Hypocitraturia in Common Bottlenose Dolphins (Tursiops truncatus): Assessing a Potential Risk Factor for Urate Nephrolithiasis

Numerous cases of urate nephrolithiasis in managed collections of common bottlenose dolphins (Tursiops truncatus) have been reported, but nephrolithiasis is believed to be uncommon in wild dolphins. Risk factors for urate nephrolithiasis in humans include low urinary pH and hypocitraturia. Urine samples from 94 dolphins were collected during April 2006 through June 2009 from 4 wild populations (n = 62) and 4 managed collections (n = 32). In addition, urine uric acid and pH were tested in a subset of these animals. Our null hypothesis was that wild and managed collection dolphins would have no significant differences in urinary creatinine, citrate, and uric acid concentrations and pH. Among urine samples from all 94 dolphins, the urinary levels (mean ± SEM) for creatinine, citrate, uric acid, and pH were 139 ± 7.6 mg/dL, 100 ± 20 mg citrate/g creatinine, 305 ± 32 mg uric acid/g creatinine, and 6.2 ± 0.05, respectively. Of the 4 urinary variables, only citrate concentration varied significantly between the 2 primary study groups; compared with wild dolphins, managed collection dolphins were more likely to have undetectable levels of citrate in the urine (21.0% and 81.3%, respectively). Mean urinary citrate concentrations for managed collection and wild dolphin populations were 2 and 150 mg citrate/g creatinine, respectively. We conclude that some managed collections of dolphins, like humans, may be predisposed to urate nephrolithiasis due to the presence of hypocitraturia. Subsequent investigations can include associations between metabolic syndrome, hypocitraturia, and urate nephrolithiasis in humans and dolphins; and the impact of varying levels of seawater ingestion on citrate excretion.