Urolithiasis

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 males due to their long, narrow urethra.
Common sites of obstruction in the different species include:

cats - the urethra

dogs - base of the os penis

bulls - the proximal end of the sigmoid flexure or the ischial arch

rams - vermiform appendage

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.

Types of uroliths

Struvite

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

Calcium oxalate

form due to supersaturation of urine with calcium
often have spine-like projections

Urate

Dalmations predisposed

Cystine

occurs with cystinuria

Calcium carbonate

occurs in horses

Silica

occurs in pastured ruminants
development of may be related to diet

Definitions

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.