Canine Uroliths (Urinary Stones)
Dog urine is a complex solution in which salts (e.g. calcium oxalate, magnesium ammonium phosphate) can remain in the solution under conditions of supersaturation. However, supersaturated urine has the potential to form solids from the dissolved salts. These solids are called crystals. If these crystals aggregate and are not excreted they form uroliths. The uroliths may damage the lining of the urinary tract resulting in inflammation of the tract. They can also predispose the dog to the development of urinary tract infection, and if they lodge in the ureters or urethra can obstruct urine flow.
Most uroliths are found in the bladder and urethra (with only 5% found in kidneys and ureters). The uroliths are normally named according to their mineral composition - approximately 50% are struvite (magnesium ammonium phosphate), 33% are calcium oxalate, 8% are urate, 1% are silicate, 1% are cystine and 7% are mixed.
How are Uroliths Formed?
Conditions contributing to the crystallisation of salts and formation of uroliths include a high concentration of salts in the urine, adequate time in the urinary tract, urine pH favourable for the salts to crystallise, a virus on which crystallisation can occur and reduced concentrations of crystal inhibitors in the urine. The combination of a high dietary intake of minerals and protein and the ability of dogs to produce highly concentrated urine contributes to the urine becoming supersaturated with salts. In some cases reduced kidney tubule resorption (i.e. calcium, cystine, uric acid) or increase production secondary to bacterial infection (e.g. phosphate and ammonium ions) also contribute to the urine becoming supersaturated.
What Signs are Seen When Uroliths are Present?
The signs seen depend on the number, type and location of the stones in the urinary tract. Most uroliths are located in the bladder and hence show signs of frequent urination, difficult urination and blood in the urine. Small uroliths may pass into the urethra in male dogs leading to partial or complete urinary tract obstruction with signs of distended bladder and straining to urinate.
Where the uroliths lodge in the kidneys, if only one kidney is affected there may be no signs or there may be blood in the urine and the dog may develop a chronic kidney infection. Where both kidneys are affected the dog may develop chronic kidney failure.
Dogs with urethral uroliths may also show no signs or may have blood in the urine and abdominal pain.
How are Uroliths Diagnosed?
Diagnosis is based on a combination of clinical history, clinical examination and X-rays or ultrasound. In males with difficult urination caused by urethral stones attempt to pass a urinary catheter is often met by a gritty feeling of resistance. Diagnosis can be confirmed using special X-ray techniques in such cases.
Stones in the bladder can sometimes be detected via palpation of the abdomen. Normal X-rays and ultrasounds are able to confirm the presence of these stones.
What are the Treatments for Canine Uroliths?
The principles in treating canine uroliths include relieving urethral obstruction and decompressing the bladder. This is achieved by passing a catheter along the urethra to the bladder, draining the bladder by syringing, dislodging the urethral stones by hydropusion or emergency surgery. The dog is placed on intravenous fluid therapy to restore water and electrolyte balance if dehydration is noted. The potassium and kidney enzymes levels should also be monitored and appropriate treatment must be given should any abnormalities be detected.
Medical breakdown of struvite, urate and cystine stones has been shown to be effective but the decision between surgical removal and medical breakdown of the stones is not always clear. The advantages of surgery include that the stones can be identified and any predisposing structural abnormalities can be corrected and urinary bladder lining samples can be collected for bacterial culture. Disadvantages include the need for anaesthetics, invasiveness of surgery and possibility of incomplete removal of the stones.
Medical treatment reduces the concentration of salts in the urine, increases salt solubility and urine volume. The major disadvantage of medical treatment of canine uroliths is that considerable owner compliance is required for weeks to months, and on a cost basis is comparable to the cost of surgery as multiple urine samples need to be analysed and cultured during the treatment period and radiographs may also be required.
Dogs with urinary obstruction cannot be treated medically, and some stones (calcium oxalate, calcium phosphate, silicate and mixed uroliths) do not respond to medical breakdown.
Once a dog has been treated for uroliths, either medically or surgically, preventive measures must be undertaken to remove the chances of recurrence. They include the induction of diuresis and eradication of urinary tract infection. Maintaining a urine specific gravity of below 1.020 is ideal and dogs should be offered frequent opportunities to urinate. The urine sediments and pH must be checked and urinary tract infection treated promptly once bacterial culture results are available.
