Comparison of potential dietary and urinary risk factors for ammonium urate nephrolithiasis in two bottlenose dolphin (Tursiops truncatus) populations

Dietary and urinary risk factors have been implicated in conditions favoring ammonium urate nephrolithiasis in managed dolphins compared with free-ranging dolphins. In this study, urine samples were collected from 16 dolphins (8 cases, 8 controls) from the U.S. Navy Marine Mammal Program for the purposes of assessing changes in urinary biomarkers after a large meal. Urinary biomarkers and nephrolithiasis presence were assessed opportunistically in 15 long-term resident free-ranging dolphins living in Sarasota Bay, Florida. Additionally, the total purine contents of fish commonly consumed by each dolphin population were measured to evaluate potential dietary risk factors. Populations were compared for total dietary purine composition, recently fed status, nephrolithiasis presence, and differences in urinary biochemical, acid-base, and physicochemical parameters via Wilcoxon rank sum analysis and least square means. Managed dolphins had higher urinary pH and ammonium (NH⁺ ₄) in both pre- and postprandial conditions and higher urinary uric acid and saturation indices of NH4U in the postprandial condition compared with free-ranging dolphins (P < 0.05). The purine content was greater (P < 0.0001) in the diet consumed by managed dolphins [7 mmol/Mcal metabolizable energy (ME)] than in the free-ranging dolphin diet (4 mmol/Mcal ME). Free-ranging dolphins did not show evidence of nephrolithiasis. Observed differences in urinary biomarkers and dietary purine content in these two dolphin populations suggest a pathophysiologic basis for the role of fish types on the risk of NH₄U stone formation. Future research should investigate fish type and feeding frequency, inhibitors and promoters, and alkalinizing therapy for reducing NH₄U nephrolithiasis in dolphins.