Abstract

Despite the prevailing misconception, sharks do get cancer. At least 24 cases have been documented in four orders, six families, and 15 different species.¹ Most reported cases were from wild-caught individuals, and with rare exceptions, diagnosed on postmortem examination.² The current case involves a neoplasm diagnosed antemortem in a captive bonnethead shark, Sphyrna tiburo (Linnaeus), representing a genus and family (Sphyrnidae) in which neoplasia has never been reported. In the spring of 2010, a 7.3 kg, 116 cm total length, mature female bonnethead shark was observed with several presumed bite wounds on the bonnet and pectoral fins. Medical treatment was initiated, and over the next month, lesions regressed though failed to completely resolve. Later that year, the shark developed proliferative lesions in affected regions. An incisional biopsy was collected, preserved in 10% neutral buffered formalin, routinely processed, sectioned at 5 µm, and stained with hematoxylin and eosin for light microscopic examination. Sections contained a poorly demarcated, unencapsulated, and infiltrative dermal mass composed of pleomorphic neoplastic round cells arranged in dense sheets. The neoplasm had regions of erosion, ulceration, and necrosis. Subsequent (excisional) biopsies were histologically similar. Clusters of neoplastic cells within small thin-walled vessels provided evidence of lymphatic invasion. Immunohistochemistry was pursued in an attempt to better characterize the neoplastic round cells. Sections were evaluated for expression of lysozyme (histiocyte marker), CD3 (T lymphocyte marker), CD79a and CD20 (B lymphocyte markers) using mammalian antibodies and archival normal bonnethead shark tissues as controls. Round cell morphology within routine sections combined with distinct cytoplasmic CD79a and CD20 immunolabeling of the neoplastic cells were compatible with a neoplasm of B lymphocyte origin. To the authors' knowledge, this represents the first report of a neoplasm in a bonnethead shark and first demonstrated validation and optimization of mammalian-derived antibodies as tumor markers in a shark species.

Acknowledgements

The authors wish to thank the aquarists at Atlantis for the care of the shark during treatment and the University of Illinois Veterinary Histology Laboratory for expertise and assistance with immunohistochemistry.

References

1.  Ostrander GK, Cheng KC, Wolfe JC, Wolfe MJ. Shark cartilage, cancer and the growing threat of pseudoscience. Cancer Research 2004; 64: 8485–8491.

2.  Borucinska JD, Harshbarger JC, Reimschuessel R, Bogicevic T. Gingival neoplasms in a captive sand tiger shark, Carcharias taurus (Rafinesque), and a wild-caught blue shark, Prionace glauca (L.). Journal of Fish Diseases 2004; 27: 185–191.