Stones in the urinary tract are common in dogs and cats. Even though dogs and cats do get kidney stones, it is bladder stones that cause more problems. The medical terms for bladder stones are urolithiasis or cystic calculi. We will use stone, calculi, and urolith synonymously in this page. The two most common calculi are calcium oxalate and struvite.
Stones can also occur in the kidneys, where they are called nephroliths. This page will limit its discussion to stones in the bladder.
There are several factor, usually working in combination that lead to urolith formation:
- Mineral Crystals
Urine that is saturated with excess amount of certain minerals is prone to form bladder stones. These minerals commonly include magnesium, phosphorous, calcium, and ammonia. Most stones consist of an organic matrix of protein surrounded by crystalline minerals.
Diet can have a major impact here, and is one of the primary methods we use to treat and prevent uroliths.
- Urine pH
pH is an indicator of acidity by measuring the hydrogen ion concentration. A pH of 7 is neutral. Below 7 is acidic, above 7 is alkaline. As a general rule, dog and cats tend to have an acidic urine. Some uroliths have a propensity to form in acidic urine, while others tend to form in alkaline urine. Urine pH needs to be measure immediately upon voiding from the bladder for it to be accurate.
Some uroliths form due to the presence of bacteria in the urine, so control of these bacteria is important. Bacteria are diagnosed by culturing the urine during a sterile urinalysis, or by culturing the inside of a stone after surgical removal. Normal urine is sterile, so any bacteria cultured from the urine is abnormal. When bacteria are cultured a pet has a urinary tract infection (UTI) and needs antibiotics. Common bacteria in UTI’s include E. coli, Staph. spp., and Proteus.
If any bacteria are cultured in the urine our laboratory will test numerous antibiotics to determine which are the best ones to eliminate the bacteria. This is called sensitivity testing. We also do a test called MIC (Minimum Inhibitory Concentration) to let us know the best antibiotic to use and the best dose of that antibiotic.
Liver shunts are an abnormality of blood flow to the liver. Blood that would normally flow through the liver now bypasses the liver. One of the many consequences of this disease, called Portosystemic shunts (PSS), is the formation of ammonium urate bladder stones.
Medications can predispose pets to forming bladder stones. Sometimes they do this by increasing the calcium level in the urine. Medications that increase or decrease the pH of the urine can also set the stage for stone formation. Some medications can actually cause formation of stones when used for long periods of time. The following list includes some of these medications:
Predisposing causes of bladder stones include pets that are not drinking enough or are not allowed to urinate frequently. Bacteria and stone forming chemicals stagnate in the urinary bladder and increase the chance of a stone forming. Mechanical flushing of the bladder, in the form of normal and frequent urination, will prevent this.
Always make sure your pet has access to fresh water, changed several times per day, and the ability to urinate frequently. As a matter of fact, if you feed dry food you should be giving more than one cup of water per cup of dry food. An easy way to get around this important requirement for water is not to feed dry food at all. Your pet’s urine should be clear, with no odor or color, and your pet should urinate every few hours. Sometimes these common sense suggestions are so obvious that we tend to forget about their importance.
Typical symptoms of bladder stones:
- Straining to urinate (stranguria)
- Blood in the urine (hematuria)
- Urinating small amounts frequently (pollakiuria)
- Often times there are no symptoms at all, and the problem is discovered on routine abdominal palpation or radiography.
There might also be excess urination (polyuria), pain in the rear quarters, reluctance to jump or play, or even lethargy and a poor appetite. Some pets can have bladder stones without any apparent symptoms at all!
The bladder stones can pass out of the bladder and lodge in the urethra, especially in male dogs due to the smaller diameter of their urethra. In some cases they can block the flow of urine, which is a medical emergency. This can cause problems with the kidneys, leading to the buildup of toxic waste products. Common areas for these urethral obstructions are the ischial arch near the pelvis and further down the urethra at the os penis. Both of these areas have small diameter urethras that do not expand to allow passage of a stone. You will learn more about this later.
Urine analysis Culture Radiography Ultrasound
There are many different types of bladder stones. Some of the most common ones include:
This urolith is diagnosed in almost half the cases of bladder stones. It is also called MAP (Magnesium-ammonium-Phosphate), or triple phosphate. Struvite uroliths come in many different shapes and sizes, are radiodense, and form in an alkaline urine.
