Urolithiasis in Small Mammals by Dr. Sarah Ozawa, DACZM: Part 2
By Sarah Ozawa, DVM, DACZM, Assistant Professor
In Part 2 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, specifically in guinea pigs and chinchillas, along with about treatment of urolithiasis. Please make sure to check out last week’s Part 1 HERE to learn all things urolithiasis!
The most common urolith type in guinea pigs is calcium carbonate.(23) Older literature may cite calcium oxalate as the most common stone type, which is likely due to the methodology of stone detection and diagnosis. Most uroliths are present in the bladder, followed by the urethra and the ureter. Uroliths can also be present within the seminal vesicles in males and there is a single case of a urolith present within a urethral diverticulum in a guinea pig.(24)
A recent retrospective study of urolithiasis in guinea pigs discussed 158 cases diagnosed between 2009-2019.(25) In this study, both males and females were represented, but males often presented at a younger age. Females more commonly developed distal urinary stones, likely secondary to their distensible urethra. Overall females had a longer survival time than males. Factors that were associated with non-survival in guinea pigs with urolithiasis included increased age, anorexia, weight loss, male sex, and lower body temperature at intake. Importantly, recurrence of urolithiasis occurred in 14% of the cases and is an important aspect of disease management to discuss with owners.
Given the high calcium absorption and excretion in guinea pigs, similar to rabbits, dietary modifications are often included in the management strategy of this disease process. However, certain breeds of guinea pigs are predisposed to fibrous osteodystrophy due to calcium or vitamin D imbalances and therefore calcium should never be eliminated from their diet.(26) In other species, excessive vitamin C may lead to the formation of calcium based stones.(27) When hypercalciuric guinea pigs were fed increasing amounts of vitamin C, bladder calcification occurred.(28) While vitamin C is required in guinea pigs, there may be evidence that excessive supplementation could be detrimental.
Urolithiasis appears to be less common in chinchillas than other small mammal species. A retrospective case series described 15 cases of urolithiasis in chinchillas over a four year period.(10) The median age was 30 months and more males (14/15) than females were diagnosed with urolithiasis. Calcium carbonate also appears to be the most common urolith in chinchillas.10 There is a single case of a semen-matrix calculi in a chinchilla.(29) Most uroliths are present within the bladder with fewer present within the urethra.
The etiology in chinchillas is unknown. Insufficient water intake may play a role. Dietary causes are considered unlikely in chillas given the low amount of calcium that is typically excreted in the urine in chinchillas.10
TREATMENT OF UROLITHIASIS
Most cases of urolithiasis in small mammals require surgical removal as medical therapy or dissolution is not typically effective. However, medical management is indicated in certain cases where urolithiasis is non-obstructive or unlikely to result in clinical consequences.(30)
In cases of obstructive urolithiasis, catheterization is often necessary. Heavy sedation utilizing benzodiazepines, opioids and/or alpha agonists or ketamine or general anesthesia is recommended. Catheterization is particularly challenging in the male ferret due to the small diameter of the penile urethra, the acute angle of the urethra at the caudal aspect of the pelvic canal, possible extraluminal compression at the level of the prostate and the J-shaped os penis. The urethral opening is on the ventral surface of the glans. Dilation with retrograde flushing, using a stylet such as small gauge suture material or a small gauge IV catheter can aid in placement. Recommended materials include silicone catheters, tom cat catheters, jugular catheters, and red rubber catheters in larger males. Body wrapping the catheter to the male is recommended in addition to e-collar placement. In female ferret and rabbits, in general ventral recumbency with the hindquarters elevated is recommended for catheterization. An otoscope, nasal speculum or endoscope may aid in placement. Female guinea pigs and chinchillas are relatively easy to place a urinary catheter due to the external urinary papillae.
