Ultrasonagraphic Evaluation of Thyroid Size and Morphology in Captive Beluga Whales, Delphinapterus leucas and Pacific White-sided Dolphins, Lagenorhynchus obliquidens
Abstract
The study of metabolic physiology is necessary to understand endocrine control and is fundamental in ensuring the health and survival of any species;9 in particular, the mammalian thyroid gland serves a crucial role in the synthesis and storage for both thyroxine (T4) and tri-iodothyronine (T3), hormones which are important mediators of metabolism, by maintaining homeostasis in response to environmental demands and normal tissue function.3,7 Beluga whales and Pacific white-sided dolphins are two common cetaceans exhibiting various seasonal activities. Previous reports revealed that seasonality and possibly life history stages may alter thyroid function and morphology. Important variables such as demographic factors, different states of reproductive physiological status in female subjects, health status, nutritional status, stress and special events (e.g., molting in beluga during summer) must be taken into account while evaluating the dynamic changes of thyroid function and morphology that occur in association with significant life history events in these two species. In addition, environmental contaminants and local environmental influences have been implicated as the cause of thyroid hormone imbalances1 and the development of detectable morphological and histological abnormalities.2,6,8 Mikaelian and his team6 reported that 17 out of 30 beluga whale specimens collected from two individual populations from the highly contaminated waterways of St. Lawrence Estuary and Hudson Bay in Canada were found to have hyperplastic and cystic thyroid lesions, which correlated with the high tissue levels of chemical contaminants. To the best of our knowledge, the formal literature is devoid of any reference to diagnosis of thyroid gland abnormalities in living beluga whales and Pacific white-sided dolphins. In order to accurately diagnose and assess thyroid abnormalities in live animals, reliable methods of assessing the normal gland morphology and volume, as well as knowledge on possible influence of various demographic and physiological factors, must be established to serve in correlation with biochemical and clinical data.
Ultrasound is a useful imaging tool in the assessment of thyroid morphology and physiology in humans4 and companion animals.10 Reliable standard ultrasonographic imaging protocol for thyroid assessment in a group of Indo-Pacific bottlenose dolphins (Tursiops aduncus) was developed.5 This study was undertaken to gather information on the thyroid gland of the beluga whale (Delphinapterus leucas) and Pacific white-sided dolphin (Lagenorhynchus obliquidens), by documenting the normal ultrasonographic characteristics and volume of the thyroid gland using the established protocol of the previous dolphin thyroid study.
All ultrasound examinations were performed with a SonoSite 180 Plus ultrasound unit in conjunction with a 2-5 MHz broadband curved array 2-D transducer. The thyroid morphology and volume were evaluated by ultrasound following the established method.5 A total of 19 individual scans were conducted in 13 D. leucas and 40 individual scans were conducted in 15 L. obliquidens.
The thyroid of the Pacific white-sided dolphin was located at the neck region, cranial to the thoracic inlet and midway between the insertions of the pectoral fins. In beluga whales, the thyroid was located further cranial at the submental triangle, proximal to the region of neck fold and midway between the insertions of the pectoral fins.
The borders of the thyroid in these two species were usually well-defined, whereas ill-defined borders were observed in five beluga whales. In the 13 beluga whales, the echogenicity of the thyroid tended to be isoechoic (n = 8, 61.5%) or hypoechoic (n=5, 38.5%), when compared to the neck blubber. In the 15 Pacific white-sided dolphins, the echogenicity of the thyroid was varied from hyperechoic (n = 2, 13.3%), isoechoic (n=10, 66.7%) to hypoechoic (n = 3, 20%) when compared to the adjacent muscles. Within a single subject, varied echogenicity was observed among different sections of the thyroid lobes.
The echopattern of the thyroid was generally homogeneous in Pacific white-sided dolphins with echogenic reticulations present in all thyroids. Hypoechoic and isoechoic foci were observed in six subjects. In beluga subjects, thyroids were usually heterogeneous, with coarse echopattern and showed echogenic reticulations with various degrees of lobulation. Further investigation is needed to determine whether these ultrasonographic characteristics can be considered as natural occurrences, or as pathological changes due to endocrine-disrupting chemicals in their original captured sites.6
The thyroid volumes of beluga whale and Pacific white-sided dolphin ranged from 351.40 cm3 to 740.29 cm3 (mean 541.97 cm3 ± 118.81 cm3) and 12.56 cm3 to 68.52 cm3 (mean 26.49 cm3 ± 11.36 cm3) respectively.
Preliminary results of this study demonstrated that ultrasound can be used to evaluate the morphology and volume of the beluga whale and Pacific white-sided dolphin thyroid gland. Substantial variations of thyroid morphology and volume were documented in both species. Investigation is still on-going to assess the possible association of thyroid morphology and volume on different demographic factors and different seasons due to the unique life history in both species.
Acknowledgements
The authors are indebted to the veterinarians, curators, trainers and veterinary technicians at all three collaborating facilities whose contribution and devotion made this project possible. This project was funded by The Hong Kong Polytechnic University Research Student Attachment Programme.
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