Neoplastic diseases similar to those found in other animals are found in fish. Their incidence frequently is higher in some geographic areas and in certain species. Some tumors are genetically mediated, such as the malignant melanoma of the gypsy-swordtail cross, and possibly the pseudobranch tumor of cod, thyroid tumors, malignant lymphosarcoma of northern pike, and fibromas or sarcomas of goldfish. The reported incidence of tumors in sharks, skates, and rays is low.

Gonadal tumors are important neoplastic disorders of koi. Typically fish present with a swollen abdomen, and depending on the severity of disease, there may be significant loss of condition. The presence of a mass can be confirmed with ultrasound. Biopsy of tissue may not offer a clear diagnosis. Laparotomy of affected fish often reveals a circumscribed mass of gonadal tissue. Fish that are not excessively debilitated are excellent surgical candidates for removal of the mass.

Lip fibromas have been reported in freshwater angelfish and a retrovirus has been seen via electron microscopy in affected tissues. The masses are easily removed in pet fish, but depopulation is recommended for fish in breeding populations.

Last full review/revision July 2011 by Ruth Francis-Floyd, DVM, MS, DACZM