged in the urethra, preventing urine from leaving the bladder entirely. Passing small amounts each time and passing nothing at all can be hard to tell apart, particularly with a cat using a litter tray or a dog urinating outdoors. A full, obstructed bladder cannot be reliably identified by feeling the abdomen at home.
Tense or painful abdomen
The abdomen may feel firm to the touch, and the animal may resent handling or palpation in that area. This can accompany bladder distension when urine cannot be voided.
Lethargy and reduced appetite
As the bladder remains full and waste products accumulate in the bloodstream, the animal may become quieter, less interested in food, and spend more time resting. This reflects the systemic effects of urinary obstruction. A rising potassium level can slow and disturb the heart rhythm, and vomiting, weakness or collapse can follow within a day or two of a complete blockage.
Vomiting, lethargy or loss of appetite in cats without straining
In cats, the great majority of stones lodged in the ureters, the tubes that carry urine from the kidneys to the bladder, are calcium oxalate. A blockage at this level does not usually cause straining. It tends instead to show as vomiting, lethargy, loss of appetite and weight loss. These signs are shared with many other illnesses and can reflect sudden kidney injury, which can only be identified through blood tests and imaging.
Click to read about the biological mechanisms
How this is usually investigated
Investigation typically begins with a detailed history focusing on urination patterns, water intake, and any previous episodes of urinary signs. Physical examination may reveal a tense or painful bladder, particularly in animals with partial or complete obstruction. Where a blockage is suspected, blood tests for kidney values and potassium, and often an ECG, are typically carried out alongside imaging, because these results shape how the animal is stabilised before the stone itself is addressed. The diagnosis is confirmed through imaging, which reveals the presence, number, size, and location of stones, often supported by urine testing to assess the chemical environment and rule out concurrent infection.
Radiography
Abdominal ultrasound
Urinalysis with sediment
Chemistry panel
Bacterial culture and sensitivity
Options & trade-offs
Management typically combines removal or monitoring of existing stones with adjustments to reduce the likelihood of new stone formation. The approach chosen depends on the size and location of stones, whether they are causing signs, the animal's overall health, and the owner's capacity to sustain long-term dietary and monitoring strategies. Different combinations suit different households, and the aim is to balance the immediate resolution of stones with practical measures to slow or prevent recurrence.
Surgical removal (cystotomy)
Cystotomy involves opening the bladder under general anaesthesia to remove stones directly. This allows complete clearance of visible stones and provides tissue for analysis to confirm the mineral composition. The procedure requires a brief hospital stay and a recovery period of around two weeks, during which activity is restricted and the incision monitored.
Trade-offs: Surgery carries the risks associated with anaesthesia and any invasive procedure, including infection and delayed healing. It does not address the underlying metabolic tendency to form stones, so recurrence remains possible without concurrent dietary or medical management. Animals with multiple small stones throughout the urinary tract may be more challenging to clear completely.
Laser lithotripsy or voiding urohydropulsion
Laser lithotripsy uses a fibre-optic scope and laser energy to fragment stones into smaller pieces that can be flushed out or passed naturally. Voiding urohydropulsion involves filling the bladder with sterile fluid and using pressure or positioning to expel small stones through the urethra. Both techniques are less invasive than open surgery and may allow same-day or next-day discharge.
Trade-offs: These techniques require specialised equipment and expertise, so availability may be limited to referral centres. They are most effective for smaller stones and may not fully clear large or numerous calculi. Incomplete removal leaves a nidus for regrowth, and recurrence rates are similar to those following surgery.
Dietary modification to reduce supersaturation
Therapeutic diets aim to dilute the urine, modify pH toward a range less favourable for calcium oxalate precipitation, and reduce dietary oxalate and calcium where appropriate. Increased water intake—through wet food, added water to meals, or multiple drinking stations—helps maintain lower urine concentration. These changes are typically continued long-term to reduce the chance of new stone formation.
Trade-offs: Dietary strategies do not dissolve existing calcium oxalate stones, so they are used alongside or after removal, or as monitoring in animals with small, asymptomatic stones. Compliance can be challenging in multi-pet households or in animals with strong food preferences. The effect on recurrence is modest, and some animals form new stones despite careful dietary management.
Monitoring without intervention
When stones are small, few in number, and causing no signs, some owners and vets choose to monitor with periodic imaging and urinalysis rather than pursue immediate removal. This approach suits animals with incidental findings, older animals with anaesthetic risk, or those whose owners prefer to avoid surgery unless signs develop.
Trade-offs: Monitoring requires regular follow-up to detect growth or the onset of obstruction, and there is a risk that stones may enlarge or move into a position that complicates later removal. In male dogs and cats, a small bladder stone can pass into the urethra and block it, which can abruptly change a monitored finding into a situation in which urine cannot be passed. It does not prevent progression, and some animals eventually require intervention as stones grow or clinical signs appear.