It is the predominant stone type in female dogs of all breeds except Dalmatians. This is because of the high association of struvite crystals with urinary tract infections, which are more common in female dogs. If a female dog gets a bladder stone, 80% of the time it is struvite because of this association. Dogs under a year of age, no matter what sex, get struvite as the most common bladder stone for this same reason. Miniature Schnauzers are the most commonly affected breed, along with bichon frise, cocker spaniels, and miniature poodles.
Canine urine is commonly supersaturated with struvite crystals due to the high protein and mineral content of current diets. This excess of these crystals in the urine, stored in the urinary bladder, combines with bacteria to set the stage from the formation of bladder stones (uroliths). Bacteria that are commonly implicated include Staphlycoccus, Enterococcus, and Mycoplasma. These bacteria increase the amount of organic debris available for crystallization.
These bacteria also produce urease as a by product of their metabolism. They split the urea in urine into ammonia and carbon dioxide. As this ammonia is broken down it changes the pH of the urine from its usual acid (pH less than 7) to alkaline (PH > than 7). High urine ammonia concentrations in the urinary bladder can damage the glycosaminoglycans that help prevent the bacteria from adhering to the bladder mucosa (interior lining of the bladder). When we treat for this disease we address these issues.
What does all of this mean?
Excess struvite crystals in the urine set the stage for the formation of the urolith.
Urinary Tract Infection (UTI)- some bacteria produce a byproduct called urease. Urease will increase the pH of the urine and promote ammonium in the urine.
The formation of an alkaline (pH greater that 7.0) urine from the diet, or from urease producing bacteria, will cause the struvite crystals to precipitate out of solution and begin the formation of a urolith.
Urine that stays in the bladder longer than usual gives the struvite crystals further opportunity to precipitate out of solution and form a urolith.
- Cats commonly form struvite bladder stones in the absence of a urinary tract infection. This is probably because cat urine is more concentrated (higher specific gravity) than dogs, so the urine is more saturated (we call this supersaturation) with magnesium, ammonium, and phosphate. This is especially true when the urine pH is highly alkaline, which can occur with the use of some drugs, in certain diets, and when the tubules of the kidney are diseases. If there is a bacteria involved it is usually Staphlycoccus.
This is the second most prevalent type of urolith after struvite, making up around 30-50% of the uroliths we diagnose, especially in male dogs of all species, except Dalmatians. They come in two versions; the monohydrate and the dihydrate. Sometimes the two are found together, sometimes they are found with other uroliths like calcium phosphate, struvite, or ammonia irate. They show up well on a radiograph. They form in an acidic or neutral urine (pH is 7 or just slightly less). Recurrence is common, so diligence on your part is needed to prevent recurrence.
Feeding an older pet with Kidney problems a food like Hills Prescription Diet K/D may slow or prevent further growth.
If this stone is small and not causing any apparent problem it can be monitored. Some pets will have high calcium (hypercalcemia) in the bloodstream.
Several predisposing factors work together to increase the chance of this urolith forming:
Increased calcium in the bloodstream (hypercalcemia)
Increased calcium in the urine (hypercalciuria) with no hypercalcemia
Concurrent Cushing’s disease
These tend to occur in neutered middle aged cats and older cats, especially Burmese, Himalayan, and Persian breeds. One of the predisposing conditions might be an increased calcium level in the bloodstream. These uroliths tend to form in an acidic urine.
This is a problem most commonly in older male dogs. It is suspected that there is a correlation with hormone changes that occur as a pet ages. Several breeds are prone:
Urate and Ammonium Urate
This is a common urolith found in Dalmatians around middle age, with males affected much more often than females. This is because they metabolize protein differently in the liver, with the end result being uric acid buildup in the urine. In addition to their high prevalence in Dalmatians, they are found in Bulldogs and Yorkshire terriers.
There does not seem to be a connection with a urinary tract infection, and they tend to form in an acidic urine.
Urate stones are radiolucent. If they get large or covered with other minerals they might become radiopaque. Urate calculi tend to be small and occur as several stones. These stones usually form in the bladder, and when passed through the urethra, can become lodged.
Most cases of struvite are treated surgically. The procedure is explained below.
Struvite bladder stones can also be be treated medically with a food called Hills S/D. The mechanism involves creating a urine that is undersaturated with the crystals that caused the struvite urolith to form in the first place. This undersaturation literally cause the urolith to dissolve in the urine, and then get urinated out. It is a slow and gradual process, although most pets get relief from straining and blood in the urine soon after starting this diet.
