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Tumors in Koi


Tumors happen in fish, just like in other animals and humans. The cause of many of the tumours and malformations found among Koi are for the most part poorly known, because studies involving fish are few and far between. The financial incentives for studies on fish are just not there. Certain malformations are caused by injuries.  Tumors occur on nearly all organs or tissues. Some tumors are caused by various ingredients in the diet. There is also a suspicion that viruses may cause certain tumors, similar to the megaloblastic growth in cells like with Lymphocystis and Lymphosarcoma. Certain chemicals are also known to be carcinogenic and may cause tumors. Tumors on the skin are very obvious and some resolve itself over time, given an increase in temperature and optimal water conditions.

Most tumors do not appear to be fatal to the fish, but may lead to complications over time. I have witnessed many fish that developed a tumor and the situation seems to stabilize after the initial disfiguring growth. In such a case the fish then leads a normal life for weeks, months and in some cases many years. In some cases a tumor will appear and then continue to grow until the skin rapture or the sheer volume of the growth will actually inhibit the normal functioning of vital organs. When the skin raptures, the end result will inevitably be a massive bacterial infection.


 In very rare cases, the tumor will actually protrude through the ruptured skin and manifest itself as a rubbery, raised, dark red growth, resembling an open, raised sore.


When a tumor develops around an organ in the body cavity and continues to grow, the end result is normally a blockage of some kind and the fish will, during the final stages, exhibit symptoms very similar to dropsy shortly before death.

The above reasons for tumors are valid, but more and more data are accumulating to implicate environmental agents as important for carcinogenesis in humans, animals and fish. Many of these agents enter the natural waters and come in contact with fish and invertebrates. These agents can be of natural, industrial and agricultural origin. They are numerous and can include natural agents such as UV light. Other agents include crude oil, various soluble metals and their salts, petroleum wastes, DDT, other pesticides, benzyl, arsenic, domestic wastes, herbicides, aromatic amines, and various components of effluent from mines, industry, and dyestuffs. The mechanisms by which these environmental agents act to generate neoplasia are presently unknown. That they probably do act additively and even synergistically in conjunction with multiple host factors is well known in mammals and certainly should be similar in fish.

There are a few studies available on the impact of genetics on the development of cancer/tumors in fish, but I could not find a specific study referring to the common carp or to Koi. There are however references to the impact of in-breeding and line-breeding in fish and the conclusion that I could reach is that the limited genetic pool in Koi breeding must surely be a contributing factor to the frequent occurrence of tumors in Koi, especially those tumors that occur on young fish.

There are various reasons for malformations in koi and therefore one should not suspect a tumor at first glance. Impacted eggs or the start of dropsy may also be the cause of bloating. A tumor can therefore only accurately be diagnosed through an autopsy. There are however certain ways to be reasonably sure that a tumor is present in a fish. The first indication is a swelling that occurs. This swelling is mostly on one side of the fish, and not a symmetrical, even swelling for example of the whole abdominal area. The second difference to impacted eggs and dropsy is that while the bloated area is normally soft, a tumor will feel like a distinctive hard lump inside the fish when firmly touched.

There is no cure for a tumor. The only solution is to remove it surgically. The removal of a tumor in normal tissue can be successful but the procedure to remove an abdominal tumor is very invasive and such a procedure mostly result in the fish dying from bacterial infection within a few days after the operation. I must haste to add that some successes have been reported, but exceptional skill is necessary where the tumor has engulfed some vital organs.


The following photos received from Pieter J. De Villiers clearly demonstrate a tumor that developed over a period of only three months from a barely noticeable bulge to a tumor measuring 11 cm long with a 4 cm diameter, causing the death of the fish.

Note the small initial bulge on the abdomen of this 40 cm Sanke about 2 cm behind the right pectoral fin.



The tumor is revealed during an autopsy. Note how the cauliflower-like tumor consists of pea size growths, clustered together to form one big lump.



A large part of the intestines and liver are engulfed by the tumor.


All organs removed from the fish.


The tumor bisected. Just a white “dead growth” with very few blood vessels.

Photos: Pieter J De Villiers

The following very clear, high quality photos were kindly supplied by Wayne Barker to demonstrate yet another excellent example of a tumour development:


This is a photo of the Shiro Utsuri when acquired. One can see the body shape is not very symmetrical at this early stage. The second photo shows the same fish with the tumour at an advanced stage

   shirotumour shirotumour_development

The euthanized fish


Biopsy shows the ovarian tumour clearly


Note the amount of dark blood as a result of internal bleeding


Blood removed to show the full extent of the tumour


The tumour is removed. Note the massive cavity occupied by the tumour. Also note the similarity of the structure of the tumours in both fish.

Photos: Wayne Barker

Last Updated on Monday, 09 November 2015 22:14