Treatment of underlying metabolic causes
In animals with documented hypercalcaemia, hyperparathyroidism, or other metabolic abnormalities, addressing the underlying condition can reduce ongoing stone formation. This may involve surgical removal of a parathyroid tumour, management of Cushing's disease, or dietary adjustments for conditions affecting calcium metabolism.
Trade-offs: These conditions are relatively uncommon contributors to calcium oxalate urolithiasis, so screening is typically reserved for animals with persistently elevated blood calcium or other suggestive clinical findings. Correcting the metabolic abnormality does not reverse stones already formed, and surgical or dietary management of stones may still be needed.
Relief of urethral obstruction
When a stone blocks the urethra, the first step is to restore urine flow rather than to remove the stone permanently. This typically involves blood tests, intravenous fluids and treatment of a high potassium level where present, alongside sedation or anaesthesia. An overfull bladder may first be drained with a needle through the abdominal wall. The stone is often then flushed back into the bladder with sterile fluid through a urinary catheter, a technique called retrograde urohydropulsion. Once the animal is stable, the stone can be removed from the bladder by surgery or lithotripsy.
Trade-offs: Flushing the stone back into the bladder succeeds in most dogs. Where the stone cannot be moved, a temporary tube placed into the bladder or surgery on the urethra may be needed. Draining the bladder with a needle carries a small risk of urine leakage or bladder injury. Relieving the blockage does not treat the stone itself, and the stone can lodge again until it is removed. The length of time a blockage has been present tends to influence how much the kidneys and heart rhythm are affected, and some animals need several days of hospital care.
Common misconceptions
"Calcium oxalate stones can be dissolved with a special diet or medication, just like some other types of bladder stones."
Unlike struvite stones, which can dissolve with dietary acidification and reduced magnesium, calcium oxalate stones are not amenable to medical dissolution. Once formed, they persist until physically removed through surgery, lithotripsy, or natural passage. Dietary modification plays a role in reducing recurrence, not in dissolving existing stones.
"Restricting all calcium from the diet will prevent calcium oxalate stones from forming."
Severely restricting dietary calcium can paradoxically increase the risk of stone formation, because low calcium in the intestine allows more oxalate to be absorbed and excreted in the urine. A moderate, balanced intake of calcium is generally safer than extreme restriction. The focus tends to be on managing oxalate intake, promoting dilute urine, and addressing metabolic factors rather than eliminating calcium entirely.
"If a dog or cat has had stones removed once, they will not form them again."
Recurrence is common, particularly in dogs, where approximately half of animals develop new stones within two to three years after removal. The metabolic tendency that led to the first episode often persists, and without ongoing dietary or environmental management, the conditions favouring stone formation can return. Lifelong attention to urine dilution and composition is typically part of the long-term strategy.
Related conditions
Acute Kidney Injury
Calcium oxalate stones that form in the kidneys can sometimes cause obstruction or damage to kidney tissue, and acute kidney injury may develop if both ureters become blocked or if a large stone burden impairs normal filtration. Conversely, certain metabolic changes during acute kidney injury can alter urine composition in ways that may influence crystal formation.
Chronic Kidney Disease in Cats
Chronic kidney disease can alter the way the kidneys handle calcium and other minerals, and some cats with kidney disease may be at higher risk of forming calcium oxalate stones. Managing both conditions together often involves careful attention to diet, hydration, and monitoring of urine and blood chemistry.
Chronic Kidney Disease in Dogs
Dogs with chronic kidney disease may develop changes in urine pH, calcium handling, and mineral balance that can favour calcium oxalate crystal formation. In some cases, dietary strategies intended to support kidney function may need to be balanced against measures to reduce stone recurrence.
Cushings Disease in Dogs
Cushing's disease can lead to increased calcium excretion in the urine and changes in urine concentration that may raise the risk of calcium oxalate stone formation. Dogs with both conditions often require coordinated management of hormone levels, hydration, and dietary mineral content.
Ectopic Ureters
Ectopic ureters can sometimes create pooling or altered flow of urine, and in rare cases this may contribute to an environment where minerals are more likely to precipitate. Both conditions can present with urinary signs, though ectopic ureters typically cause incontinence from an early age rather than the intermittent straining or haematuria more common with stones.
Understanding the factors that influence urine concentration and mineral balance can help frame ongoing decisions about diet, water access, and monitoring frequency. The broader context of metabolic health—including how the kidneys handle calcium and how the body metabolises oxalate—connects to a range of other conditions that share overlapping risk factors. Conversations about recurrence risk, the practicalities of long-term dietary change, and what to watch for at home can be useful as part of routine follow-up visits.
Last reviewed: 13 September 2026 · Dr Alastair Greenway MRCVS