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Also known as: Urethral Calculi — Urinary Calculi


Struvite crystals (sourced from Wikipedia.org)
Calcium oxalate (sourced from Wikipedia.org)

Urolithiasis is the main cause of obstruction in the lower urinary tract. Although it occurs in both males and females, urolithiasis is more common in mammalian males due to their long, narrow urethra.

Common sites of obstruction in the different species include:

cats - the terminal urethra
dogs - base of the os penis
bulls - the proximal end of the sigmoid flexure or the ischial arch
rams - vermiform appendage
birds - cloaca

The most common location for urolith formation is the bladder lumen although they may also occur in the renal pelvis, ureters or urethra.

Consequences include obstruction, ulceration, cystitis, urethritis, nephritis, hydronephrosis and rupture of the bladder or urethra.

Predisposing factors include bacterial infection, alterations in pH of the urine, heredity, sex, species and decreased water intake.

In birds, the condition arises during breeding or following egg incubation. Some birds sit very tight on their eggs and fail to void their cloacal contents regularly. Urates can precipitate in the cloaca and lead to the formation of a urolith.


Urolithiasis - the presence of calculi, or uroliths, in the urinary passages.
Urolith (calculus, stone) - polycrystalline concretion of precipitated urinary solutes, urinary proteins and proteinaceous debris.

Types of Uroliths


most common urolith in small animals commonly associated with infection by urease producing organisms such as Staphylococcus or Proteus
more common in females due to increased susceptibility to urinary tract infections
formation enhanced by the presence of alkaline urine
struvite uroliths may contain calcium and become radiodense on radiographs

Calcium oxalate

form due to supersaturation of urine with calcium
often have spine-like projections
markedly radiodense on radiographs


Dalmations predisposed, urolith of birds
urolith associated with hepatic disease, especially portosystemic shunts
radiolucent on radiographs


occurs with cystinuria
radiodense on radiographs, but may occasionally be radiolucent

Calcium carbonate

occurs in horses
radiodense on radiographs


occurs in pastured ruminants, development may be related to diet
radiodense on radiographs

Clinical Signs

Clinical signs are due to the macroscopic uroliths in the lower urinary tract which interfere with the flow of urine or irritate the mucosal lining. This leads to signs such as: dysuria, stranguria and haematuria.

Nephroliths are often asymptomatic unless pyelonephritis develops or the stone passes into the ureter.

Ureteroliths may present as pain which is difficult to detect, or signs of vomiting, lethargy, especially if there is distension of the renal capsule and developing hydronephrosis. Clinical signs may not be apparent unless both ureters are affected, there is contralateral chronic kidney disease, or a renal infection develops.

Urethral obstruction may occur suddenly or may develop over days or weeks. There will be dysuria and stranguria, and complete obstruction leads to: depression, anorexia, vomiting, dehydration, coma and death.

Birds may present weakened, straining frequently and passing only small volumes of faeces and uric acid.


Abdominal palpation is helpful in finding bladder stones. Rectal palpation may help identify urethral calculi.

However, multiple uroliths can be present throughout the urinary tract, and radiographic examination is always indicated.

Radiodense calculi >3mm in diameter can usually be seen, but contrast radiography and ultrasonography may be necessary to identify radiolucent calculi.

Contrast studies include: excretory urogram for nephroliths and ureteroliths, double contrast radiography for bladder stones, retrograde urethrogram for urethroliths.

Urinalysis, including examination of urine, urine culture and sensitivity and crystal identification on microscopic examination, is also indicated.

Urethral obstruction is diagnosed by palpating a distended, hard and painful bladder.

Once removed, stones can be sent for quantitative assessment.


Nephroliths and ureteroliths: if only one side is affected, benign neglect can be used if the other kidney is functional. If hydronephrosis has occurred, ureteronephrectomy should be performed on the affected side.

Surgical removal of the stone may be possible, and dietary dissolution can be considered if the ureter is not obstructed.

Bladder stones can be dissolved medically, or stones removed by voiding urohydropropulsion or cystotomy.

Urethral stones should be flushed back into the bladder and removed by cystotomy, or a urethrostomy can be performed in refractory or recurrent cases.

Medical management can be attempted if there is no obstruction, and the stone is amenable to dissolution (struvite, cystine or urate). Options include: diet or drug therapy, increasing water intake, changing uring pH.

Treatment of a urethral obstruction is an emergency and the animal should be stabilised prior to sedation and manipulation, due to the hyperkalaemia which predisposes to cardiac arrhythmias.

In birds, cloacal urolithiasis is treated by breaking up the urolith with a pair of forceps and removing it piecemeal. Antibiotics should be administered for 3-5 days after due to the local trauma which can be caused by the removal.

Urolithiasis Learning Resources
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Test your knowledge using flashcard type questions
Feline Medicine Q&A 21
Avian Medicine Q&A 18

Sample Book Chapters
Free chapter
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Struvite stones
A Colour Handbook - Urinary Stones in Small Animal Medicine
Albrecht Hesse, Reto Neiger
Buy book


Kahn, C. (2005) Merck veterinary manual Merck and co

Forbes, N. Self-assessment colour review: avian medicine Manson Publishing

Syme, H. (2009) Urogenital system: urolithiasis RVC student notes

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