Certain types of urinary tract stones (or uroliths) are relatively common in pets, especially bladder stones in dogs. To prevent complications to the urinary tract, urinary stones must be promptly diagnosed and treated by your vets. Some stone types may require surgery will others can be dissolved with a prescription diet.
Keep reading to learn the signs that your dog or cat may have a urinary stone and how stones are diagnosed, treated, and prevented.
What causes urinary stones to develop?
The kidneys help rid the body of excess minerals and other products, such as magnesium, phosphorus, calcium, and certain proteins, by filtering them out of the bloodstream and into urine, where they are to be expelled by the body via urination. If urine becomes excessively saturated with these minerals, mineral precipitation can occur, and crystals can develop. If this process continues, particularly in very concentrated urine (such as in dehydrated animals), the crystals can precipitate further and form a solid stone, known as a urolith. The presence of a nidus (or structure upon which these mineralized crystals attach) can cause the crystals to accumulate very quickly with a more rapid formation of a urolith. Bacterial infections can serve as a nidus for the formation of some stones (i.e. struvite stones in dogs).
Ensuring adequate hydration (minerals will have a harder time accumulating in dilute urine), preventing excessive mineral and protein content from entering the urinary tract, and treating urinary tract infections are all essential to help reduce the frequency of urinary stones.
Definitions
- Urinary tract: consists of the upper urinary tract (kidneys and ureters) and lower urinary tract (urinary bladder and urethra)
- Urolithiasis: the formation of urinary tract stones
- Urolith: a stone anywhere in the urinary tract
- Calculus or urolith: single stone
- Calculi or uroliths: multiple stones
- Nephroliths: stones in the kidney
- Ureteroliths: stones in a ureter (the two narrow tubes connecting each kidney to the bladder)
- Urocystoliths: stones in the urinary bladder (not to be confused with cholecystoliths, which are stones in the gallbladder)
- Urethroliths: stones in the urethra (the short tube connecting the bladder to the outside world)
Clinical Signs of Urinary Stones
Signs of uroliths of the lower urinary tract (bladder and urethra) may be mistaken for those of a urinary tract infection (UTI):
- Increased urge to urinate small amounts of urine (pollakiuria)
- Pain or vocalizing when attempting to urinate (stranguria)
- Straining to urinate
- Bloody urine (hematuria)
- You may also observe grit or a small stone expelled from your pet in his or her urine (spontaneous voiding).
Signs of uroliths of the upper urinary tract (kidneys and ureters) may be more difficult to observe. The following may occur with nephroliths or ureteroliths:
- Blood in the urine (hematuria),
- Abdominal discomfort/pain (your pet may whine or yelp when picked up and experience general malaise with reduced appetite)
- Signs of secondary renal failure (increased thirst and urination, vomiting, decreased appetite, weight loss, etc.) may occur.
Complications of Urinary Stones
If urinary stones are left untreated, the following complications may arise:
- Persistent or recurrent urinary tract infections (UTI): While only struvite-type stones in dogs are caused by a pre-existing UTI, all bladder stones can irritate the bladder lining and lead to the development of a secondary UTI. Severe infections can cause sepsis, which is life-threatening.
- Urinary blockage – a medical emergency!: If your pet is straining or crying upon urination, especially if you do not observe a normal flow or urine, take your pet to a vet ASAP. If untreated, complications include: cardiac disturbances due to electrolyte imbalance, a ruptured bladder, secondary renal failure.
- Complications of nephroliths and ureteroliths: Uroliths more commonly affect the bladder and urethra. However, nephroliths an ureteroliths can block the flow of urine, causing it to back up into the kidneys where pressure and toxins cause renal damage. Chronic renal failure can result, especially in cats. Over half of cats that develop chronic renal failure secondary to uroliths of the upper urinary tract do not recover from renal failure even after the stones are resolved.
