Ron Hines DVM PhD
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Our pets suffer from kidney and bladder stones (calculi or uroliths) very similar to those found in humans. Most of these stones are composed of the mineral salts of common elements found in our bodies, calcium, magnesium, ammonia, phosphorus and carbonates. Their composition and consistency are similar to that of limestone.
What Causes Bladder And Kidney Stones?
Several things contribute to the formation of urinary tract stones. If the concentration of mineral salts are too high in the urine, they precipitate out, layer upon layer. This can happen in the kidneys or farther down the tract in the bladder. The urine of dogs and cats should be naturally acidic. Most mineral salts are less soluble in alkaline urine so any factors that make the urine more basic or alkaline contribute to stone formation.
Many bacteria decompose the urea in urine into ammonia and in doing so, alkalinize the urine. Like pearls, mineral salts come out of solution easier if they have small objects to attach to. That is why the debris of urinary tract infections often serve as foci for forming stones.
Certain breeds and lines of dogs are susceptible to stones of different compositions. These stones are formed because the salts of amino acids and urates that occur in the blood of these dogs is too high in concentration. The best-known example of this are the ammonium urate stone formed by Dalmatians.
To this day we do not know why some dogs form bladder and kidney stones while others do not. Dehydration increases the concentration of minerals within the urine and can increase stone formation. Female animals of all species are more susceptible to urinary tract infections and subsequent stones due to their shorter urethra. Other dogs and cats are born with mucosal immunity defects that leave them more susceptible to urinary tract infection.
What Are The Signs Of Bladder And Kidney Stones?
The two most common signs of bladder stones are blood in the urine and painful urination. Blood in the urine or hematuria occurs when stones irritate the bladder’s sensitive lining causing bleeding. The inflamed bladder lining is quite painful as are the small sand-like stones that pass out with the urine. Pets with bladder stones attempt to urinate much too frequently. They will squat and strain with no apparent success. The urine may be red or port wine in color. When they do urinate, the quantity of urine is small. In between urination these pets are restless. They walk with their loins down in a crouched position. Owners may misinterpret these signs and assume the pet is constipated. When I palpate the abdomen of these animals I can often feel small stones in the bladder grinding together like marbles. Large solitary stones in the bladder are hard for me to miss.
If a veterinarian does not attend to this problem immediately, one of the stones may block the urethra, the tube leading from the bladder to the penis or vagina. When this happens, urine backs up into the body causing uremia, depression and vomiting. The bladder stretches to several times it normal size and may even burst. It may take weeks after the problem is relieved for the bladder’s tone and size to return to normal.
Blood taken from obstructed animals has elevated urea and creatinine and its ionic balance is disrupted. Despite these changes, dogs and cats with stones do not run high fevers. Analysis of the urine of these pets usually finds blood and white blood cells. Bacteria may also be present in the urine. Any urinary tract infection that reoccurs frequently in your pet should be checked carefully for the presence of bladder or kidney stones. These stones are porous and bacteria reside within them where antibiotics and the body’s immune system cannot easily reach them. That is why infections frequently reoccur.
Somewhat less common are stones that form in the kidneys. These stones can cause sudden colic and intermittent bloody urine or may occur without any visible symptoms. Occasionally, stones will leave the kidney plugging the ureters, the tubes that lead to the bladder. This event is marked by severe pain, agitation and straining until the stone has passed or been surgically removed.
administering enemas, I X-ray all patients that might have kidney,
bladder or urethral stones. Most of these stones are quite visible
on x-rays. Occasional the stones contain more organic material than
mineral and are difficult to visualize. If I suspect this form of
stone, I fill the bladder with air or dye before the x-ray is taken.
Ultrasound will also detect these radio-lucent stones.
When bladder or kidney stones are large and unlikely to plug the ureters or the urethra, we may attempt to manage them medically. To do this, we must first have an idea what the stones are made of. Struvite (magnesium ammonium phosphate or triple phosphate) is the most common of all urinary tract stones. Schnauzers and dachshunds seem particularly susceptible to struvite stones. Struvite crystals have a characteristic shape in the urine. Specialty diets are commercially available and can be made at home that encourage stones to dissolve. These diets have reduced protein so less ammonia is formed in the urine. They also contain less magnesium and phosphorus, the building blocks of struvite. Since struvite dissolves in acid urine, these diets contain ingredients to keep the urine acidic. A common trade name for such a diet is S/d. There are many competing brands. A formula for a similar home made diet is given in another article in this series. I generally keep these cases on trimethoprim/sulfa or another urinary antibiotic during this period. I usually give these diets three to five months to work. If the stones have not dissolved completely by then I remove them surgically. Dogs should not be left on this ultra low-protein food for more than five or six months. Cats can stay on their diets longer. Pets that will not eat the commercial diet will often eat the home cooked one.