S/D has several modifications in its ingredients to set up this undersaturated urine. Its reduced in protein, so there is less ammonia buildup in the bladder from bacteria. Magnesium and phosphorous are restricted also. With less contents of the minerals that form the struvite urolith (magnesium, ammonia, and phosphorous- MAP) the urolith starts dissolving.
S/D also has an increased amount of salt (sodium chloride). This promotes drinking and urination and literally helps flush the struvite crystals out of the bladder. It also changes the pH to a more acidic state, which further makes the struvite stone dissolve.
The rate of dissolution is proportional to the surface area of the stone expose to this now undersaturated urine and the control of the urinary tract infection in dogs. Feline struvite stones tend to dissolve faster than canine because bacteria are not implicated.
We can measure the size of a stone with our digital x-ray to make sure it is getting smaller early in the process of dissolving it
S/D must be the only food fed for it to work. We can monitor whether or not an owner is doing this by looking at the pH of the urine along with the specific gravity of the urine. also, the BUN (blood urea nitrogen) of a pet on S/D should be lower than normal.
We have clues from other diagnostic tests to help decide if a urolith found in the urinary bladder on a radiograph is truly struvite. The urinalysis gives us an idea of the composition of the urolith by looking at the crystals in the sediment. The pH of the urine will be alkaline. Also, the presence of bacteria on a culture in a breed that is prone to struvite uroliths is also a strong indication.
If bacteria are found on a urine culture in dogs then antibiotics must be used simultaneously while a pet is on S/D. It must be the correct antibiotic, so the importance of the urine culture is obvious. Both S/D and antibiotics are used for one month after the stone is no longer visible on a radiograph. If there is a persistent urinary tract infection we might use a urease inhibitor called acetohydroxamic acid. Urinary acidifiers are usually not used.
If the urolith is still present after 2 months of S/D and antibiotics, then surgery should be performed. Most pets need to be fed S/D for 4-6 months for complete resolution. It should be fed for an addition 30 days after there is no longer any stone based on a radiograph.
S/D is restricted in protein, so it is not a complete diet for long term use in dogs. It also should not be used pregnant dogs, lactating dogs, ground dogs, and after surgery due to this protein restriction. While on S/D your dog’s blood should be monitored to ensure there are no side effects of the restricted protein.
S/D diet tends to work faster in cats because bacteria are not usually involved. S/D should also be fed for 30 days after there is no more evidence of a stone in the bladder when radiographed. Feline S/D is not protein restricted, so it can be used for the rest of your cat’s life.
The only treatment for these uroliths is surgical removal. This is the easy part, its preventing their recurrence that is difficult. The protocol in how to do this was shown earlier.
If there is no PSS these uroliths can sometimes be handled medically also. A food called Hills U/D should be used. It is low in purines and has restricted protein. A medication called allopurinol is given which helps minimize the amount of uric acid produced in the urine. Potassium citrate is also used to make the urine less acidic, although this might occur with the use of U/D alone. Adding salt to the diet or mixing water with the food will also promote urination of the urate crystals. Recurrence is common, so this diet needs to be used for the life of your pet, especially in Dalmatians. We will talk more about this later in the prevention section.
Many bladder stones are treated surgically.
Medical dissolution or surgical removal of the bladder stone is the first step in the process. The next, and just as important step, is the prevention of the stone’s recurrence. Some of these stones will require a diet change only, while others might require long term medication. Some breeds are predisposed to forming stones in spite of what we do to prevent them. Our doctors will set up a specific protocol for your pet based on the breed and type of stone removed.
Long term follow up is important. Your pet will have to return periodically to recheck a urinalysis, culture the urine for bacteria, and x-ray the bladder. Many stones recur because owners forget the importance of long term prevention.
At all times make sure your pet has access to fresh water and the ability to go to the bathroom. This treatment alone will go a long way to prevent recurrence. Giving your pet food that has more moisture will increase the flow of water through the urinary system and minimize crystal formation in the bladder. As a general rule, we do not recommend adding salt (sodium chloride) to the food to get your pet to drink more water because it might promote calcium formation in the urine, thus setting the stage for a stone to form. Your doctor will let you know if this applies in your case.
Pets that have struvite urolithiasis, whether treated surgically or medically, have a chance of recurrence if not monitored carefully. If the urine is alkaline on a urinalysis it should be cultured to check for bacteria. Antibiotics are indicated if the culture is positive. If the pH stays alkaline in spite of antibiotic therapy, then Hills C/D should be used. It will help maintain an acidic urine, and has some mild restrictions in protein and minerals. As a general rule, we recommend C/D for all pets that have had struvite crystals. It can be used on a long term basis, and is helpful in preventing FLUTD in cats. S/D can also be used long term in cats.
If your dog gets recurring urinary tract infections, thus predisposing it to a struvite bladder stone, it is important to examine it for abnormal anatomy of the vulva, for obesity, and problems with the urethral sphincter.
Recurrence rates with this urolith are high, so a long term plan and commitment to sticking with it are important.
Baseline data consisting or radiographs, urinalysis, urine culture, and blood panel needs to be obtained. Particular attention is paid to the blood calcium level.
A diet with reduced quantities of protein, calcium, and sodium, that also does not make an acidic urine, is the ideal diet to feed. Too much of a reduction in these nutrients can be detrimental, so no supplements should be added. The primary diet in the cat that accomplishes this ideal is Hills Prescription Diet K/D. In dogs, K/D, W/D, and U/D are used. The use of U/D might preclude the use of potassium citrate as long as there are not calcium oxalate crystals in the urine and the pH is alkaline. There is a C/D that treats calcium oxalate uroliths in cats.
Certain foods that are high in oxalates or calcium, or foods that increase calcium levels, must be avoided. They include high protein foods, spinach, rhubarb, parsley, milk products, and table salt.
When we diagnose a calcium oxalate stone we will check the calcium level in the bloodstream to make sure it is not abnormally high. This normal calcium level was from the dog above that had the surgery to remove the calcium oxalate stones from its bladder and urethra.
Vitamin C and D, along with drugs that make the urine more acidic, should not be added to the diet. Cortisone also should be used cautiously because it increases the calcium level in the urine.
Potassium citrate will inhibit calcium oxalate crystal formation and keep the pH of the urine in the ideal range. We will use this if U/D does not keep the urine alkaline.
After surgery your pet should return in 2 weeks for a urinalysis. If there are calcium oxalate crystals in the urine we will add potassium citrate as per above. If there are no crystals we will not need to do a recheck for 3 months. At this 3 month recheck of the urine we will also re-radiograph the bladder and check a blood panel noting the calcium level.
Two weeks after surgery this is the urine sample of the dog with the above stones. This is what we want to see- no crystals, no bacteria, and an alkaline pH.
Ammonium Urate and Urate – Medical care for Dalmatians after surgery includes:
- Increasing water consumption so the specific gravity of the urine is near 1.018. adding water to dry food, feeding more canned food, and adding encapsulated light salt (KCl) all will help.
- Feeding a diet low in purines. Diets low in purines include the Hills Prescription Diets K/D or U/D dry and U/D canned.
- Increasing the pH of the urine to 7.0-7.5. The above diets might accomplish this pH. If not, adding oral potassium citrate to the diet will help.
- Controlling urinary tract infections, if any, by performing urine culture and sensitivity tests every 3-6 months, and using antibiotics on a long term basis if needed.
- Adding allopurinol to the diet to prevent the conversion of purine to uric acid.This therapy needs to be followed diligently for the life of your Dalmatian.
Medical care for cats and non-Dalmatian dogs after surgery is similar. Most of these uroliths are caused by a PSS, thus surgery to correct the liver problem will help prevent the recurrence of these stones in the bladder. In some PSS’s the liver problem cannot be completely resolved, adding to the risk of formation of these bladder stones in the future. Long term therapy is similar to Dalmatians:
- Increasing water consumption so the specific gravity of the urine is near 1.018 for dogs and 1.025 for cats. adding water to dry food, feeding more canned food, and adding encapsulated light salt (KCl) all will help.
- Feeding a diet low in purines for dogs. Diets low in purines include the Hills Prescription Diets K/D or U/D dry and U/D canned. For cats feed diets that do not acidify the urine, which unfortunately, many do to prevent FLUTD.
- Increasing the pH of the urine to 7.0-7.5. The above diets might accomplish this pH. If not, adding oral potassium citrate to the diet will help.
- Controlling urinary tract infections by performing urine culture and sensitivity tests every 3-6 months, and using antibiotics on a long term basis if needed.
Credit to Long beach animal hospital www.lbah.com