Urolithiasis in Small Mammals by Dr. Sarah Ozawa, DACZM: Part 1
By Sarah Ozawa, DVM, DACZM, Assistant Professor
In Part 1 of this two-part VETgirl online veterinary continuing education blog, Dr. Sarah Ozawa, DACZM, Assistant Professor at North Carolina State University discusses urolithiasis in small mammals. Please make sure to check out next week’s Part 2 HERE to learn all about treatment of urolithiasis in small mammals too!
Urolithiasis is a common condition in exotic small mammal species. While the diagnosis may be an incidental finding, many cases are symptomatic and may present as an emergency. Much of our knowledge is extrapolated from small animal medicine keeping in mind the limitations of patient size and differing anatomy and physiology. Understanding the common stone types, predisposing factors, signalment, diagnostics and treatment of this disease is instrumental in the management of urolithiasis in exotic small mammal species.
ANATOMY AND PHYSIOLOGY
The ferret bladder is relatively small and thin walled and easily distensible. The male (hob) has a baculum that is J-shaped.(1) This runs from near the tip of the penis to caudal to the ventral rim of the pelvis. The urethral orifice opens on the ventral surface of the glans penis and the preputial opening is caudal to the umbilicus. Additionally, the prostate is fusiform in shape and surrounds the proximal urethra. Prostatic disease can occur as a sequela to hyperandrogenism associated with adrenal disease and mimic the presentation of urolithiasis. These anatomic variations result in a male ferret predilection for obstructive urolithiasis and may make catheterization challenging. The urethra in the female opens into the vaginal vestibule.
In ferrets, the range for creatinine is small and the total creatinine is lower compared to other species. Therefore, small elevations in creatinine may be significant in ferrets. Given their carnivorous diet, the urine pH is normally acidic.
The bladder normally lies within the caudal abdomen to pelvic canal but is very distensible. There is no baculum in the male. The female urethra exits within the vaginal vestibule. Pigment from endogenously synthesized compounds or ingested plants are commonly excreted in the urine resulting in a yellow to red coloration. The urine is normally cloudy in color due to ammonium magnesium phosphate and calcium carbonate precipitates. Rabbits have a unique calcium metabolism compared to other mammals. Serum calcium is directly related to dietary calcium and intestinal absorption is independent of vitamin D levels. The urine is the main route of calcium excretion.(2,3) Rabbits may have less carbonic anhydrase within the thick ascending loop of renal tubules. However literature supporting this is sparse.(4,5) Rabbits have high water intake and urine output with daily fluid requirements of approximately 100 ml/kg/day.(6)
Males have numerous accessory sex glands including large vesicular glands in which stones can become lodged.(7) The penis is S-shaped and males have a baculum. There is intromittent sac containing two horny styles located caudoventral to the urethral opening at the tip of the glans penis. The female urethra is readily distensible, and the urinary papilla is located outside of the vagina, like most rodents. Females have a vaginal membrane. Guinea pigs also have a high urinary excretion of calcium.
Male chinchillas possess a baculum. Males also have well developed accessory sex glands and produce a post-mating plug. Chinchillas can produce a wide range of urine specific gravities, but are often concentrated.(8,9) Chinchillas excrete the majority of their calcium in the feces, not urine.(10)
UROLITHIASIS BY SPECIES
Historically sterile struvite uroliths were the most common urolith type in the United States. In a study evaluating urolithiasis from the Minnesota Urolith Center from 1981-2007, 67% of the stones submitted were struvite.(11) However the prevalence of this stone type has shifted over time. In a more recent retrospective study (2010-2018), the most common stone type in ferrets in North America is now cystine, with 92.6% of submissions being primarily cystine in composition.(12) This is in contrast to uroliths in other countries with cystine urolithiasis making up only 26.8% of the submissions from Europe and Asia.(12) In this study, the odds of submission of a cystine urolith decreased with age with the median age of ferrets with cystine urolithiasis being 2 years of age and those with other stone types, 4 years of age.
Overall male ferrets appear to present more often with urolithiasis, likely due to their anatomical characteristics that predispose them to urinary obstructions.(11,12) Urinary pH may influence the type of stone that develops as well. Diets higher in plant proteins may produce more alkaline urine which favors the formation of struvite stones. While the naturally more acidic pH of ferret urine makes cystine insoluble.(1) In a retrospective case control study of ferrets with cystine urolithiasis, 94% of the cases with cystine stones received a grain-free diet.(13) While dietary management or prevention of this disease may be challenging, emphasis should be put on a high quality animal protein diet. Cystinuria is associated with an inherited genetic mutation in humans, dogs and cats.(14) Ferrets in the United States have minimal genetic diversity, and a similar underlying genetic mechanism likely contributes to this disease process.(15,16) Research is ongoing in this subject.
The most common urolith in rabbits is calcium carbonate. Other stone types include ammonium magnesium phosphate and calcium oxalate. Rare reports include silica, struvite and a single cases of calcium sulfate dihydrate from a rabbit eating gypsum based plaster.(17) The prevalence of urolithiasis in pet rabbit populations is unknown but estimated to be 2-10%.(18)
Most stones are located within the bladder, however urethroliths, ureteroliths and renoliths also occur. Rabbits can also develop uroliths within the vaginal vestibule secondary to urine pooling or uroliths that were urinated into the vestibule.(19)
In addition to true urolithiasis, rabbits may develop micro urinary calculi (MUC) which is more commonly referred to as bladder “sludge.” This is differentiated from normal calciuria as it precipitates to form microscopic calculi within the bladder. This appears to be a relatively common condition in rabbits.
Predisposing factors for naturally occurring urolithiasis are less well described in rabbits than in ferrets. Things like obesity, lack of voiding, infection and urinary nidus may universally predispose animals to stone formation. Ureteral obstruction itself can predispose rabbits to nephrolithiasis of the contralateral kidney.(20,21) Certain biochemical changes associated with urolithiasis in rabbits include increasing plasma calcium and sodium, likely due to their high calcium absorption and excretion.(18) A study evaluated the effect of dietary calcium on urolithiasis in rabbits fed diets of varying calcium concentrations for a total of 25 weeks. Rabbits fed primarily alfalfa, a high calcium food item, had more urinary sediment on ultrasound, but none of the rabbits in this study developed urolithiasis or tissue mineralization.(22) While diet is often blamed as a cause of urolithiasis in rabbits, calcium should not be excluded from the diet as it is a necessary nutrient. Instead, priority should be placed on increasing water intake and limiting dietary items with excessive calcium levels.
Check back next week for Part 2 HERE to learn more about guinea pigs and chinchillas, along with treatment of urolithiasis!
References for Part 1 and 2:
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