Urate urinary stone can also sometimes be managed medically. To dissolve these stones we feed diets low in purines with restricted protein. We also administer a drug, allopurinol that reduces the amount of uric acid the body produces.
Certain stones such as calcium oxalate and all those that did not dissolve through medical treatment are removed surgically (cystotomy). Sometimes I choose surgery to end the problem quickly. In female dogs I have been able to reach up into the bladder and crush small stones with a medical instrument called a biopsy punch. This approach requires the use of a fluoroscope. But in most cases, I open the bladder through an incision in its thickest part and manually scoop out the stones. I pass a catheter out the urethra to be sure no stones block it. When a stone is present in the urethra or in the ureter, I move it back into the bladder or kidney and remove it from there. This is so neither the urethra nor the ureters are scared ,constricted and obstruct urine flow. When I am certain all the stones have been removed I flush the bladder with dilute acetic acid and antibiotics and sew it shut like a football. I am very careful that none of my sutures actually protrude on the inner surface of the bladder because they can form the basis for a new stone. To be sure the bladder does not leak, I suture a patch of the veil-like omentum over the bladder incision. To avoid pressure on the bladder incision, I leave a catheter in the bladder for the next two days so that urine does not expand and inflate it.
How Can I Prevent The Reoccurrence Of Stones?
Stones should be sent to a laboratory for analysis. All stones are less likely to form if your pet has free access to water and frequent opportunities to urinate. Stones are less likely in dilute urine.
Eliminating bacterial urinary tract infections and checking (bacterial culture and sensitivity) the urine of dogs prone to them twice a year helps prevent stones that occur due to the alkaline urine infections cause.
Adding additional salt to your pets water or food will not encourage more drinking or make urinary tract stones less likely to reoccur. Contrary to what we once thought, salty foods do not encourage thirst - they do the opposite. (ref1, ref2, email1)
Diets high in grain and vegetables produce alkaline urine, which allows certain stones to form. This is one of many reasons to see to it that your pet eats a commercially prepared diet.
Calcium Oxalate stones seem to be getting quite common and are very frustrating to treat. They differ from struvite stones in that they form in acidic urine (see another article on this website). Unfortunately, they are harder to prevent through dietary manipulation than struvite or urate calculi (stones). Because of this, these stones must be removed surgically and they often reoccur within a few years. They are much more common in male dogs (75%) and Burmese, Himalayan and Persian cats. The most common dog breeds affected are Yorkshire terriers, poodles, shih tzus, schnauzers, lhasa apsos and bichon frises. These stones are associated with excess calcium in the blood and urine. Sometimes these stones are the result of other diseases such as Cushing’s disease or the excessive use of corticosteroid medications.
Many mineral salts are excreted in urine at concentrations at which they would normally fall out of solution and form granules or stones. One substance, produced by the kidneys, called nephrocalcin (an acidic glycoprotein) inhibits stone formation. When too little of this chemical is produced, stones may result.
Some diets recommended for dogs that produce oxalate stones are CNM's (Purina) NF diet, Hills U/D and K/D diets and Waltham's Low Protein Diet. If at all possible feed the canned form of this diet rather than the dry form because dogs rarely drink enough water after eating dry kibble to make up for the water consumed in canned foods. Just be especially sure to brush you dog's teeth when feeding canned diets. Do not feed human table foods in any apreciable quantity.
Life is a trade-off of risks. The drug, furosemide (Lasix) used to treat heart disease in dogs and cats may increase the likelyhood of oxylate stone formation. The same regards corticosteroid drugs such as prednisone and prednisolone and vitamins C and D.
Supplementing your pet's diet with oral potassium citrate (20-37.5mg/pound body weight) given twice a day is thought to lessen the incidence of calcium oxalate stone formation.
If urine specific gravity does not consistently approach 1.025 or less, providing the pet with a thiazide diuretic will increase thirst and produce a diluted urine (hydrochlorothiazide @ 1-2mg/pound body weight twice a day). A blood electrolyte analysis should be conducted several times during the first few months on this medication.
have a propensity for forming urate stones. Male, older dogs are
most often affected but I have seen it in litters of Dalmatian puppies.
These stones are also found in Yorkshire terriers and English bulldogs.