Urolithiasis in Ferrets, Rabbits and Rodents

Introduction

Urolithiasis is characterized by single or multiple calculi throughout the urinary tract or by the presence of sandy material within the bladder and urethra.  Uroliths are fortunately more of a historical disease in the ferret, while calculi are still an important problem in rabbits and rodents (Fig 1).

Figure 1. Large urolith in the urethra of a female guinea pig (Cavia porcellus). Photo credit: Uwe Gille via Wikimedia Commons. Click image to enlarge.

Urolithiasis in the ferret

Incidence

Urolithiasis is seen most commonly in adult males. The most common urinary calculus reported in the domestic ferret is magnesium ammonium phosphate or struvite.

Pathogenesis

Dietary factors are believed to play an important role in struvite crystal formation in the ferret. Urine pH is greatly influenced by diet, specifically by the source of dietary protein. Metabolism of animal protein tends to produce acidic urine, while plant-based protein diets, like dog food or inexpensive cat foods, produce relatively alkaline urine. Struvite crystals commonly form at urine pH exceeding 6.6. Significant crystalluria leads to the development of calculi or sandy material in the bladder and urethra. Urinary calculi used to be a common cause of stranguria in ferrets; however improvements in diet have made urolithiasis rare in ferrets on ferret food or high-quality cat food. Urolithiasis is also rare in pet ferrets on fresh meat diets in New Zealand, Australia, and Europe.

Urolithiasis may also be associated with ascending cystitis in pregnant jills. Infection is usually caused by urease-positive bacteria like Staphylococcus or Proteus spp.

Although most uroliths are struvite, mixed uroliths have also been reported in ferrets including 60% struvite and 40% calcium oxalate and there are also rare reports of cystine stones. The cause of cystine urolithiasis in ferrets is unknown, but has been speculated to be dietary or hereditary.

Clinical disease

Clinical signs of urolithiasis in the male or hob ferret may include stranguria, dysuria, pollakiuria, urine dribbling, frequent licking of the prepuce, and hematuria. Ferrets with urethral obstruction may strain violently or cry when attempting to urinate, and owners may misinterpret the straining observed as “constipation”. Tenesmus may even lead to diarrhea in some cases. Occasionally, a ferret with blockage will present for lethargy, weakness, anorexia, and even collapse without obvious signs of dysuria. If left uncorrected, urinary obstruction can result in severe metabolic disturbances, coma, and death.

Affected female ferrets or jills may be asymptomatic or show intermittent straining for days or weeks. Once the cystic calculus reaches a large size, the jill will eventually show signs of real distress. By this time, there may also be evidence of urine dribbling and urine scald. Although urethral obstruction is more common in male ferrets, females can also become obstructed potentially straining hard enough to cause rectal or vaginal prolapse and potentially fatal hemorrhage.

Diagnosis

• Obtain a complete history, including dietary history, from the owner.
• Cystic calculi or sand are often palpable in ferrets without obstruction, while a distended bladder is readily palpable in obstructed ferrets.
• Abdominal radiographs serve as a valuable diagnostic tool. Evaluate the entire urinary tract for radiodense uroliths and other abnormalities. Calculi lodged at the os penis can be difficult to detect.
• Use abdominal ultrasound to evaluate the urinary tract, prostate, and adrenal glands.
• Collect samples for complete blood count, serum biochemistry, urinalysis, and ideally urine bacterial culture and sensitivity. The reported range for normal ferret urine pH is 6.0 to 7.5. Urine should be acidic (approximately 6.0) in ferrets fed a high-quality, meat-based diet.³⁵ Laboratory results may include azotemia, hyperkalemia, hyperphosphatemia, and metabolic acidosis.

Prognosis and therapy

With aggressive treatment, the prognosis is good for urethral or cystic calculi.

