Use of Ultrasonography to Establish a Technique for Hepatobiliary Imaging and to Evaluate Metabolic Disease-Associated Liver Changes in Bottlenose Dolphins (Tursiops truncatus)
Abstract
Bottlenose dolphins (Tursiops truncatus) are susceptible to metabolic diseases that affect the liver, including insulin resistance, hyperlipidemia, hemochromatosis (iron overload) and fatty liver disease.1,2 Ultrasound is a diagnostic imaging modality used frequently in marine mammal medicine and a basic technique for evaluation of the liver has been previously reported.3 However, a more detailed technique is needed to evaluate subtle changes and disease states. This study aimed to (a) establish a comprehensive technique for ultrasound examination of the dolphinhepatobiliary system; (b) apply this technique to 30 managed dolphins; and (c) determine what, if any, sonographic changes were associated with specific blood changes, including elevated 2h post-absorptive glucose (≥ 112 mg/dl), insulin (≥ 15 µIU/ml), triglycerides (≥ 186 mg/dl), liver enzymes (≥ 46 and 294 U/L ALT and AST), and transferrin saturation (≥ 66%). An in-water ultrasound technique was developed, which included measuring the distance from the caudal liver edge to the umbilicus (- or +; a proxy for liver size); bile duct diameter, portal and hepatic vein diameters; and periportal pancreatic thickness. Dolphins with elevated glucose, insulin, and triglycerides had larger livers compared to dolphins with non-elevated levels. Using stepwise, multivariate regression, glucose was the best predictor of liver size (p = 0.005). There was a positive linear association between glucose levels and liver size (R2 = 0.25; p= 0.005). Elevated insulin dolphins had a thinner pancreas compared to non-elevated insulin dolphins (2.25 and 3.02 cm, respectively; p= 0.046). Dolphins with elevated triglycerides had smaller hepatic veins compared to those with non-elevated triglycerides (2.17 versus 2.74 cm, respectively; p= 0.02). In conclusion, this study facilitated the establishment of a much-needed technique for a rapid, diagnostic, and non-invasive evaluation of the dolphin liver using ultrasonography. In addition to this technique, the study led to the understanding that hepatic and pancreatic changes are associated with metabolic perturbations in dolphins, and these changes are detectable using ultrasonography. Future investigation will strive to understand the pathophysiological explanations for these changes.
Acknowledgments
The authors wish to thank the dedicated and skilled veterinarians, veterinary technicians and animal trainers of the United States Army, Navy Marine Mammal Program and the National Marine Mammal Foundation for their technical and logistical support of this project. The authors are grateful to the Morris Animal Foundation and the IAAAM Medway Scholarship program for assisting the student author with funding.
* Presenting author
+ Student presenter
Literature Cited
1. Venn Watson S, Benham C, Carlin K, et al. Hemochromatosis and fatty liver disease: building evidence for insulin resistance in bottlenose dolphins (Tursiops truncatus). Journal of Zoo and Wildlife Medicine. 2012;43(3 Suppl):S35–S47.
2. Venn Watson S, Carlin K, Ridgway S. Dolphins as animal models for type 2 diabetes: sustained, post-prandial hyperglycemia and hyperinsulinemia. General and Comparative Endocrinology. 2011;170(1):193–199.
3. Brook F, Vann Bonn W, Jensen E. Ultrasonography. In: CRC Handbook of Marine Mammal Medicine. Boca Raton, FL: CRC Press LLC; 2001:593–620. Print.