Abstract

The application of radiology in the field of marine mammal medicine is advancing. Different modalities may be used individually or in combination for the ante- and post mortem diagnosis of disease. Furthermore, through the use of advanced imaging (magnetic resonance imaging and computed tomography) and 3-D reconstruction techniques, imaging can aid in the understanding of anatomy and the implications of lesions. Herein we describe three interesting cases in which radiology was used to help reach a diagnosis in both alive and deceased marine mammals.

A neonatal harbor seal (Phoca vitulina) pup presented to The Marine Mammal Center (TMMC), Sausalito, CA with head tremors and muscle fasciculations. Neurological signs progressed over a period of two weeks resulting in the seal's inability to right itself from lateral recumbency, inability to swim and decreased levels of consciousness. Magnetic resonance imaging was performed after serum biochemistry, hematology, serology (Toxoplasma gondii, West Nile Virus) and empirical medical treatment failed to elicit a diagnosis. Imaging findings demonstrated an abnormal occipital bone, cerebellar herniation and dorsal deviation of the dens of the atlas causing spinal cord compression. Histopathology confirmed gliosis of the cerebellum and axonal degeneration and dilation of myelin sheaths of the dorsal funiculus of the spinal cord consistent with the imaging findings.

An emaciated pilot whale (Globicephala melas) stranded in Cape Cod, MA. During necropsy evaluation a firm bulbous swelling was noted circumferentially bridging the intervertebral disc space between the fused cervical and 1^st thoracic vertebrae and the 2^nd and 3^rd coccygeal vertebrae. Computed tomography was used to image the spine prior to further dissection. At both locations there was mineralization of the annulus and nucleus of the intervertebral disc. In addition, at the caudal lesion site there was collapse of the disc space and circumferential protrusion of the annulus was identified. The caudal spine was sectioned and gross observations confirmed the imaging findings. This finding may be similar to bone pathology previously described in this species.²

Stranded, free-ranging, young California sea lions (Zalophus californianus) periodically present to TMMC following jaw trauma. There is frequently a comminuted fracture of the mandible. Computed tomography can be used to assess the fracture for the purposes of surgical planning if appropriate, or to document catastrophic trauma. Three-dimensional reconstructions are extremely useful through this region in order to appreciate the extent of the damage and determine prognosis. Assessment of damage to associated structures such as the mandibular canal and the teeth can be made by comparison to normal CT anatomy of this region in this species.¹

To enhance the use of radiology in marine mammal medicine, we introduce http://www.marinemammalradiology.com, a website that has been developed as a free source to those working in the field of marine mammal medicine. The goals of the website are to provide examples of normal diagnostic imaging studies as well as to showcase cases of interest to the marine mammal professional community and to provide information on the appropriate application of different diagnostic imaging modalities for the diagnosis of disease.

Acknowledgements

We thank the veterinary staff and volunteers at The Marine Mammal Center; Dr. Darlene Ketten, Julie Arruda and Scott Cramer and WHOI CSI; University of Wisconsin radiology techs and faculty; WHOI Guest student program.

References

1.  Dennison S.E., and T. Schwarz. 2008. Computed tomographic imaging of the normal immature California sea lion head (Zalophus californianus). Vet Rad Ultr 49(6): 557-63.

2.  Sweeny M.M., M.P. Price, G.S. Jones, T.W. French, G.A. Early and M.J. Moore. 2005. Spondylitic changes in long-finned pilot whales (Globicephala melas) stranded on Cape Cod, Massachusetts, USA, between 1982 and 2000. J Wildl Dis 41(4): 717-727.