Surgical treatment will depend on the location of the stone, size of the patient and emergent necessity of the surgery. A cystotomy is relatively straightforward in most species and preferred over a urethrotomy. If a stone is present within the urethra, retrograde flushing should be attempted to move the stone into the bladder for removal. Urohydropropulsion can also be utilized in cases of microurinary calciuli in rabbits to attempt to dilute or remove the sludge like material from the bladder. If the stone is located within other areas of the urinary tract a urethrotomy, ureterotomy, pyelolithotomy or nephrotomy may be required. The feasibility of those surgeries is dependent on patient anatomy and size. Other reported surgeries include perineal urethrostomy and tube cystotomy, especially in cases where urethral catheterization is not possible. There are now several reports of the long-term use of low-profile tube cystotomies in ferrets.(31) However, complication such as chronic urinary tract infections and myocarditis have been reported.
In cases of end stage hydronephrosis or obstructive ureterolithiasis, a nephrectomy may be considered. A lateral flank approach has been reported in rabbits.(32) However, if functional renal tissue remains, it is preferred to leave the kidney in place and treat medically. Alternatively, in other species a subcutaneous urinary bypass (SUB) systems or ureteral stent may be recommended in these cases. In rabbits, utilization of implantable devices has proven challenging due to their high urinary excretion of calcium and predilection for mineralization. In three rabbits with ureteral stent placement, encrustation occurred of the stent in all cases resulting in re-obstruction.(33) Additionally, in a case of a SUB placed in a rabbit, the contralateral kidney obstructed and the shunting port developed an obstruction.(34) At present, there are concerns over long-term prognosis with these devices in rabbits.
In larger animals, minimally invasive endoscopic techniques may be feasible. Cystoscopy can be performed in larger female rabbits and guinea pigs.(35) There is a report of vaginoscopy to remove a vaginal calculi in a rabbit.19 Due to the distensible nature of the guinea pig female urethra, manual removal, with or without the utilization of retractors may be possible with distal urethroliths. Percutaneous cystolithotomy has been successfully performed in ferrets with a positive outcome in three out of four cases.(36) Extracorporeal shock wave lithotripsy has been used in rabbits as an experimental model. This resulted in histopathologic damage to extrarenal tissue such as the liver and lung and is therefore not recommend in clinical veterinary patients at this time. Cystoscopic lithotripsy is feasible in the female guinea pig however.(37) While not yet utilized in small mammal patients, burst wave lithotripsy shows promise and warrants further research.(38)
Medical dissolution of stones is not often effective in small mammal species. Struvite dissolving diets made for cats and dogs are not palatable for ferrets and compliance is challenging. Additionally, these diets are unlikely to have sufficient protein for ferrets. Other medications to alter urinary pH have similar challenges with palatability. Anecdotally, medications like potassium citrate and hydrochlorothiazide have been used in rabbits and guinea pigs to reduce the formation of calcium-based stones. In a retrospective study of lower urinary tract disease in guinea pigs, potassium citrate was associated with a positive response to treatment and may help reduce the recurrence of urolithiasis in at risk animals.(39) Interestingly, in this retrospective study, the majority of the guinea pigs with urolithiasis were medical managed (72%) with a combination of fluids, nonsteroidal anti-inflammatory drugs, analgesics and benzodiazepines.39 Other medical management treatments may include fluid therapy and alpha blockers to relax the urethra (e.g. prazosin). Recently however, prazosin has been suggested to increase the rate of urinary obstructions in cats and its use may require additional consideration in small mammal species. All these drugs are not approved in small mammals and their use and dosage are often extrapolated from small animal medicine.
Prevention of urolithiasis is challenging, as the etiology in most species is unknown. In all species, an appropriate diet should be recommended as previously discussed. Increasing water intake is also helpful as urolithiasis is more likely to occur in concentrated urine. This can be done by increasing water rich food sources or increasing drinking water intake. Certain species have water intake preferences with rabbits and chinchillas favoring open dishes and guinea pigs and degus having more individual preferences.(40,41) Attempting to treat or minimize predisposing factors may help prevent disease. This may include increasing exercise and promoting weight loss in cases of obesity or treating underlying mobility or sanitation issues. However, as there is likely a large physiologic and genetic component to the development of urolithiasis in these species, recurrence may be common.
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