Abstract

Ammonium urate (AAU) nephrolithiasis has been reported in bottlenose dolphins to be associated with increased morbidity.^1,2 A case-control study was performed with dolphins cared for by the Navy Marine Mammal Program to investigate potential differences in urinary biomarkers of nephrolithiasis. Control animals had no history of renal azotemia or ultrasound evidence of nephrolithiasis (n = 4). Case animals had histories of renal azotemia and ultrasound evidence of nephrolithiasis (n = 4; CT confirmation in 3). After an overnight fast, spot urine specimens were obtained. Following sampling, animals were fed one-third of their daily ration and then re-sampled two hours later. At fasting state, urinary pH and ammonium (NH₄⁺) did not differ between the case and control animals, however, urinary uric acid (UA) was higher in the case than in control animals (p = 0.043). Postprandially, urinary pH and UA increased with the same magnitude in both groups. Urinary NH₄⁺ rose modestly in the case animals, but did not change in the controls. With the elevated urinary UA, the fasting supersaturation of AAU was higher in case dolphins by both the supersaturation index (SI) and the relative supersaturation ratio (RSR). After feeding, the urinary SI of UA and AAU further increased, as did the RSR of AAU. These results suggest that the rise in urinary pH postprandially, accompanied with significant rise in urinary UA, increases the urinary supersaturation with respect to most insoluble AAU in the urinary environment in dolphins and increases the risk of AAU stone formation.

Acknowledgments

The authors are grateful to the Office of Naval Research for funding this work (ONR Award N000141110203) and to the Navy Marine Mammal Program management team for their support. Special thanks to Kathy Hill, Paulette Padalino, Randall Dear, Brenda Bauer, Elaine Allen, Veronica Cendejas, Mark Baird, and Kevin Carlin, who provided essential technical support.

* Presenting author

Literature Cited

1.  Venn-Watson S, Smith CR, Daniels R, Townsend F. 2010. Clinical relevance of urate nephrolithiasis in bottlenose dolphins (Tursiops truncatus). Dis Aqua Org 89: 167–177.

2.  Venn-Watson S, Townsend FI, Daniels RL, Sweeney JC, McBain JW, Klatsky LJ, Hicks CL, Staggs LA, Rowles TK, Schwacke LH, Wells RS, Smith CR. 2010. Hypocitraturia in common bottlenose dolphins (Tursiops truncatus): Assessing a potential risk factor for urate nephrolithiasis. Comp Med 60:149–153.