Diagnosis of Urolithiasis
Your veterinarian will be suspicious of urinary stones based on your description of your dog or cat’s clinical signs at home. To confirm diagnosis, your vet will perform the following diagnostic tests:
Physical exam- Urinalysis
- Helps identify blood in urine, evidence of infection, and crystal type
- Involves the following tests: urine dipstick, microscopic examination of urine sediment (solid cells and crystals that settle out in urine)
- Your vet may also use the IDEXX SediVue Dx Urine Sediment Analyzer to assess urine sediment. Results are used with caution since some stones can produce multiple crystal types.
- A urinalysis should ideally consist of multiple days’ worth of urine samples. A first morning sample is the ideal specimen, collected prior to eating.
- Radiographs (X-rays) of the urinary tract
- May require contrast, a dye instilled into the urinary tract to see stones more clearly)
- Radiographs are to be repeated following surgery or during stone dissolution therapy to ensure all uroliths are adequately removed or dissolved
- Ultrasound may also be required
- Following cystotomy (bladder surgery to remove a stone), your vet should submit the stone for quantitative stone analysis (to determine the stone type and potential cause of formation) and bacterial culture (if infection is suspected)
- Note: The University of Minnesota Urolith Center does not charge for urolith analysis in exchange for epidemiological data.
Treatment Options for Uroliths
Depending on the type, location, and severity/size of urinary stones, your vet will recommend one of the following options for treatment.
(from least to most invasive)
Spontaneous voiding: when smaller urinary stones are expelled from the body without intervention during an animal’s normal urination- Medical dissolution diet (a.k.a. calculoytic diet): The use of a specially formulated (often prescription) urinary diet to help dissolve certain types of urinary stones to offset the need for surgery.
- Dissolution only works on certain types of nephroliths and urocystoliths (not ureteroltihs or urethroliths) because the stones must be bathed in undersaturated urine in order to dissolve, and the only pools of urine in the urinary tract are the renal pelvis of the kidneys and bladder.
- May also require concurrent antibiotic therapy (chosen via culture and sensitivity) if uroliths are induced by infection – Antibiotics are used the entire time that dissolution diets are used because bacteria can be continuously released from within stones as they dissolve.
- Duration: The dissolution diet +/- antibiotic are continued for 2-4 weeks after resolution seen on radiographs to ensure complete treatment.
- No other food or treat should be fed or else the dissolution diet will be ineffective.
- These diets are often high in fat, which can trigger GI upset or pancreatitis in some pets.
- Following successful dissolution, pets with certain types of stones may require lifelong management of a prescription diet for the prevention of future stone formation.
- Once stones become small enough, there is a potential for them to become lodges in the urethra, but this is uncommon.
Voiding urohydropropulsion: saline is used to fill the bladder, which is then manually expressed to force bladder stones through the urethra and out
- Should not be done for urethroliths because those stones are already too large to pass through the urethra on their own
- Other non-surgical / minimally invasive options:
- Catheter retrieval
- Stone basket retrieval
- Lithotripsy: the use of shock waves to break up nephroliths and ureteroliths
- Types: extracorporeal vs. intracorporeal
- Ureteral stent: a specialty procedure in which a medical device is placed to expand the diameter of a ureter to allow a stone to pass through into the bladder
- Cystotomy: surgery of the urinary bladder; used to remove bladder stones
- If urethroliths, cystotomy may be preceded by retrograde urohydropropulsion: saline is flushed through a urinary catheter to propel urethroliths into the bladder, where they can be removed via cystotomy
- Potential cystotomy risks are uncommon yet include anesthetic complications as well as surgical complications, such as bladder tear or infection.
- Surgery of the ureters or kidneys: performed as a last resort due to potential complications
Ancillary therapies
- Other non-invasive therapies or supportive medical therapies for nephroliths and urethroltihs:
- Diuresis: increasing the flow of urine through IV fluid therapy, mannitol, or other medications (such as furosemide, thiazides, or spironolactone) in an attempt to flush stones from the kidneys and/or ureters into the bladder where they can be more readily retrieved/treated
- Smooth muscle relaxers (such as prazosin or diazepam) to aid in the passage of nephroliths and ureteroliths into the bladder
- Along with the above therapies, antibiotic administration is also to be considered for cases with concurrent UTI.
