Dermocystidium spp

From Fish
Dermocystidium nodules on koi[1]
Wet mount (phase contrast) of spores showing eccentric nucleus and large vacuole

Dermocystidium koi is an uncommon fungal disease of koi.

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.

Life cycle

The life cycle is maintained from year to year by overwintering cysts. Young perch acquire first infections in their first summer of life, and fish over 3 years possibly develop some immunity. Cysts of D. percae occurred in the skin of abdominal fins and rarely elsewhere. Cysts develop from thin-walled, round plasmodium mother cells, about 10 mum in diameter, by elongating and increasing in size. The nucleus of the plasmodium mother cell degenerates, and a reticular chromatin-containing structure with dense centra spreads out between conspicuous lipid droplets in the plasmodium. Nuclei reappear during fragmentation of the plasmodium. The sporonts thus formed divide to form sporoblasts, which in addition to small lipid droplets acquire a non-lipid 'central' inclusion. The inclusion grows in size to the typical inclusion of the ultimate Dermocystidium spore. Sporogenesis takes place mostly in summer, and can continue until autumn[2].

Clinical signs

During outbreaks, Dermocystidium spp cysts initially become visible breaking through the skin they are usually around 1mm. They grow in size until ultimately they rupture releasing infectious spores they into the water to then seek a new host fish. The size of each cyst can vary but they seldom rupture at less than 6mm in size however few remain intact to reach 10mm and cysts over that size are extremely rare and usually due to two individual lesions that appear as one.

Treatment

There is no known treatment.

Fish infected can be housed in malachite green or acriflavine which should be used in conjunction with salt in order to reduce the secondary risks of fungus and bacteria into the lesions and this can also lower the level of cross-infection. Topical treatment can be applied after the cyst has ruptured but this can be discontinued once the crater or hole the cyst creates develops a slight gloss which indicates that healing has begun. Secondary bacterial infection may require antibiotics. The recovery period is temperature related and therefore it is not possible to be accurate about how long this will take but weeks rather than days should be anticipated.

References

  1. Fishdoc.uk
  2. Pekkarinen, M. & Lotman, K. (2003) Occurrence and Life Cycles of Dermocystidium Species (Mesomycetozoa) in the Perch (Perca fluviatilis) and Ruff (Gymnocephalus cernuus) (Pisces: Perciformes) in Finland and Estonia, University of Helsinki. Journal of Natural History 37(10): 1155-1172