Specific Treatment for Struvite Uroliths
Struvite uroliths can be dissolved using Hill's Canine S/D Prescription Diet as it contains greatly restricted protein, calcium and magnesium content, high salt content and it results in the production of acidic urine. The diet needs to be fed for a minimum of 30 days after the stones are no longer visible on X-ray, with the average treatment period being 8 to 10 weeks. The rate at which the stones dissolve is proportionate to the surface area of the stone exposed to the unsaturated urine.
In adddition to Hill's Canine S/D Prescription Diet, any bacterial urinary tract infection must be eliminated using antibotics. The antibiotics must be continued throughout the period of stone breakdown to destroy bacteria that may be liberated from the stone as it dissolves. The antibiotics selected should be based on urine culture findings.
Measures to prevent struvite uroliths recurrence include preventing and controlling urinary tract infection, maintaining acidic urine and reducing the intake of stone forming salts. Hill's Canine C/D Prescription Diet is able to fulfil the requirements. The diet contains moderately restricted protein, magnesium, calcium and phosphorus and produces an acidic urine.
Half a gram of salt should be supplemented daily to increase water consumption and urine production. The urine must also be assessed every 2 to 4 months routinely to ensure there are no crystals, and culture performed in dogs with signs of urinary tract inflammation.
(Hill's Canine S/D Prescription Diet should not be fed routinely as a maintenance diet, and not be given to pregnant, lactating or growing dogs or after surgery. It should not be fed to dogs with congestive heart failure or kidney disease.)
Specific Treatment for Calcium Oxalate Uroliths
There is no medical treatment available to break down calcium oxalate uroliths. After surgical removal a moderate restriction of protein, calcium, oxalate and sodium together with normal intake of phosphorus, magnesium and vitamins is advisable to prevent recurrence. This can be achieved by feeding the dog with Hill's Canine U/D Prescription Diet. Potassium citrate at recommended doses given orally may also help to prevent recurrence.
Specific Treatment for Urate Uroliths
Where urate uroliths are not associated with liver insufficiency, the dog should be fed with Hill's Canine U/D Prescription Diet as it contains reduced protein and purine and produces alkaline urine. Similar to the S/D diet, the U/D diet reduces the liver formation of urea and hence the urine concentrating ability. However, recommended doses of salt must be supplemented daily to increase water consumption and urine production as the U/D diet contains a restricted salt content. Sodium bicarbonate or potassium citrate is sometimes necessary to be added to the diet to maintain a neutral urine as well.
As with management of struvite uroliths any urinary tract infection needs to be treated accordingly, as urease producing bacteria will increase the concentration of urine and subsequently potentiate urine crystal production.
In cases where the stones are secondary to severe liver insufficiency the underlying problem needs to be corrected where possible, and in some cases the stones may spontaneously dissolve.
Specific Treatment for Silicate Uroliths
Medical breakdown of silicate uroliths is also unavailable and they are often required to be removed surgically. It is advisable to feed the dog with Hill's Canine U/D Prescription Diet after surgery as it contains low quantities of silicates and produces alkaline urine. This helps to prevent recurrence. Salt can also be added (half gram daily) to assist in increasing urine volume.
Specific Treatment for Cystine Uroliths
Cystine uroliths can be broken down medically. It is recommended to feed the affected dog with Hill's Canine U/D Prescription Diet to dissolve the uroliths and as a preventive measure. Sometimes 'thiol containing' drugs are also used as they increase the solubility of cystine in the urine. Long term urinary pH must be maintained at approximately 2 to 5 and it can be achieved by feeding an appropriate amount of potassium citrate.
Whenever medical breakdown of uroliths is attempted the dog must be examined by a veterinarian at least monthly and a complete urine analysis must be carried out as well as X-rays or ultrasound to assess the size of the urolith. If urine analysis is suggestive of a urinary tract infection the urine must be cultured and appropriate antibiotic treatment administered. Where the urolith has not reduced in size after two months of treatment owner compliance, control of infection and the type of urolith should be assessed and surgical removal should be considered.
Uroliths occur in up to 25% of dogs and they can have up to 3 or more episodes during their life. Recurrence is more likely in dogs with metabolic uroliths (i.e. calcium oxalate, urate and cystine uroliths) or where there is a breed predisposition. Appropriate preventive measures and frequent re-evaluation are important in such dogs.