Dermocystidium an uncommon fish disease

  Parasites Microscopic

Dermocystidium. koi

Identifying Dermocystidium

Dermocystidium. koi  is an uncommon fish disease affecting koi although there are reports that other species may be affected. As its name suggests, Dermocystidium is a disease that affects the skin of koi and can be found on the gills, fins or body. It causes raised swellings varying in size from 1-2 cms to large lesions up to 10 cm. The lesions are pinkish to red and vary in shape from circular to long elongated ovals.

There is minimal inflammation around the lesion. Because of the presence of both spores and hyphae it is currently believed to be a fungal infection, although some authorities class it with the protozoa. As the lesion swells, the epidermis covering the swelling gets progressively thinner, at which stage it is sometime possible to see the white hyphae inside. These give the swelling a whitish, cloudy look. When the lesion matures, it ruptures spreading thousands of spores into the water. These are the infective agent.

It is not known how the spores infect fish, whether they have to find entry into the epidermis, either by way of an open wound, or whether they are able to burrow under the skin. The main diagnostic feature is a microscopic examination of hyphae from a lesion. These are filled with hundreds of spores of 8-12  µm diameter – about the same size as a red-blood cell. Each spore has a large central fluid-filled vacuole that displaces the nucleus to one side.


Dermocystidium nodules. Top of picture shows ‘cloudy’ nodule ready to rupture.

Wet mount (phase contrast) of spores showing eccentric nucleus and large vacuole

Microphotograph x400 (H&E) showing  hyphae in transverse section filled with spores. Red blood cells (erythrocytes) between hyphae give an  indication of size

Microphotograph x1000 (H&E) D.koi spores showing vacuole with eccentric nucleus

Microphotograph x1000 (giemsa) D.koi spores showing vacuole with eccentric nucleus

photos: Frank Prince-Iles


There is no known treatment. Direct applications of malachite green and potassium permanganate sealed into ruptured lesions with waterproof paste have had no effect against the spores. Similarly, using a long-term bath of malachite green had little effect on either the spread of the disease or the spores in ruptured lesions. Surgical removal of smaller, shallow lesions was effective, but the majority of lesions did not lend themselves to surgery, because of location and shape.

Infectivity and re-infection

It is infective and likely to affect very large numbers of fish. When the lesion ruptures the lesion left, which can be substantial, seems to heal fairly well and quickly, leaving little sign of the infection. It seems to be a spring-time disease, lasting some 6-8 weeks. It has been reported that re-occurrences can sometimes happen in previously infected ponds.

Although Dermocystidium does not seem to be fatal in most cases, it does bring with it a very real threat of secondary infections. Antibiotic treatments and regular cleaning of the wounds will help prevent secondary infections and aid recovery. It is also important, as with all diseases, to maintain optimum conditions to prevent stress causing additional complications.