Abstract

An approximately 29 year old, 175 pound, female, captive, wild-born California sea lion, Zalophus californianus, presented with a firm mass palpable beneath the right caudal mammary gland nipple, approximately 6.5 cm in diameter. An initial voluntary fine-needle aspirate was performed revealing epithelial hyperplasia/neoplasia with moderate atypia. One week later, a voluntary biopsy via super core needle was collected using local anesthesia. Histopathologic finding was mammary adenocarcinoma. Surgical removal was recommended. She did not show any signs of clinical illness and bloodwork was unremarkable, hypercalcemia was not seen. Four months later a board certified veterinary surgical oncologist performed a partial mastectomy, removing an approximately 10 cm diameter mass nearly directly beneath the right caudal mamilla. Complete mastectomy was not possible due to the lack of readily identifiable organized glandular tissue. Due to the close approximation of the subcutis/neoplasm to the body wall, the external fascial sheath was also removed to preserve a margin of normal tissue. The vasculature encountered were direct myocutaneous type with no apparent axial mammary vessels as seen in carnivores. Complete excision of the mass was accomplished as a margin of normal tissue surrounding the cancerous tissue was identified with post-operative dissection. Abdominal ultrasound and thoracic radiographs revealed no apparent abdominal abnormalities or pulmonary metastasis. Mild complications, including anorexia and bilirubinemia, resolved one week post-operation when she was moved from a dry-dock enclosure to a free-swimming, open pool enclosure.

Histopathologically, the mammary mass was composed of encapsulated islands of neoplastic epithelial cells surrounded by abundant elongate myoepithelial cells occasional islands of cartilage and bone: the neoplasm was diagnosed as complex mammary adenocarcinoma. Capsular invasion and suspect lymphatic invasion was present, increasing the risk of systemic metastasis. Complex carcinomas consisting of malignant epithelium and myoepithelium occur predominantly in the dog, thus making the dog a good model for the sea lion.² Two distinctive mammary carcinomas have been previously reported in a single California sea lion, one with and without myoepithelial cell proliferation. Both masses metastasized to regional lymph nodes; however myoepithelial neoplastic cells were also seen in distant viscera.¹ That individual died 1.5 months after the onset of clinical disease. In dogs, tumors with extensive myoepithelial differentiation tend to be lower grade and less aggressive.² Due to the nature of the sea lion mammary neoplasms, continued observation for metastasis was recommended. Post-operative chemotherapy was not pursued. Since the effects of progestins in mammary disease have been well documented, it was recommended to limit exposure to synthetic progestins, such as medroxy-progesterone acetate (MPA) and megesterol acetate (MA) in this individual.²

This is the first report of surgical intervention to treat mammary tumors in sea lions. Neither a recurrence of the mammary mass nor clinical evidence of metastasis has been detected nine months post-operatively. Future studies include detailed evaluations of the sea lion mammary gland anatomy to aid surgeons in performing complete mastectomy and characterizing histological grade and outcome of more cases.

Acknowledgements

The authors wish to thank the trainers at Florida's Gulfarium for assisting in collecting diagnostic samples and providing compassionate and high quality post-operative care. Gulf World is acknowledged for providing the surgical suite.

* Presenting author

Literature Cited

1.  Matsuda M, Hashiura S, Une Y, Sirouzu H, Nomura Y. 2003. Two distinct carcinomas of mammary gland origin in a California sea lion. J Wildl Dis 39: 241–243.

2.  Munson L, Moresco A. 2007. Comparative pathology of mammary gland cancers in domestic and wild animals. Breast Disease 28:7–21.