Abstract

In February 2001, a Kannume (Mormyrus kannume) from the Lake Victoria exhibit at New England aquarium presented with a dermal mass of unknown origin and duration. This fish was placed in the exhibit one year prior to presentation with no history of prior illness or lesions. This fish was from a group of 25 wild-caught mormyrids that completed a standard 30-day quarantine before being placed on exhibit, which include skin scrapes, gill clips, visual inspection, and prophylactic treatments. On presentation the fish appeared healthy except for a 1.5 x 1.2-cm mass on the left side of the proboscis, which had normal pigmentation, an irregular smooth surface and did not appear to affect the fish's appetite or attitude. Fine-needle aspirate of the mass was performed, and microscopic findings suggested possible epithelial hyperplasia or neoplasia. A blood sample revealed a PCV of 34 and plasma protein of 3.5.

At surgery the mass extended deep into the underlying tissue and bone. The mass was de-bulked and the tissue submitted for histopathology. One-week later, liquid nitrogen was used to freeze the prior surgical site since complete margins were not possible due to the invasiveness of the lesion. Histology was consistent with the diagnosis of squamous cell carcinoma. A poor prognosis was given based on a high mitotic rate of greater than three mitoses per 400x.

Two weeks later a slight swelling at the caudal aspect of the surgical site was noted and even though the rest of the wound appeared to be healing, this area gradually grew in size over the next month. Two months after the initial surgery, the new mass was de-bulked followed with cryosurgical freezing of the lesion. Over the next five months the area appeared to heal completely and there was no evidence of reoccurrence of the tumor. A follow up biopsy of the area was planned, but unfortunately the Kannume became ill and was euthanized. Necropsy revealed no significant findings except a tiny bump below the left eye. Further histopathology identified squamous cell carcinoma infiltrating sections of bone, skeletal muscle and skin from the left periorbital area.

Although uncommon, squamous cell carcinomas have been reported in a variety of fish involving the oral cavity and tissues. In some animals with oral squamous cell carcinoma, aggressive surgery including the removal of underlying bone and possible chemotherapy is recommended. In this case, even though there was no obvious gross observation of the tumor for five months, there was still microscopic evidence of the neoplasia on necropsy. This case shows that aggressive surgical treatment is possible in fish and that advances in oncology may be successfully applied in aquatic animal medicine.

Acknowledgements

Robert Cooper, Kristen Dube, Katarina Peterson, Casey Sugerman and the volunteers and staff at The New England Aquarium. Dr. Mac Law, North Carolina State University, College of Veterinary Medicine, Raleigh, NC. Dr. Sal Frasca, Connecticut Veterinary Diagnostic Laboratory, University of Connecticut, Storrs, CT