Chapter 47 Uroliths and Gastroenteroliths in Camelids
Obstruction of the tubular urinary or digestive tract may cause acute colic signs and may be life threatening in any species of animal. Veterinarians are well aware of the syndromes in traditional livestock and companion animals. Similar conditions occur in zoo animals.
Historically, camelids have always been considered zoo animals, but with the advent of the public fancy for llamas and alpacas, the prevalence of uroliths and gastroenteroliths has been more frequently reported. Information gleaned from these cases may be of value to those dealing with camelids in zoos.
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.
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
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.
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.
Diagnosis is based on assessment of clinical signs, pertinent history, and special diagnostic tests. A differential diagnosis should include any disease condition in which colic is a sign. The major hematologic and serum chemistry changes found in camelids with obstructive urolithiasis include hemoconcentration, elevated blood urea nitrogen (BUN), hypophosphatemia, hypercalcemia, hypermagnesemia, hyperkalemia, hypercreatininemia, and hypochloremia.20 It is not always easy to identify the source of fluid in the abdominal cavity. Urine should have the odor of ammonia, but it may be necessary to heat the fluid to concentrate the odor before it becomes perceptible. The ability of individuals to smell ammonia varies. Abdominal fluid caused by urine has a creatinine concentration greater than 15 mg/dL (normal serum <3 mg/dL). Urine potassium is greater than185 mmol/L (normal serum <5 mmol/L). The latter is the most important biochemical urine detector.
Urethral catheterization and double-contrast radiography are routinely used to diagnosis urethral calculosis in carnivores. It is impossible to pass a catheter in a camelid male because of the restrictive urethral diameter and the dorsal urethral recess. The value of radiography and ultrasonography has been less than satisfactory. Uroliths that do not contain calcium may be radiolucent, especially when surrounded by an inflammatory reaction.
The narrow urethra may predispose the accumulation of organic debris. A llama developed a swelling of the preputial area that was treated initially as a trauma and later as a cellulitis. At necropsy the urethra had ruptured proximally to an obstruction at the glans penis caused by a plug of nonmineralized organic material. The urethra may also become obstructed by external pressure on the urethra caused by a hematoma of the corpus cavernosum urethra after penile trauma.