- Additionally, promoting increased water consumption to maintain hydration is also advised for all cases to keep urine dilute. For cats, consider a water fountain. For dogs and cats, consider wet canned food, which has more moisture content than dry kibble.
Urolith Composition: Types of Stones
Uroliths may differ in their mineral composition and underlying cause of formation. The most common types are struvite stones and calcium oxalate stones. Uncommon types include urate and cystine stones. Dogs or cats may be affected by any type although one species or certain breeds may be predisposed to a particular type of stone.
Struvite (a.k.a. magnesium ammonium phosphate or MAP stones)
- Common type of stone
- Cause:
- Dogs:
- Induced by infection: formed by urease-producing bacteria (ex. Staphylococcus or Proteus) that convert ammonia into ammonium, which binds to magnesium and phosphorus to form stones
- More common in females
- Cats: tend to be metabolic or nutritional in nature due to high protein or ash intake in food; rare to be caused by infection (i.e. are sterile)
- Dogs:
Diagnostic test results:
- Urinalysis
- Form in alkaline urine (pH >7)
- Evidence of UTI in dogs
- Struvite crystal appearance: coffin-shaped or cross-shaped
- Radiographs: stones are somewhat radiodense (somewhat white on radiographs); usually large and singular with smooth surface and edges
- Urinalysis
- Treatment options:
- Dissolution diet
- With a prescription calculytic diet, struvite stones will dissolve over time without the need for surgery. Therapy is continued for 2-4 weeks past radiographic resolution.
- Dissolution diets work by acidifying urine and reducing the amounts of magnesium, ammonium, and phosphate in urine.
- Prescription options:
- Dogs: Hill’s s/d for dissolution, Royal Canin SO for dissolution and maintenance
- Cats: Hill’s c/d Multicare for dissolution and maintenance
- For dogs, antibiotics are also required (ex. amoxicillin). Pets will become more comfortable within 48 hours of starting antibiotics even if stones are still present.
- should be given or else the diet will not be effective
- Radiographs and urinalysis should be repeated monthly throughout dissolution therapy to ensure stones are reducing in size and infection has cleared.
- Dogs: takes about 3 months
- Cats: takes 4-6 weeks
- Cystotomy can also be performed instead. Surgery is about same cost as dissolution diet therapy and rechecks over time. Cystotomy is recommended for cats with severe straining or discomfort and for dogs experiencing severe signs that persists over 48 hours following the start of antibiotic therapy.
- Prevention:
- Dog: Treat any recurrent UTI (recheck urinalysis every 3-6mo). Following treatment, dogs do not need to remain on dissolution diet for prevention.
- Cat: Keep on Hill’s c/d Multicare and increase water consumption.
- Dissolution diet
Calcium oxalate (CaOx)
- Common type of stone
- Cause:
- Can be idiopathic (without a known cause) or can occur in pets with hypercalcemia (high blood calcium) or hyperadrenocorticism (Cushing’s disease).
- Pets may also be deficient in endogenous crystallization inhibitors (such as glycosaminoglycans, citrates, and pyrophosphates).
- Dog breeds predisposed: miniature schnauzers, Lhasa Apsos, Shih Tzus, miniature poodles, Bichon Frises
- More common in males
Diagnostic test results:
- Urinalysis:
- Form in acidic urine (pH <7)
- Calcium oxalate crystal appearance: X-shaped or kite-shaped
- Radiographs: very radiodense (appears bright white on X-rays); usually multiple stones with an irregular shape, bearing points and sharp edges
- Urinalysis:
- Treatment:
- Surgery (cystotomy) is required since calcium oxalate stones will not dissolve.
- Radiographs and urinalysis should be repeated in 1 month following surgery and then every 3-6 months thereafter.
- If urolithiasis is detected early enough when stones are small, lithotripsy or voiding urohydropropulsion can be attempted in some cases.
