Abstract

Urolithiasis, specifically the presence of renal calculi, has been described previously in small cetaceans. While often subclinical, clinical presentations have included gross hematuria and non-specific signs referable to renal colic (including anorexia, abdominal splinting and genital rubbing, among others).

Diagnosis of urolithiasis in small cetaceans typically is based on the presentation of clinical signs and ultrasound plus/minus radiographic imaging. To assist in the determination of the chemical composition of the calculi, analysis of urine has been performed for measurements such as uric acid:creatinine ratios. Calculi that have been analyzed from postmortem sampling have been described as being uric acid in composition. Based on such studies, uric acid salts appear to be the most commonly described renal calculi in these animals.

In 1994, bilateral renal calculi were diagnosed in a mature female Pacific white-sided dolphin at the John G. Shedd Aquarium. Diagnosis was made based on clinical presentation, ultrasound imaging and urinalysis. Initially, treatment was pursued with 1) urinary alkalinizing agents, 2) xanthine oxidase inhibitors and 3) fluid administration to encourage diuresis.

Serial analyses of urine for supersaturation, with respect to uric acid (sodium acid urate, potassium acid urate and ammonium acid urate), under fasted and postprandial conditions have led us to theorize that urate transport mechanisms in the kidney of these animals cause rapid net urate secretion in the postprandial state. The net result is that uric acid moves quickly from the bloodstream into the urine, serum uric acid levels remain relatively low, and high urine supersaturation levels are reached episodically throughout the day. The magnitude of these super-saturations, and the tendency towards calculus deposition, appears to be inversely related to the frequency of feeds and directly related to the level of purine loading at each feed.

Based upon this work, we believe that manipulating the feeding schedule for these animals is critical to creating a chen-deal environment that is not conducive to the formation of uric acid calculi. Our model suggests that frequent, low-volume feedings spread over several hours will decrease the propensity for uric acid calculus formation. Ultimately, therapy directed toward either dissolution of renal calculi or prevention of this syndrome may, in fact, involve this model, in conjunction with components of the three-pronged therapeutic approach described above.

A survey has been developed and is being administered to ascertain the prevalence and significance of this syndrome in small cetaceans in zoological parks. In addition, we hope to further characterize the etiopathogenesis and subsequently develop effective treatment regimens for uric acid urolithiasis in small cetaceans.

Acknowledgements

The authors wish to thank the staff of the marine mammal department and laboratory at the Aquarium for their patience and support. In addition, Randy Randazzo M.D. and Teresa Robot affiliated with St. Francis Hospital in Evanston, IL, provided diagnostic support and consultation. The University of Chicago graciously donated the analytic services of the Nephrology laboratory.