Abstract

Bottlenose dolphins (Tursiops truncatus) under human care are susceptible to ammonium urate (NH₄U) kidney stone formation. Kidney stone disease in dolphins can cause significant morbidity attributed to azotemia, hematuria, renal obstruction, reduced renal function, and renal failure.¹⁵ Diagnosis and quantification of kidney stones in bottlenose dolphins is best achieved using a combination of ultrasound and computed tomography (CT).^7,9 CT with IV contrast as well as renal scintigraphy have been successfully utilized to assess renal perfusion, function and renal obstructions in dolphins.^7,10

Medical management of cases with partial ureteral obstruction has recently been accomplished with aggressive fluid support and low-dose tamsulosin.⁸ If the problem worsens or complete obstruction develops, surgical intervention may be required. Surgical intervention of ureteral obstruction has been successfully performed using laser lithotripsy in two adult female dolphins.^6,8,12 In male dolphins, ureteroscopy has not been successful due to the unique urethral anatomy, including an extreme curvature and excessive length.¹²

Recent investigations of ammonium urate stone solubility combined with challenges associated with allopurinol therapy suggest that stone dissolution may not be possible in dolphins.³ Instead, current therapy is aimed at managing clinical evidence of disease and preventing future stone formation. Hydration therapy is important to increase urine production, subsequently decrease saturation of urine crystals, and improve renal azotemia.¹⁴ Additionally, various formulations of potassium-citrate and sodium-citrate have been administered orally, aiming to alkalinize the urine with limited success.^13,15,16

Comprehensive studies of both free-ranging and managed dolphin populations have been conducted to identify risk factors for stone formation, revealing differences in urinary physicochemistry and renal stone prevalence.^4,9,11 The risk for NH₄U stone formation in dolphins may be due, in part, to a marked increase in urinary ammonium (NH4⁺), uric acid, and pH following a large fish meal, thus creating a favorable urinary environment for the precipitation of NH₄U salt.⁹ Further, nutrient analyses of fish species comprising the diets of free-ranging versus managed dolphin populations have also revealed differences in key nutrients, including mineral cation and anion concentrations and purine content, which may increase the risk of stone formation in managed dolphins.^1,2 Recent modified diet feeding trials, targeting a lower purine composition and acid-ash content, have demonstrated a dietary impact on improving urinary risk factors, implicating the role of nutrition in the pathophysiology of stone formation.⁵

Current best practices for management of kidney stone disease in dolphins consist of a combination of hydration, diet, and treatment aimed to alkalinize the urine and reduce the risk factors associated with stone formation. Further dietary and treatment optimization is currently underway.

Acknowledgments

We acknowledge the ONR grants that supported this work (N00014-15-1-2131 and N00014-15-1-2327). The authors also sincerely thank all of the veterinarians, technicians, trainers, and research team members who assisted in acquiring the samples and data for this project.

Literature Cited

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