Diet & Stone Formation in Ferrets

An 8-month-old female ferret presented with hematuria and stranguria of 8 days' duration. The animal's diet had been changed to an unknown commercial ferret food within the past 2 months. For 6 months before this change, the diet had consisted of a dry kitten food with lower levels of protein, fat, and calcium than are recommended for ferrets. Upon physical examination, the abdomen was painful and a small, hard, round structure was palpable caudally. A free-catch urine sample was obtained and urinalysis conducted. Relevant results were as follows: specific gravity, 1.032; pH, 7; urinary protein, 100 mg/dL (++); occult blood, 80 red blood cells/mL (++); and sediment with 2 to 3 epithelial cells per high-powered field (HPF), 5 to 6 struvite crystals/HPF, 5 to 6 erythrocytes/HPF, and a few bacteria. Urine culture was not performed. Blood analysis findings were within the normal reference range. Radiographs showed a radiopaque urolith in the urinary bladder. Cystotomy was performed to remove the urolith, which was 60% struvite and 40% calcium oxalate dihydrate. Struvite urolithiasis is a frequent condition in ferrets fed a low-protein diet. As an obligate carnivore, ferrets should be fed a meat-based high-protein diet, and the urine pH should be acidic (< 6). In some cases, ferrets fed an appropriate diet can also develop struvite urolithiasis due to kidney disease or bacterial infections caused by urease-producing bacteria. In the case described, the struvite component of the urolith could be related to a previous poor-quality diet and a possible dietary imbalance, combined with a possible urinary tract infection. An explanation for the presence of calcium oxalate is more difficult. In humans with a normal excretion of citrate and a pH of 6 or more, the presence of hypercalciuria favors the formation of calcium oxalate dihydrate. The authors therefore hypothesize that the ferret described in this case was deficient in some inhibitors of crystal formation, aggregation, or both, predisposing it to the development of a mixed urolith.

COMMENTARY: This is an interesting case-struvite urinary calculi are rarely seen in ferrets today because of improvements in commercial diets. The authors state that the struvite portion of the calculus was probably due to a 6-month history of a diet with lower levels of protein, fat, and calcium. They then hypothesize that the calcium portion of the calculus may have been due to a deficiency in inhibitors of crystal formation or aggregation, or both. However, since the ferret was fed a different, unknown diet for 2 months before presentation, the influence of this diet on the calcium stone formation cannot be absolutely discounted. In domestic cats, diets with lower protein, sodium, potassium, moisture, calcium, phosphorus, and magnesium are linked with calcium-containing calculi. This case emphasizes the importance of diet composition in reducing risk for urolithiasis in the pet ferret.Mixed urolith (struvite and calcium oxalate) in a ferret (Mustela putorius furo). Del Angel-Caraza J, Chávez-Moreno O, García-Navarro S, Pérez-García C. J VET DIAGN INVEST 20:682-683, 2008.