Calcium Oxalate Bladder Stones in Dogs

By Courtney Barnes, BSc, DVMTammy Hunter, DVM; Ernest Ward, DVM

What are calcium oxalate bladder stones?

Bladder stones (uroliths or cystic calculi) are rock-like formations of minerals that form in the urinary bladder and are more common than kidney stones in dogs. There may be a large, single stone or a collection of stones that range in size from sand-like grains to gravel.

One of the more common uroliths in the dog is composed of calcium oxalate crystals.

What causes calcium oxalate bladder stones?

The exact cause of calcium oxalate bladder stones is complex and poorly understood at this time. Normal dog urine is slightly acidic and contains waste products from metabolism, including dissolved mineral salts and other compounds. These mineral salts remain dissolved in the urine as long as the pH stays within a narrow range, and as long as the urine does not become too concentrated.

Current research indicates that urine high in calcium, citrates, or oxalates that is also acidic predisposes a pet to developing calcium oxalate urinary crystals and stones. Diets higher in carbohydrates, lower in phosphorus, and those that promote a more acidic urine can lead to the development of this type of bladder stone. Male, small breed dogs are overrepresented.

Disorders of calcium metabolism, such as hyperadrenocorticism, hyperparathyroidism, or certain cancers, can lead to high levels of calcium in the bloodstream, which then leads to excess calcium excretion in the urine. Genetics also play a role, and gene mutations in several breeds have been identified.

How common are calcium oxalate bladder stones?

Bladder stones are somewhat common in dogs, and calcium oxalate bladder stones are the second-most common type of stone, second only to struvite stones. Together, struvite and calcium oxalate uroliths have been found to comprise over 85% of all canine uroliths. The number of struvite bladder stones has been declining in dogs, while the number of calcium oxalate stones has been increasing over the past ten years. Struvite uroliths were noted to be more common in female dogs and calcium oxalate uroliths in male dogs. Breeds most diagnosed with struvite and calcium oxalate bladder stones include Shih Tzus, miniature schnauzer, bichon frise, Lhasa Apso, and Yorkshire terrier.

What are the signs of calcium oxalate bladder stones?

The signs of bladder stones are similar to the signs of an uncomplicated bladder infection or cystitis. The most common signs of bladder stones are blood in the urine and straining to urinate.

Blood in the urine occurs because the stones rub against the bladder wall, irritating and damaging the tissue and causing bleeding. Straining may result from inflammation and swelling of the bladder walls or the urethra (the tube that transports the urine from the bladder to the outside of the body), from muscle spasms, or from a physical obstruction to urine flow.

Veterinarians assume that the condition is painful, because people with bladder stones experience pain, and because many clients remark about how much better and more active their dog becomes following surgical removal of bladder stones.

Large stones may act almost like a valve or stopcock, causing an intermittent or partial obstruction at the point where the bladder attaches to the urethra. Small stones may flow with the urine into the urethra where they can become lodged and cause an obstruction. If an obstruction occurs, the bladder cannot be emptied fully; if the obstruction is complete, the dog will be unable to urinate at all. If the obstruction is not relieved, the bladder may rupture or the kidneys may fail.

A complete obstruction is potentially life threatening and requires immediate emergency treatment. Complete obstruction is more common in male dogs due to their longer, narrower urethras.

How will my veterinarian diagnose oxalate bladder stones?

If your dog presents to the veterinarian for urinary signs, your veterinarian will first perform a urinalysis. This test involves obtaining a small sample of urine for biochemical analysis and examination under the microscope. If your dog has oxalate bladder stones, the urinalysis will likely show the presence of a low urine pH, red blood cells (due to bladder trauma), white blood cells (consistent with inflammation), and increased numbers of oxalate crystals in the urine.

Your veterinarian will also likely perform blood tests, including a complete blood cell count and serum biochemistry profile. These tests will assess your dog’s overall health and rule out other medical conditions that may be contributing to your dog’s urinary signs. Some dogs with oxalate stones have high blood calcium levels, which can be detected on bloodwork.

"The only way to be sure that a bladder stone is made of calcium oxalate is to have the stone analyzed at a veterinary laboratory."

In some cases, if your dog is relaxed and the bladder is not too painful, your veterinarian may be able to feel stones in the bladder. However, some stones are too small to be felt this way. Often, bladder stones are diagnosed by an X-ray of the bladder. Calcium oxalate stones almost always show up on an X-ray, but sometimes bones or other overlying body parts interfere with the ability to see bladder stones on regular X-rays, in which case, your veterinarian may recommend a bladder ultrasound or a contrast study, a specialized technique that uses dye to outline the stones in the bladder.

These imaging procedures identify the presence of a bladder stone but do not definitively indicate composition. The only way to be sure that a bladder stone is made of calcium oxalate is to have the stone analyzed at a veterinary laboratory.

In some cases, your veterinarian may make an educated guess about the type of stone, based on the radiographic appearance and results of a urinalysis. For example, if X-rays show there are one or more stones in the bladder, and the urinalysis shows the presence of acidic urine along with numerous calcium oxalate crystals, your veterinarian may make a presumptive diagnosis of calcium oxalate bladder stones and recommend treatment accordingly.

What is the treatment for oxalate bladder stones?

There are two primary treatment strategies for treating calcium oxalate bladder stones in dogs: non-surgical removal by urohydropropulsion, and surgical removal.

With urohydropropulsion, the bladder stones are flushed out using a special urinary catheter technique. This procedure is only possible when the stones are very small in diameter. In some cases, this may be performed with the dog under heavy sedation, although general anesthesia is often necessary. Small stones in the bladder or urethra can sometimes be removed with a cystoscope, avoiding a surgical procedure to open the bladder. Either of these procedures may also be used to obtain a sample stone for analysis so your veterinarian can determine if the stones can be dissolved by changing the dog’s diet.

"Your veterinarian will discuss the appropriate treatment strategy on your dog's individual situation."

Surgical removal is commonly recommended in cases where the bladder stones are too large for urohydropropulsion, when there are many stones in the bladder, if there is an increased risk that the patient will develop an obstruction of the urinary tract, or if the client wishes to have the problem resolved as quickly as possible. Male dogs are at a much higher risk of developing an obstruction in the urinary tract due to bladder stones, so when bladder stones are diagnosed in a male dog, surgical removal is often strongly recommended. Your veterinarian will discuss the appropriate treatment strategybased on your dog's individual situation.

Are there any other treatment options?

In some selected referral centers, ultrasonic dissolution may be available to treat bladder stones. With this technique, high frequency ultrasound waves are used to disrupt or break the stones into tiny particles that can then be flushed out of the bladder. It allows for immediate removal of the offending stones without the need for surgery. Your veterinarian will discuss this treatment option with you if it is available in your area.

My dog is not showing any signs. What will happen if I do nothing?

Sometimes, bladder stones are found incidentally when an X-ray is performed for another reason. In such cases, where only a few small bladder stones are present and the dog is not experiencing clinical signs, it might seem reasonable to do nothing. However, it can be very risky to adopt a "wait-and-see" approach, because calcium oxalate bladder stones are most diagnosed in male dogs, and male dogs are at an increased risk of urinary obstruction due to a small stone becoming lodged in the urethra.

If the patient cannot undergo surgical treatment or non-surgical stone removal, and you are willing to assume the risks, it may be acceptable to delay the treatment for a short while. During this time, the diet is often changed to one less likely to contribute to calcium oxalate stone formation. However, if there is any indication that your dog's condition is worsening, or if the dog develops a urinary obstruction, you must seek immediate veterinary attention.

How can I prevent my dog from developing calcium oxalate bladder stones in the future?

Dogs that have developed calcium oxalate bladder stones in the past will often be fed a therapeutic diet for life. Diets that promote less acidic, more dilute urine are recommended. Diets that may help include Royal Canin® Urinary SO, Purina® ProPlan® Veterinary Diet UR Ox™/St™, Hill's Prescription Diet® c/d® Multi-Benefit, or Rayne Clinical Nutrition Adult HealthRSS™.

Table food may be a problem for these dogs. Most dogs should be fed a canned or wet diet to encourage water consumption. Dilute urine with a concentration (urine specific gravity less than 1.020) is an important part of the prevention of calcium oxalate bladder stones. In certain cases, medications to increase the urinary pH, such as potassium citrate, may be required. Dogs that repeatedly develop calcium oxalate bladder stones without high blood calcium levels may benefit from a medication called hydrochlorothiazide.

"Dogs that have developed calcium oxalate bladder stones in the past will often be fed a therapeutic diet for life."

Dogs diagnosed with calcium oxalate stones should avoid calcium supplements unless specifically advised by your veterinarian. They should not be fed high oxalate foods such as nuts, rhubarb, beets, green beans, and spinach. Additionally, vitamin C supplements should be avoided.

The urine will be frequently monitored to detect problems early on. Bladder X-rays and urinalysis will typically be performed one month after treatment and then every three to six months for the remainder of the dog’s life. Dogs displaying any clinical signs should be evaluated immediately (e.g, frequent urination, urinating in unusual places, painful urination, or the presence of blood in the urine). Unfortunately, calcium oxalate stones have a somewhat high rate of recurrence, despite careful attention to diet and lifestyle.

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