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Epistylis Picture

Epistylis is also know as the "Red Sore Disease" is a ciliate protozoan which is not visible to the naked eye. Identication must be done by a skin scrape examined under a microscope.

It looks bell-shaped with a long “handle” connected to it. Tiny hair-like cilia on the end of the bell shape may be seen. Cilia are used for controlling movement, and in the case of Epistylis are also used to feed on waterborne bacteria. It may also be seen in its contracted form and in this instance it will simply look circular.

The organisms secrete an enzyme that breaks down tissue at the attachment site and leaves a wound that is vulnerable to bacterial and fungal invasion.

The genus Epistylis, also called Heteropolaria, belongs to the order Peritrichida, a group of ciliated vase-shaped protozoans. They are from 200-250 um long and colonies can reach up to 2mm long.


This disease appears as whitish or off-white tufts of fluffy growth that spring up all over the affected fish or from small lesions made by trauma or by concurrent biting parasites. Most often, the fins and eyes are affected and the Koi owner may mistake the pathogen for Fungus because fungus infections also appear as tufty whitish growths.

Epistylis usually is not fatal to the Koi. It is debilitation that will eventually kill the fish.

Epistylis Picture

In the early stages of infection no external visible signs may be seen on the infected Koi, but behavioral changes such as flicking or hanging in the water may be observed. At this point microscope examination of a skin scrape is the only way to make an exact diagnosis.

As the infection worsens, small white patches may appear on the skin which can develop in size up to 0.2in (5mm) or so. At first these may be limited in number, but as they become larger they will spread and more will become apparent on the body of the infected Koi, as well as on the gills.

As these small white patches spread, the skin will become redder and this eventually leads to scales lifting and (if not treated) falling out. This leaves the affected area susceptible to secondary infections.

At this stage of infection it is vital to check regularly for other parasites as these will quickly take advantage of the situation and worsen the problem.

As the Epistylis infection spreads, it is quite common to see ulceration caused by bacteria attacking areas from where scales have fallen off.

Epistylis infections are very unlikely to occur when the pond temperature is below 54º F (12º C), but as the temperature rises the level of infection will increase. Water temperatures of over 68º F (20º C) lead to high levels of parasite activity.


Episylis is never the primary infection.

Poor water conditions are the main cause for this Koi disease.

Good Koi husbandry and system maintenance can reduce the likelihood of Episylis.

Removal of fish wastes and massive water changes are sufficient for clearance of Epistylis.

To learn more about water quality and it's importance, click here.

In catfish the lesion will involve the spines and bones that underlie the skin of the head and pectoral girdle. This protozoan parasite has also been observed on eggs.

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