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Chilodonella Cyprini

Chilodnella cyprini

Chilodonella cyprini attack the skin and gills of Koi, and severe infections can result in rapid fish losses. Chilodonella is a ciliated protozoan, which has tiny hair-like parts called cilla that are used for controlling movement, and sometimes feeding. They cannot be seen with the naked eye and can only be identified with a microscope, which means you will need to do a skin scrape. When viewed under a microscope Chilodonella will appear as a kind of heart-shaped parasite with rows of cilia running along its longer side.

The parasite spreads through division and, as it can swim, infection from fish to fish can occur easily and very quickly in heavily stocked or poorly maintained systems. Infection can occur at all temperatures, although temperatures under 68°F (20°C) are favorable for this parasite.


Koi infected with Chilodonella may display a number of symptoms which will differ depending upon whether the infection is limited to the body or also affects the gills.

General symptoms of both types of infection include:

  • Koi appear lifeless
  • Hanging in water with fins clamped
  • Spend time near oxygen sources (water returns & airstones)
  • Flashing or rubbing against sides of pond
  • Skin may take on an opaque appearance
  • Skin will lose protective mucus and become rough to the touch
  • Lose their appetite
  • Weight Loss

The area between the head and dorsal fin is generally the most severely affected. In advanced cases skin begins to swell, eventually shredding and falling off in strips.

An important factor to consider when identifying Chilodonella is that many of the symptoms exhibited by your Koi are the same for many other parasitic infections, so it is vital that a correct identification is made so that the right treatment can be administered. In some cases more than one species of parasite is present, and then a suitable treatment must be chosen to eradicate all identified parasites.


Chilodonella can be treated with Chloramine-T or malachite green and formalin.

Pond Salt can be used to control Chilodonella, either as a pond treatment or a bath, the rate of which will depend upon the stage of infection. If opting to use salt as a pond treatment, be sure not to use medication such as formalin until the salt has been removed. It may also necessary to treat any secondary infections and this can be done with a topical treatment like propolis, or malachite green and propolis in more severe cases.

All fish as well as the pond they inhabit should be treated.


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Highly infectious and able to kill in great numbers, Chilodonella may very well be the most dangerous skin parasite there is. Overcrowding increases the risk of infection.

As Chilodonella can swim, they may attach themselves to nets, live food, wildlife and even the hands of fish keepers. Care must be taken to sterilize all equipment and you should wash your hands before handling food to avoid cross contamination.

If the pond is heavily stocked, rapid infection will occur, so sensible stocking levels should be observed. Good water quality and system husbandry will also help in preventing an outbreak of this disease as it is only at times of stress that the parasite will be able to multiply at a rate quick enough to become a major concern.

To help reduce stress the use of a pond heating system is also desirable to prevent fluctuations in temperature as this disease is more common in the spring and autumn when the water temperature warms after winter and then cools as winter approaches. Chilodonella is a greater threat at lower water temperatures. If the water temperature is kept above 50° F, and good husbandry techniques are applied, the chances of a major outbreak are reduced.

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