- May also be prescribed hydrochlorothiazides, medications to reduce calcium content in urine
- Alkalinizing medications (ex. potassium citrate) may also be prescribed to further increase the urine pH.
- Surgery (cystotomy) is required since calcium oxalate stones will not dissolve.
- Prevention
- Prescription prevention diet
- Lifelong dietary management is imperative since these stones will recur (40-50% by year 3).
- However, prescription prevention diets, while helpful, have not been proven to be 100% effective at preventing calcium oxalate stones in dogs or cats.
- Prevention prescription diets keep urine pH neutral to alkaline and reduce the amount of calcium and oxalate content in urine.
- Prescription options:
- Dogs: Royal Canin Urinary SO or Hill’s u/d
- Cats: Hill’s c/d Multicare
- Also, keep urine dilute by increasing water consumption.
- Prescription prevention diet
Urate (or ammonium urate stones)
- Uncommon type of stone
- Cause:
- Dogs: Urate stones form in dogs due to an impairment in the way that purines, a group of amino acid building blocks of protein, are metabolized. This condition tends to be genetic.
- Dog breeds predisposed to impaired genetic metabolism of purines: Dalmatians, English bulldogs, French bulldogs
- These breeds do not get gout like affected humans can.
- Dog breeds predisposed to a portosystemic shunt (PSS) of the liver: Yorkshire terrier, Shih Tzu, miniature schnauzer
- 97% are male
- Dog breeds predisposed to impaired genetic metabolism of purines: Dalmatians, English bulldogs, French bulldogs
- Cats: idiopathic (without a known cause)
- Dogs: Urate stones form in dogs due to an impairment in the way that purines, a group of amino acid building blocks of protein, are metabolized. This condition tends to be genetic.
Diagnostic test results:
- Urinalysis: form in acidic urine (pH <7)
- Radiographs: radiolucent (difficult to identify on radiographs) or slightly radiopaque; very smooth
- Treatment and Prevention:
- A low-purine diet is the most important treatment tool.
- 30-40% of urate stones are dissolvable with a prescription diet.
- These diets keep urine pH neutral and have reduced purine content because they are high in plant-based proteins.
- Dogs with genetic impairment of purine metabolism will require lifelong management with a prescription low-purine diet in order to prevent future urate stones from developing.
- Prescription example for dogs: Hill’s u/d
- High in fat, which some patients cannot tolerate well due to GI upset or pancreatitis.
- Vegetarian diets are naturally low in purines. However, this diet is not recommended in dogs because it is not nutritionally balanced for canines. Vegetarian diets are deadly to cats and should be strictly avoided.
- In addition to a low-purine diet, allopurinol may also be prescribed. Allopurinol is a medication which prevents the conversion of urea (a waste product of protein metabolism) into uric acid (which can cause urate stones to develop); instead, the urea byproduct is kept as water-soluble allantoin instead, which dissolves in urine instead of forming a solid stone. Allopurinol should not be given on its own; it must be administered with a concurrent low-purine diet. Otherwise, sole use can convert urate stones into xanthine stones instead, a very rare type of urolith.
- If a dog has a portosystemic shunt, the PSS should be surgically corrected.
- Cystotomy can also be performed.
- Also, keep urine dilute with adequate hydration by water consumption.
- A low-purine diet is the most important treatment tool.
Cystine
- Rare type of stone
- Cause: diets overly high in sulfur-containing proteins (especially boutique diets)
- Dog breeds predisposed: English bulldogs, French bulldogs, Dachshunds, Newfoundlands
- 98% are male: Cystine stones are androgen-responsive, so castration is recommended to help prevent them from recurring.
- Diagnostic test results:
- Radiographs: radiolucent (difficult to visualize on radiographs)
- Treatment: cystotomy (surgery)
- Prevention:
- Dietary change to an AAFCO-approved commercial diet. Your vet may also recommend a prescription urinary diet.
- Castration.
If you suspect your pet may have a urinary stone, schedule an appointment with your veterinarian.
– Maranda Elswick, DVM
