Koi Information » Koi Diseases

In this section, we have provided some information on a few of the most common diseases which afflict Koi. We naturally can not cover them all here, but it is a great start for any hobbyist.

We would also like to suggest Advanced Koi Care and Koi Health and Disease as additional valuable sources of reading. To see more information about these books, click on the links above, or you can order them below.

Important Note:
The information contained on this site is not intended or implied to constitute medical advice or diagnosis or treatment, or to be a substitute for medical advice.

Koi hobbyists are advised to consult other sources and confirm the information contained on this site. Consumers are cautioned that this site is not intended to provide medical advice about any specific disease that your Koi may have or treatment they may need. You are advised to seek the advice of a qualified Veterinarian regarding any health-related questions you may have, and should never disregard medical advice or delay seeking help as a consequence of something you have read on this, or any other site.

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Anchor worm

Crustacean parasite, Lernaea. The juvenile stages settle in the gills of Koi, when they mature they mate and the male leaves the Koi, the fertilized female settles on the body of the Koi and continues to grow, becoming the familiar worm shape.

The female buries into the skin and underlying tissue to hold on. The damage caused can become a target for bacterial or fungal infection which can spread.

Available remedies:

Cotton Wool Disease

Another bacterial infection. The common name comes from the white tufts that develop around the mouth and spread to the body and fins, often leading to ulcers and a thin appearance.

Flexibacter is the bacterium which causes this disease.

Treatment with anti bacterial medicine is usually effective.

Available remedies:

Dropsy

Raised scales (rather like a pine cone) and eyes standing out from the head.

A sign of a number of conditions, may be congenital heart or kidney failure or an internal bacterial infection.

Bacterial dropsy is infectious so treat with an anti bacterial remedy and if possible isolate affected Koi.

Available remedies:

Fin rot and ulcers

A number of bacteria are associated with fin rot, lesions and internal hemorrhaging, notably Aeromonas and Pseudomonas. Ulcers usually start at the site of an injury, the bacteria then infect it causing further damage, and fungal infection can also occur.

Such holes result in osmoregulatory problems, leading to damaged kidneys and death if not treated. It is worth adding a weak salt solution to the pond as well as anti bacterial remedy, a concentration of 3gm per litter will help to restore the osmotic balance and reduce strain on the kidneys (make sure that the salt is fully dissolved before you add it to the pond).

Finrot is easily noticeable, the fins and/or tail look chewed and are red at the edges. Secondary bacterial and fungal infections can develop.

Available remedies:

Fish Lice

Argulus another crustacean parasite, round and up to 1cm wide. They have a sucker to hold on to the Koi with needle-like mouth parts which they stick into the Koi and inject a toxin. This causes intense irritation to the Koi and they scratch and jump and can cause bacterial infection.

If they infect the gills they cause severe damage and often death. Most antiparasite remedies will not kill fish lice, a strong chemical is needed which is not freely on sale. Ask a professional dealer or vet.

Available remedies:

Gill maggots

Gill maggots are the mature females of the parasitic crustacean Ergasilus.

Heavy infestations can cause severe damage, eroding the gill filaments and allowing secondary infections to develop.

Saprolegnia fungus

One of the most common fungal infections of Koi. The fungal spores will grow anywhere on the Koi, including the gills, initially germinating on dead tissue. Their threadlike hyphae release digestive juices which break down the tissue so the fungus can absorb it, as the fungus grows these juices start breaking down living tissue.

Fungus on the body appears as cotton wool like growths, it is hard to tell if a Koi has it in the gills, but if it hangs at the surface gulping for air it is likely.

Carp pox. A virus that produces solid waxy lumps on Koi. It will not kill Koi and is generally harmless, but can look unsightly. It is most often present in small Koi and in cold weather, clearing up disappearing when Koi grow and in the spring when water temperatures rise.

Available remedies:

Skin and gill flukes

These are the fish equivalent of fleas.

They are two different types of flukes, but despite of their names both can be found on the body and the gills, feeding on mucus.

They use hooks to hold onto the Koi, this causes irritation and in a weak, sick or stressed Koi can become a problem.

Available remedies:

White spot (Ich)

Caused by Ichthyopthirius multifiliis. The white spots on the skin, gills and fins are individual protozoan cells that are under the skin and feed on the body fluids and cells. They then punch out of the skin and fall to the bottom of the pond, collect together and begin breeding, the offspring then re-invest the fish.

As well as white spots symptoms are scratching and swimming into the water inlet, failure to feed and lethargy. It is fatal if untreated. Fortunately commercial white spot remedies are widely available.

Available remedies: