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Bladder stones or
uroliths are a common occurrence in animals, especially in domestic animals such as
dogs and
cats. Occurrence in other species, including
tortoises,
[1] have been reported as well. The stones form in the
urinary bladder in varying size and numbers secondary to
infection, dietary influences, and
genetics. Stones can form in any part of the urinary tract in dogs and cats, but unlike in humans, stones of the
kidney are less common and do not often cause significant disease, although they can contribute to
pyelonephritis and
chronic renal failure. Types of stones include
struvite,
calcium oxalate,
urate,
cystine,
calcium phosphate, and
silicate. Struvite and calcium oxalate stones are by far the most common.
Particularly large (510 gram) canine bladder stone
X-ray of a single, large bladder stone in a dog with a bladder located more to the rear than is usual
X-ray of bladder stones in a dog
X-ray of a struvite bladder stone in a cat
Formation of bladder stones[edit source | edit]
Oversaturation of
urine with crystals is by the far the biggest factor in stone formation
[clarification needed].
[2] This oversaturation can be caused by increased excretion of crystals by the kidneys, water reabsorption by the
renal tubules resulting in concentration of the urine, and changes in urine
pH that influence crystallization. Other contributing factors include diet, frequency of urination, genetics, current medications, and the presence of a
urinary tract infection.
The stones form around a
nidus, which can consist of
white blood cells,
bacteria, and organic matrix mixed with crystals, or crystals alone. The nidus makes up about two to ten percent of the mass of the stone.
[3] It is possible for the nidus to be made of a different type of crystal than the rest of the stone, also known as
epitaxial growth.
Bladder stones may cause blood in the urine (
hematuria) (giving the appearance that the animal is urinating blood) but sometimes there may be no signs at all. Painful urination or straining to urinate are other signs. Urinary tract infections are commonly associated with bladder stones. Smaller stones may become lodged in the
urethra, especially in male animals, causing
urinary tract obstruction and the inability to urinate. This condition causes
acute renal failure,
hyperkalemia,
septicemia, and death within a few days.
When symptoms indicate bladder stones, the first step is usually to take an
x-ray. Most types of stones will appear readily in an x-ray, urate and occasionally cystine stones being the most common exceptions. Stones smaller than three millimeters may not be visible.
[3] Ultrasonography is also useful for identifying bladder stones. Crystals identified in a
urinalysis may help identify the stones, but analysis of the stones is necessary for identification of the complete chemical composition.
Reasons for treatment of bladder stones include recurring symptoms and risk of urinary tract obstruction. Some stones can be dissolved using dietary modifications and/or medications. Small stones in female dogs may possibly be removed by
urohydropropulsion, a nonsurgical procedure. Urohydropropulsion is performed under sedation by filling the bladder with saline through a catheter, holding the dog vertically, and squeezing the bladder to expel the stones through the urethra. Bladder stones can be removed surgically by a
cystotomy, opening of the bladder. Stones lodged in the urethra can often be flushed into the bladder and removed, but sometimes a
urethrotomy is necessary. In male dogs with recurrent urinary tract obstruction a
scrotal urethrostomy creates a permanent opening in the urethra proximal to the area where most stones lodge, behind the
os penis. In male cats, stones lodge where the urethra narrows in the penis. Recurrent cases can be treated surgically with a
perineal urethrostomy, which removes the penis and creates a new opening for the urethra.
To prevent recurrence of stones, special diets can be used for each type of stone. Increasing water consumption by the animal dilutes the urine, which prevents oversaturation of the urine with crystals.
Struvite stones are also known as magnesium ammonium phosphate stones due to their chemical composition - MgNH
4PO
4·6H
20. Often there is a small amount of calcium phosphate present.
[4] They form at a neutral to alkaline pH of the urine. Bacterial infections contribute to their formation by increasing the pH of the urine through the
urease enzyme in dogs. More than 90 percent of dogs with struvite stones have an associated urease-producing bacterial infection in the urinary tract, but in cats struvite stones usually form in sterile urine.
[5] The appearance of the stones vary from large solitary stones to multiple smaller stones. They can assume the shape of the bladder or urethra.
