Calcium Oxalate Bladder Stones in Dogs

By Tammy Hunter, DVM; Ernest Ward, DVM

My dog has calcium oxalate bladder stones. What are they?

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 to form?

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 will 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 and is acidic predisposes a pet to developing calcium oxalate urinary crystals and stones. Recent studies have shown diets that cause high urine acidity (urine pH less than 6.5) may predispose dogs to develop this type of bladder stone.

There are likely other causes of calcium oxalate bladder stones. Over-usage of antibiotics may reduce numbers of the intestinal bacteria Oxalobacter formigenes whose sole nutrient is oxalate. In dogs with low populations of Oxalobacter, excess oxalate is secreted in the urine, increasing the likelihood that calcium oxalate crystals and stones can form if the urine is highly concentrated or becomes acidic.

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. Based on the results of tens of thousands of stone analyses, it has been found that the number of struvite bladder stones has been declining in dogs, while the number of calcium oxalate stones has been increasing during the past ten years. Struvite uroliths were noted to be more common in female dogs and calcium oxalate uroliths in male dogs. Breeds most commonly diagnosed with struvite and calcium oxalate bladder stones included 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 very similar to the signs of an uncomplicated bladder infection or cystitis. The most common signs that a dog has bladder stones are hematuria (blood in the urine) and dysuria (straining to urinate). Hematuria occurs because the stones rub against the bladder wall, irritating and damaging the tissue and causing bleeding. Dysuria 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 neck of the bladder, 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. A complete obstruction is potentially life threatening and requires immediate emergency treatment.

How are calcium oxalate bladder stones diagnosed?

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

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

These imaging procedures will identify the presence of a bladder stone, but will not definitively tell your veterinarian the composition of the stone. 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 that is present, based on the radiographic appearance and results of a urinalysis. For example, if X-rays show that there are one or more stones present in the bladder, and the results of the urinalysis show 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.

How are calcium oxalate bladder stones treated?

There are two primary treatment strategies for treating calcium oxalate bladder stones in dogs: non-surgical removal by urohydropropulsion, and 2) surgical removal.

Small stones may be removed non-surgically in some cases by urohydropropulsion. In simplest terms, the bladder stones are flushed out of the bladder using special urinary catheter technique. This is only possible when the stones are very small in diameter. In some cases, this procedure may be performed with the dog under heavy sedation, although general anesthesia is often necessary. If your veterinarian has a cystoscope, small stones in the bladder or urethra can sometimes be removed with this instrument, therefore avoiding a surgical procedure to open the bladder.

Either of these non-surgical procedures may also be used to obtain a sample stone for analysis so that your veterinarian can determine if dietary dissolution is possible.

Surgical removal is commonly recommended in cases where the bladder stones are too large for urohydropropulsion, when there are a large number of 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 as a result of bladder stones, so when bladder stones are diagnosed in a male dog, your veterinarian will often strongly recommend surgical removal. Your veterinarian will discuss the appropriate treatment strategy for your dog, based on your dog's individual situation.

 Are there any other treatment options?

In some selected referral centers, another option may be available to treat bladder stones. This option is ultrasonic dissolution, a technique in which 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 has the advantage of 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?

In cases where only a few small bladder stones are present and the dog is not experiencing clinical signs (painful or frequent urination, blood in the urine, etc.), it might seem reasonable to do nothing. The most common scenario for this situation is when bladder stones are found as an 'incidental' finding when an X-ray is performed for another reason. Since calcium oxalate bladder stones are most commonly 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, it can be extremely risky to adopt a "wait-and- see approach". However, if for some reason 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 and 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 Health-RSS™. 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 low urine specific gravity (urine specific gravity or USpG less than 1.020) is an important part of the prevention of calcium oxalate bladder stones. In certain cases, medications to lower 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 hydrochlorothiazide treatment.

"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.

In addition, careful routine monitoring of the urine to detect any signs of bacterial infection is also recommended. 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 such as frequent urination, urinating in unusual places, painful urination or the presence of blood in the urine should be evaluated immediately. Unfortunately, calcium oxalate stones have a somewhat high rate of recurrence, despite careful attention to diet and lifestyle.

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