Overview and Clinical Importance
Neoplasia in fish represents an important multisystemic disease category that is increasingly recognized in aquatic veterinary practice. Fish develop tumors similar to those found in higher vertebrates, with both benign and malignant forms occurring across multiple organ systems. Neoplastic diseases occur with some frequency in both wild and captive fish populations, with the skin being the most commonly affected site. Among ornamental fish, koi (Cyprinus rubrofuscus) and goldfish (Carassius auratus) are the most frequently diagnosed species with neoplasia, representing approximately 18.5% and 12.5% of all fish cancer cases respectively.
In bony fishes, connective tissue neoplasms such as fibroma and fibrosarcoma are among the most common tumor types. The mean age at presentation for fish with neoplasia is approximately 8.25 years (99.45 months), with a range of 12 to 300 months. Soft tissue sarcoma is the most prevalent histologic diagnosis, accounting for 26.4% of all cases, followed by chromatophoromas (pigment cell tumors) and gonadal tumors.
Etiology and Pathophysiology
Causative Factors
Known and suspected factors contributing to neoplasia in fish include genetic predisposition, viral infections, environmental carcinogens, chronic inflammation, parasites, and UV radiation. Some tumors are genetically mediated, such as the malignant melanoma of the platy-swordtail cross. Retroviruses have been associated with several fish neoplasms including walleye dermal sarcoma, lymphosarcoma in northern pike, and swim bladder fibrosarcoma in Atlantic salmon.
Etiologic Factors by Tumor Type
Classification of Fish Neoplasms
Fish tumors are classified similarly to mammalian neoplasms based on tissue of origin. Benign tumors (lipomas, fibromas, seminomas) cause local pathology such as irritation or compression but do not metastasize. Malignant tumors (sarcomas, carcinomas) may cause systemic disease through metastasis or metabolic dysfunction.
Fibrosarcoma in Goldfish
Fibrosarcoma is a malignant mesenchymal tumor of fibroblast origin that is well-documented in goldfish (Carassius auratus). While cutaneous fibromas are not uncommon in fish, fibrosarcomas are less frequently reported but represent an important differential diagnosis for cutaneous masses in goldfish. These tumors are commonly associated with viral infections and chemical agents in wild and laboratory fish.
Clinical Presentation
Goldfish with fibrosarcoma typically present with exophytic, firm, non-ulcerated cutaneous masses that are dull white in appearance with a smooth surface. Masses may be located anywhere on the body but commonly occur near the dorsal fin, caudal peduncle, or flanks. Multiple tumors may be present. The cut surface appears homogeneous and white, with tissue arranged in whorled bundles.
Histopathologic Features
- Proliferating spindle cells in interlacing, loose bundles
- Eosinophilic fibrillar matrix (collagen)
- Nuclear pleomorphism and mitotic figures (indicating malignancy)
- Masson's trichrome stain: Blue staining collagen fibers
- Invasion into underlying musculature and adjacent tissues
Differential Diagnosis for Spindle Cell Tumors in Goldfish
Other Common Fish Neoplasms
Chromatophoromas (Pigment Cell Tumors)
Chromatophoromas are neoplasms of dermal pigment cells originating from the neural crest. They are classified according to their pigment cell origin: melanophoroma (melanoma, black pigment from tyrosine), erythrophoroma (red pigment), xanthophoroma (yellow pigment), and iridophoroma (guanophoroma, reflective purine crystals). In koi and goldfish living in outdoor ponds, chromatophoromas commonly develop on the dorsal aspect of the eyeballs or around dorsal fins. UV radiation exposure is a suspected contributing factor.
Treatment: Cryotherapy has been used successfully for ocular chromatophoromas. Advanced cases with globe involvement may require enucleation (eye removal).
Gonadal Tumors
Gonadal tumors are important neoplastic disorders reported primarily in koi and northern pike. Female koi aged 8-12 years are particularly susceptible. Patients typically present with progressive coelomic (abdominal) distension, asymmetric body shape, and loss of condition. Ovarian tumors are most common (predominantly sex cord-stromal tumors including thecoma), while testicular tumors (seminoma) are rare but reported.
Clinical Signs of Gonadal Tumors
- Progressive abdominal distension (may be mistaken for egg-binding)
- Asymmetric body shape ("lopsided" appearance)
- Buoyancy disorders and swimming abnormalities
- Weight loss and poor body condition
- Lepidorthosis (raised scales) from cutaneous edema in advanced cases
Peripheral Nerve Sheath Tumors (PNSTs)
PNSTs including neurofibroma and schwannoma are commonly reported in goldfish. Externally visible tumors in goldfish are almost always peripheral nerve sheath tumors, appearing as raised, pale, lobulated cutaneous masses. These are typically benign and do not significantly impact quality of life despite poor aesthetics. Unlike fibrosarcomas, PNSTs are positive for S100 protein on immunohistochemistry and contain areas rich in mucin (positive with Alcian blue staining).
Diagnostic Approach
A definitive diagnosis of neoplasia in fish is currently limited to biopsy or necropsy followed by histopathology. Cytologic examination of impression smears or fine needle aspirates may provide preliminary information but is less reliable due to the peculiarities of piscine cytology, including normal findings such as diatomaceous algae on seahorse integument, alarm cells in epithelial depths, and circulating mitotic figures.
Diagnostic Modalities
Treatment Options
Treatment of fish neoplasia is limited compared to mammalian oncology. Only 2.9% of fish tumors in one large study received any form of treatment. Surgical excision is the most commonly performed and effective treatment, with a mean post-treatment survival time of 8.85 months. Fish that are not excessively debilitated are excellent surgical candidates.
Anesthesia for Fish Surgery
Fish are anesthetized using water-based immersion agents. Tricaine methanesulfonate (MS-222, Tricaine-S, Syncaine) is the only FDA-approved anesthetic for food fish in the United States. It is dissolved in water at 50-200 mg/L (species-dependent) and buffered with sodium bicarbonate to maintain pH 6.5-7.5. During surgery, anesthetized water is continuously perfused over the gills to maintain anesthesia while the fish is positioned out of water.
Treatment Modalities by Tumor Type
Key Surgical Considerations
- Remove scales along incision site before cutting into skin
- Use absorbable or non-absorbable suture; suture removal at 10-14 days
- Post-operative antibiotics (intramuscular) and local analgesics for invasive procedures
- Maintain excellent water quality for recovery; fish may recover in home tank if quality is good
- Remove trapped air from under skin post-operatively if present
Chemotherapy Note: Very few clinical reports exist regarding chemotherapy in fish. Limited cases have used intralesional bleomycin for myxoma and oral prednisolone for round cell neoplasia with variable success. Chemotherapy remains largely investigational in fish oncology.
Prognosis
Prognosis depends on tumor type, location, extent of disease, and whether treatment is feasible. Benign tumors (fibromas, peripheral nerve sheath tumors) are generally not life-threatening and may not require treatment. Malignant tumors have variable prognosis based on invasiveness and metastatic potential. Surgical excision when feasible improves outcomes, with mean survival of 8.85 months post-treatment compared to shorter survival without intervention.