CONCRETIONS OF URINARY TRACT

Urinary calculi (urolithiasis; uroliths, nephroliths, bladder stones, cystoliths) are formed in either the calices of the kidney or, more often, the urinary bladder. Small uroliths may enter the ureter or urethra and cause partial or complete obstruction of urine flow.^* No specific studies on the pathogenesis of urinary calculi formation in camelids have been reported. There are clinical reports of disease caused by the obstruction produced by the calculi. Zoo veterinarians must rely on information extrapolated from studies of cattle, sheep, and goats, which have urine of similar composition (Table 47-1). Box 47-1 lists the composition of urinary calculi recovered from llamas and alpacas at the Veterinary Medical Teaching Hospital, University of California. Others have reported similar findings in camels, vicu-a, and guanacos. Urethral obstruction may also occur with nonmineral concretions.

Table 47-1 Characteristics of Normal Urine

Box 47-1 Composition of Urinary Calculi ^*

Silicon dioxide (SiO₂)—Crystobalite

Magnesium ammonium phosphate (MgNH₄PO₄ 6H₂O)—Struvite

Basic calcium phosphate (Ca₅[PO₄]₃[OH])—Apatite

Calcium carbonate (CaCO₃)—Carbonate

Uric acid (C₅H₄N₄O₃)—Urate

^* Recovered from llamas at the Veterinary Medical Teaching Hospital, University of California.

Urinary calculi are formed in males and females equally, but the bore (diameter) of the female urethra generally allows free passage of a calculus that may enter the urethra. Thus, obstructive urolithiasis is rare in the female. Urolithiasis has been associated with a diet high in concentrated feeds, as often used in zoos. Cattle pastured on grasses containing high levels of silicates may sometimes develop silicate urolithiasis, and presumably, camelids grazing on such pastures may be at risk as well.

A basic understanding of the camelid urethra is required to locate sites of possible obstruction and develop approaches to management.Figure 47-1 is a diagram of the camelid urethra and associated structures. The prostate gland does not surround the pelvic urethra as in carnivores. The pelvic urethra is expansive, but at the reflection around the ischium, only a tiny orifice allows passage of urine beyond this point. The anatomy of this area is further complicated by a dorsal urethral recess that precludes any possibility of passing a catheter into the bladder from the tip of the penis. Whereas the sigmoid flexure is the probable site of the majority of bovine urethral obstructions, this is not the case in camelids. The orifice from the pelvic urethra into the penile urethra is a common site of obstruction; another site is where the penile urethra narrows as it enters the glans penis.

Fig 47-1 Diagram of urinary tract of male camelid. A Dorsal urethral recess; B, bulbourethral gland; C, pelvic urethra; D, narrowed junction of pelvic and penile urethra; E, penile urethra; F, sigmoid flexure; G, pelvis; H, prostate gland; I, bladder; J, ureter; K, kidney; L, calix of kidney; M, reflection of prepuce onto penis. This is the beginning of the glans penis: N, narrowing of urethra at origin of glans penis; O, cartilaginous projection at tip of penis; P, urethral orifice; Q, preputial opening.

Clinical Signs

Uroliths in the bladder or calices of the kidney rarely cause discomfort, although large and rough-surfaced uroliths may initiate a cystitis. The signs of urethral obstruction caused by calculi vary with the stage of the disorder.⁹ Signs prior to bladder rupture include colic, straining stance to urinate, dribbling urine, blood-tinged urine, anuria, distended bladder, and possible pulsation of the urethra. Signs after bladder rupture are absence of colic, with depression, anorexia, anuria, uroperitoneum, and possible distention of the abdomen and uremia (muscular weakness, dehydration, dyspnea, tremors, uremic breath odor, tachycardia, recumbency, coma, death).²⁴

Uroperitoneum may result from trauma when the bladder is distended or from rupture of the bladder after urethral obstruction. In the llama the immediate response to urine flushing into the abdominal cavity is excruciating pain. Llamas become frenzied and thrash about violently. This initial pain subsides, and the pain associated with a distended bladder disappears. Urine in the abdomen may arise from a single or multiple tears in the bladder wall, but also from seepage through the stretched-thin bladder wall. A ureter may also rupture, but such a condition has not been diagnosed in a camelid.