1. Cystotomy:  If the ferret is not obstructed, provide supportive care, including fluid therapy, and then schedule cystotomy to remove cystic calculi and flush the bladder. Submit calculi for mineral analysis, and send crushed calculi and bladder mucosa for bacterial culture and sensitivity. Begin antibiotics after surgery or pre-operatively, if you suspect infection. Select a broad-spectrum antibiotic that reaches high levels in the urinary tract until culture and sensitivity results are available. Administer antibiotics for a minimum of 10 to 14 days, ideally using urinalysis and urine culture results to guide the duration of therapy.
2. Urethral catheterization:  Treatment of urethral obstruction in male ferrets is a challenge. Place a urinary catheter, then flush the urolith into the urinary bladder for future removal via cystotomy.
3. Dietary modification:  Convert the ferret to an animal protein-based diet (i.e. ferret food or a high-quality cat food). Attempts to feed feline magnesium-restricted acidifying diets (i.e. feline s/d [Hill’s Pet Nutrition, Topeka, KS] or feline Urinary SO (Royal Canin, St. Charles, MO) are generally unsuccessful. These diets probably also contain insufficient protein for long-term use in ferrets, although use of a protein-restrictive diet for advanced renal disease (Hill’s Prescription diet u/d) has been described for dietary management of cystine urolithiasis. The ferret was also fed a protein supplement and hemoglobin and albumin levels were monitored. Two cases of cystine urolithiasis in which owners did not modify diet postoperatively have also been reported, and calculi did not recur.
4. Because a ferret on a high-quality diet has a urinary pH of approximately 6.0, urinary acidifiers are usually unnecessary.

Urolithiasis in rabbits and rodents

Urolithiasis is an important disease condition in chinchillas, guinea pigs, and particularly, rabbits.  Disease most commonly affects the bladder, but there have also been reports of stones in the urethra, ureter, and kidney.

Pathogenesis

Calcium-containing stones such as calcium carbonate (calcite) and calcium oxalate are most commonly reported. The rabbit is most frequently affected because of its unique calcium metabolism. High dietary calcium levels (i.e. alfalfa-based diets) lead to hypercalcemia.  Calcium levels that exceed bodily requirements are then excreted almost entirely by the kidney which can in turn lead to hypercalcuria. High urinary calcium levels can cause bladder sludge or sand and potentially stone formation.

The cause of urolithiasis in rodents also appears to be related to dietary calcium intake. A recent survey of 127 guinea pigs with urinary calculi by Hawkins et al found that 93% of calculi were composed of 100% calcium carbonate. Interestingly, although many guinea pigs were on antibiotics before urine culture samples were collected, Corynebacterium renale was isolated from 5 urine samples.

Clinical disease

Some, but not all owners, may report signs of stranguria or hematuria.  More commonly, clinical signs may reflect abdominal pain, such as a hunched posture or bruxism, or non-specific signs of disease such as anorexia, lethargy, and weight loss. On physical examination, a red, swollen prepuce may be identified. Although presentation may be acute, it is common for signs to progress over days to weeks in affected rabbits or rodents.

Diagnosis

• Obtain a complete history, including dietary history, from the owner. Consider urolithiasis in any rabbit or rodent with changes in their urinary output or with non-specific signs of illness.
• Laboratory findings may reflect a high calcium diet (hypercalcemia) or evidence of cystitis (crystalluria, hematuria, or leukosuria.
• Abdominal radiographs are also valuable in detecting bladder sand or stones.
• Abdominal ultrasound can help to confirm a presumptive diagnoses made using radiographs or it may be necessary to diagnose stones in the urethra, ureter, or kidneys.

Therapy

1. Cystotomy: If the rabbit or rodent is not obstructed, provide supportive care, including fluids, and then schedule cystotomy for surgical removal and analysis of the urolith. Approach culture/sensitivity testing and antibiotic therapy as listed above.
2. Dietary modification:  Reduce dietary calcium levels (i.e. switch to a grass-hay based diet).
3. Diuresis:  In rabbits with bladder sludge or sand, administer high volumes of crystalloids (i.e. 0.9% NaCl) to promote flushing of the bladder. Concurrent antibiotic therapy may also be indicated.

References