Myasthenia Gravis Secondary to Thymoma in a Polar Bear (Ursus maritimus)
American Association of Zoo Veterinarians Conference 2001
Jeff Baier, MS, DVM; David E. Kenny, VMD; Felicia Knightly, DVM
Denver Zoological Foundation, Denver, CO, USA


A 15-year-old, 290-kg, female polar bear presented with weakness of the hind legs in June of 2000. The animal was able to bear weight on the hind limbs, but could not climb out of the pool in which she had been found. As the day progressed, the animal’s hind legs became weaker. The animal was no longer able to bear weight on the hind limbs by early afternoon.

The following day the bear was found in sternal recumbency and was exhibiting forelimb weakness. The animal was anesthetized using 5 mg/kg Telazol IM (Telazol, Fort Dodge, Fort Dodge, IA, USA) via pole syringe. Physical examination did not reveal the cause of the problem. A blood sample was obtained for complete blood count, serum chemistry, and serum banking. No abnormalities were detected. Lateral radiographs of the thoracolumbar spine were obtained using a portable x-ray unit. The films were of poor quality and not diagnostic. The animal was treated using 11 IU/kg vitamin E (Vital E-300, Schering-Plough Animal Health, Union, NJ, USA) IM, 1.1 mg/kg flunixin meglumine (Banamine, Schering-Plough Animal Health, Union, NH, USA) IM, 0.7 mg/kg dexamethasone (Dexamethasone Solution, Phoenix Pharmaceutical, Inc., St. Joseph, MO, USA) IM, 80,000 IU/kg procaine penicillin G SC (Pen-G, Phoenix Pharmaceutical, Inc., St Joseph, MO, USA) SC, and 1.7 mg/kg calcium gluconate (calcium gluconate 23% solution, Phoenix Pharmaceutical, Inc., St. Joseph, MO, USA). One half of the amount of calcium gluconate was given IV, the remainder was given SC. The bear’s condition was unchanged after recovery from anesthesia.

The animal’s condition remained unchanged the following day. A visual exam was performed by a veterinary neurologist later that day. A spinal lesion was suspected, but could not be confirmed by a complete neurologic exam. Plans were made to perform spinal radiographs, spinal tap and myelogram the following day.

The bear was anesthetized using 5 mg/kg Telazol IM via pole syringe. The animal was intubated and anesthesia maintained using isoflurane (IsoFlo, Abbot Laboratories, North Chicago, IL, USA). Fluids were administered throughout the procedure. A physical exam was repeated, again revealing no pertinent abnormalities. A blood sample was obtained for complete blood count, serum biochemistry, and serum banking. Lateral radiographs of the thoracolumbar spine were obtained. No spinal lesions were noted on these films. Numerous unsuccessful attempts were made to perform a spinal tap in the lumbar region. A successful spinal tap was performed at the cisterna magna. Twelve ml of cerebrospinal fluid were collected and utilized for cytology, total protein, and bacterial culture. No abnormalities were detected. A myelogram was then performed by a veterinary radiologist. Radiographs were taken of the thoracic and lumbar spine. Although the films were not high quality, no spinal lesions were evident. At this time, the decision was made to euthanatize the animal based on a poor prognosis and the difficulty associated with providing the nursing care for a polar bear.

A necropsy was then performed by a consulting pathologist. A mass 6.0 × 4.0 × 6.0 cm was located in the cranial mediastinum. The mass was para-aortic in location and consisted of friable, apparently necrotic material. A degenerative intervertebral disc was identified at L7/S1. The spinal cord in this area appeared edematous. No signs of traumatic injury could be identified.

Histopathologic examination of the mass led to a presumptive diagnosis of thymoma, spindle cell variant. Immunohistochemical stains confirmed the presence of a thymoma. Serum collected after the onset of clinical signs was submitted to be analyzed for acetylcholine receptor antibodies (Comparative Neuromuscular Laboratory, University of California, San Diego, La Jolla, CA, USA). In addition, a serum sample from the affected bear collected prior to the onset of signs and serum samples from two clinically normal female bears were also submitted. The sample collected from the affected polar bear after the onset of clinical signs exhibited a titer of 0.86 nmol/L. Utilizing canine or feline reference values this sample would be compatible with a diagnosis of myasthenia gravis. All other samples submitted would be considered to be within normal limits, using either reference range.


The authors would like to acknowledge the contributions of Steven Colter, DVM, Dan Steinheimer, DVM, DACVR, Alameda East Veterinary Hospital, Denver, CO, USA and David Getzy, DVM, DACVP, Idexx Veterinary Services, Inc., Broomfield, CO, USA.


Speaker Information
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Jeff Baier, MS, DVM
Denver Zoological Foundation
Denver, CO, USA

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