Do Fish Get Cancer?

Our service is commonly askes, “do fish get cancer?” Unfortunately, fish get cancer like most other animals. And sharks, being fish, are included in this group. Like tumors in all other species, some are treatable, while others are not. Here are some of the most common cancers we seen in our daily practice.

Goldfish Nerve Sheath Tumors

Mario the Goldfish – after receiving cryotherapy treatment for multiple growths

These “warts” are a type of benign tumor known as a nerve sheath tumor. Although we seen them more commonly in standard, comet goldfish, we have had fancy cases as well. Aggressive treatment with excision and cryotherapy do not help correct them for the long-term. Some mild cases have been corrected, but for the most part, your fish will just have to live being a little bit ugly. Other than being less than aesthetically perfect, they do not cause any other health issues.

Hikui in Koi

A more severe case, you can see the improvement between cryotherapy sessions from left to right.

For many years, this disease was thought to be a parasite. Newer research was able to determine this disease is caused by a cutaneous perivascular wall tumor. The first of its kind to be discovered, this cancer affects the tissues surrounding blood vessels in the skin. Although similar to other skin cancers, this disease can be treated with cryotherapy. Mild cases have seen complete resolution, whereas some severe cases have mild improvement, but not been cured. DO NOT attempt cryotherapy on your own fish using kits from the drug store. All of our patients receive antibiotic therapy and pain management during our sessions where actual liquid nitrogen is applied to the skin. DO NOT attempt this at home.

Chromatophoromas in Koi and Goldfish

Chromatophoromas is the technical name for a pigment cell tumor. In koi and goldfish, mostly living in outdoor ponds, they tend to get these on the top of their eyeballs. We have seen other cases where they will have growths around their dorsal fins or sides of their face. Our colleagues at UC-Davis Veterinary School have been able to treat these with cryotherapy. More advanced cases will require enucleation, or removal of the eye.

Koi Gonadal Sarcomas


Koi that passed with gonadal sarcoma. A tumor this size is not deemed operable.

Although we are not able to prove 100% these arise from gonadal tissues, given the fact that ovaries are highly replicating tissue, ovarian tissue is usually not found and histology cannot rule it out, it’s our best guess at this time. With more sampling of early cases, detected by ultrasound, we’ll have the 100% guarantee some day. We see these most often in female koi around 8-12 years of age and usually, we don’t know about them until it is too late. Female koi can grow very wide, especially when reproducing, and it is hard to tell eggs from tumor until the fish starts to look lopsided.

The only treatment for a coelomic tumor is surgery. DO NOT TRY THIS AT HOME. (We say this because someone did, and decided to call us mid-surgery for “guidance.” Please, do not attempt surgery unless you are a qualified veterinarian. It is not as easy as I make it look on YouTube) Of the many cases we have treated with surgery, a few have grown back, a few are still doing well and a few have passed from other causes.

And Others

Occasionally, we’ll see something that is not recorded in the pet fish files. We’ve had coelomic tumors that turned out to be 100% liver tumors, but with that surgery, there were multiple nodes spread throughout the fish, unlike the others that have one concentrated growth. Another fun case is of this little koi…

We don’t know what it is, but cryo and antibiotics didn’t alter it at all. It does not hinder the fish’s life at all, so the owner is fine leaving it and not performing further diagnostics. Our vet checked in with him a few weeks ago, and it’s gotten a little bigger, but the fish is fine overall.

Since this is a newer niche of veterinary medicine, we are constantly discovering fun new things. We like to share them with you and our colleagues so we can all learn more about our amazing patients and pets together.

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