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Trichodina fish disease picture

Trichodina and other trichodinid parasites are probably among the most familiar protozoans to the Koi keeper. These saucer shaped organisms have distinctive rings of tiny hairs (cilia) around the body which enable the cell organism to move. (These cilia are clearly visible at a magnification of x100-200 through a microscope.) Inside each parasite there is a concentric circle of "teeth" known as the denticulate ring. This is clearly visible with a microscope, which in the living parasite can be seen to rotate. Viewes from the side, Trichodina has a dome like shape.

Koi infected by Trichodina develop the familiar grey film over the body in response to the infestation. This excess mucus simply encourages the Trichodina to reproduce at a faster rate. Trichodinids are found on even the most healthy Koi, but if the fish is diseased or in poor condition, they reproduce unchecked and allow secondary bacterial infections to occur.

There are at least three genera matching this description: Trichodina, Trichodonella, and Triparciella are seen in freshwater fish. In low numbers these parasites are generally considered harmless, feeding upon suspended particles in the water or grazing across he surface of the skin, possibly consuming dead skin cells. In large numbers they are irritants and can cause serious damage to the skin.

Trichodinids can also infect the gills, where damage to the delicate filaments makes the gills susceptible to bacterial infection. In the early stages of infection, Koi repeatedly scratch themselves and leap out of the water. As the infection progresses, the fish stop feeding and will stay at the surface of the pond, usually where air enters the pond. They may be seen to gasp and frequently gulp air at the surface. At this stage, there is very little chance that an infected Koi will survive, even with treatment.

They reproduce by binary fission and have a direct life cycle. They are freely motile on the surface of the fish and can swim through the water. They live on the skin and gills where they damage tissue with their rotating denticular (toothed) ring. Affected fish may produce excessive mucus and develop a white cast to the skin. Small hemorrhages may appear on the skin and fins. In rare cases internal infestations have been described, with parasites being found in the kidneys, oviducts and intestinal tracts. Trichodina is often found in the company of other ectoparasites such as Gyrodactylus (skin flukes) and other protozoa.


Trichodiniasis is more likely to occur with poor water conditions along with high levels of suspended fecal and mulm particles. High stocking densities increase the parasite's ability to move on to new hosts at a quicker rate. This parasite can be introduced into aquaria and ponds on plants and substrate materials such as gravel.

Decreasing organic debris and stocking densities will reduce risk of infestation.

The installation of a heating system in the pond will help prevent fluctuations in temperature, and thus remove the prime time when Trichodina may be a problem. Good husbandry and regular inspections of the filters also ensure that only limited amounts of debris are allowed to collect as debris build up can contribute to heavy infestations.


If left untreated severe outbreaks will kill. Even if it is not fatal, the bacterial infections which generally follow often are. If you suspect Trichodina, a skin scrape will need to be carried out and examined under a microscope. If a positive identification is made, and appropriate course of medication needs to be followed.

ProForm-C is an effective treatment for Trichodina.

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