Dissolution of the struvite stones depends on acidification of the urine through diet or urinary
acidifiers. Special diets for dissolution also have reduced
protein,
phosphorus, and
magnesium, as well as increased salt to increase water consumption and dilute the urine. The diet needs to be fed exclusively, but it can only be fed for a few months total due to potential side effects.
Contraindications to this diet include
heart failure,
liver failure,
kidney failure,
pancreatitis,
hypertension (high blood pressure), and
hypoalbuminemia (low
serum albumin).
[6] Prevention of struvite stones is with a similar diet with milder restrictions.
Certain dog breeds are predisposed to struvite stones, including
Miniature Schnauzers,
Bichon Frises, and
Cocker Spaniels.
[2] They are the most commonly reported bladder stone in female dogs
[7] and in
ferrets (pregnant ferrets may be especially predisposed).
[8] For frequency in cats, see below.
Calcium oxalate stones form in an acidic to neutral urine. Two types naturally occur, calcium oxalate monohydrate, or
whewellite (CaC
2O
4·H
2O), and calcium oxalate dihydrate, or
weddellite (CaC
2O
4·2H
2O). Their appearance can be rough, smooth,
spiculated, or
jackstone. Calcium oxalate stones form more readily in animals with
hypercalcaemia, which can caused by
Addison's disease or certain types of
cancer. Hypercalcaemia results in
hypercalciuria, which can also be caused by
Cushing's syndrome or
hyperparathyroidism.
There is no recommended diet to dissolve calcium oxalate stones. For prevention a diet low in protein and
oxalates and high in magnesium, phosphorus, and calcium is recommended. Increased dietary magnesium and phosphorus decreases the amount of calcium in the urine, and increased dietary calcium reduces absorption of oxalates from the intestines.
[7] Potassium citrate has been recommended as a preventative for calcium oxalate stone formation because it forms a soluble complex with oxalates and promotes the formation of alkaline urine.
[2]
Dog breeds possible prone to calcium oxalate stones include
Miniature Schnauzers,
Lhasa Apsos,
Yorkshire Terriers,
Miniature Poodles,
Shih Tzus, and
Bichon Frises.
[6] They are the most common stone in male dogs.
[7] Calcium oxalate stones are also common in
domestic rabbits. Rabbits are prone to hypercalciuria due to intestinal absorption of calcium not being dependent on
vitamin D and a high fractional urinary excretion of calcium.
[8] The urine will appear thick and creamy or sometimes sandy. Small stones and sand can be removed using urohydropropulsion. Prevention is through reducing calcium intake by feeding more hay and less commercial rabbit pellets, and by not using mineral supplements.
Frequency of struvite and calcium oxalate stones in cats[edit source | edit]
The
Minnesota Urolith Center at the
University of Minnesota College of Veterinary Medicine has done detailed analysis of uroliths from animals since 1981 and has noted changing trends in feline uroliths. In 1981, struvite stones were the most common type in cats, making up 78 percent of submitted samples, with only 2 percent comprising calcium oxalate stones. In the mid 1980s there was a substantial increase in the number of calcium oxalate samples, and between 1994 and 2002, 55 percent of feline stones were calcium oxalate and 33 percent were struvite. This may have been caused by the use of dissolution diets for struvite stones in cats and modification of other diets to prevent struvite crystal formation. These modifications predisposed to calcium oxalate crystal formation. However in 2004, struvite stones once again surpassed calcium oxalate stones 44.9 percent to 44.3 percent, and in 2006, 50 percent of stones were struvite and 39 percent were calcium oxalate. This may have been due to the increased use of diets designed to prevent calcium oxalate crystal formation, which because of increased magnesium in the diet and decreased acidity of the urine help promote struvite crystal formation.
[9]
Urethral plugs in cats are usually composed of struvite crystals and organic matter.
[10]
Urate (C
5H
4N
4O
3) stones, usually ammonium urate (NH
4·C
5H
4N
4O
3) or sodium urate monohydrate (Na·C
5H
4N
4O
3×H
2O), form in an acidic to neutral urine. They are usually small, yellow-brown, smooth stones. Urate stones form due to an increased excretion of
uric acid in the urine.
Dalmatians (especially males)
[11] and to a lesser extent
Bulldogs are genetically predisposed to the formation of urate stones because of an altered metabolism of
purines. Dalmatians have a decreased rate of urate hepatic transport, leading to only about 30 to 40 percent conversion of urate to
allantoin, compared with greater than 90 percent conversion in other breeds.
[6] Dogs with
portosystemic shunts or endstage liver disease also have increased uric acid excretion in the urine due to reduced conversion of uric acid to
allantoin and
ammonia to
urea. Urate stones make up about six percent of all stones in the cat.
[12]
Urate stones can be dissolved using a diet with reduced purines that alkalinizes and dilutes the urine.
Allopurinol is used in dogs with altered purine metabolism to prevent the formation of uric acid. Feeding a diet high in purines while simultaneously administering allopurinol can result in the formation of
xanthine (C
5H
4N
4O
2) stones.
Cystine ((SCH
2CHNH
2COOH)
2) stones form in an acidic to neutral urine. They are usually smooth and round. They are caused by increased urine excretion of cystine (a relatively insoluble amino acid) in dogs with a defect in
renal tubule reabsorption of cystine. Dietary reduction of protein and alkalinization of the urine may help prevent formation. Medications such as
D-penicillamine and
2-MPG contain
thiol, which forms a soluble complex with cystine in the urine.
[7] Dog breeds possibly predisposed to formation of cystine stones include Bulldogs,
Dachshunds,
Basset Hounds,
Chihuahuas, Yorkshire Terriers,
Irish Terriers, and
Newfoundlands. In Newfoundlands, cystinuria is inherited as an
autosomal recessive trait, but in the other breeds it is a
sex linked trait and found primarily in male dogs.
[6]
Calcium phosphate, also known as hydroxyapatite (Ca
10(PO
4)
6(OH)
2), stones form in neutral to alkaline urine. They are usually smooth and round. Calcium phosphate is usually a component of struvite or calcium oxalate stones and is infrequently a pure stone. They form more readily with hypercalcaemia. Dog breeds possibly predisposed to calcium phosphate stone formation include Yorkshire Terriers, Miniature Schnauzers, and Cocker Spaniels.
[2]
Silicate (SiO
2) stones form in acidic to neutral urine. They are usually
jackstone in appearance. There is possibly an increased incidence associated with dogs on diets that have a large amount of corn
gluten or
soybean hulls. Dog breeds possibly predisposed include
German Shepherd Dogs,
Golden Retrievers,
Labrador Retrievers, and Miniature Schnauzers.
[2]
- ^ Giant tortoise cheats death ("Evening Express", Aberdeen, 31/01/2009)
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- ^ a b "Urolithiasis: Overview". The Merck Veterinary Manual. 2006. Retrieved 2007-04-14.
- ^ Buffington, Tony (2004). "Nutrition and Urolithiasis". Proceedings of the 29th World Congress of the World Small Animal Veterinary Association. Retrieved 2006-07-16.
- ^ Hoskins, Johnny D. (November 2006). "Feline Urolithiasis". DVM (Advanstar Communications): 6S–7S.
- ^ a b c d "Canine Urolithiasis". The Merck Veterinary Manual. 2006. Retrieved 2007-04-14.
- ^ a b c d Zoran, Debra L. (2006). "Role of Diet in Feline and Canine Urolithiasis". Western Veterinary Conference Proceedings.
- ^ a b Hillyer, Elizabeth V.;Quesenberry, Katherin E. (1997). Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery (1st ed.). W.B. Saunders Company. ISBN 0-7216-4023-0.
- ^ Osborne, Carl A.; Lulich, Jody P. (February 2007). "Changing trends in composition of feline uroliths and urethral plugs". DVM (Advanstar Communications): 38–40.
- ^ Escolar E, Bellanato J (2003). "Analysis of feline urinary calculi and urethral plugs by infrared spectroscopy and scanning electron microscopy". Vet Rec 152 (20): 625–8. doi:10.1136/vr.152.20.625. PMID 12790167.
- ^ Albasan H, Lulich J, Osborne C, Lekcharoensuk C (2005). "Evaluation of the association between sex and risk of forming urate uroliths in Dalmatians". J Am Vet Med Assoc 227 (4): 565–9. doi:10.2460/javma.2005.227.565. PMID 16117063.
- ^ "Feline Urolithiasis and Feline Lower Urinary Tract Disease". The Merck Veterinary Manual. 2006. Retrieved 2007-04-14
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