Dermocystidium

This uncommon parasitic organism causes skin diseases in a few species of pet fish. The lesions vary in appearance but are mainly small swellings that enlarge slowly and burst, exposing white thread-like tubules packed with microscopic spores.  In koi, the disease occurs mainly in spring, and the swellings grow to about 1cm in size in a few weeks. There is no effective treatment and once burst, the lesions usually heal on their own. Occasionally, they may need to be removed surgically

Dermocystidium koi
Dermocystidium koi

This lesion on the tail of a koi is starting to burst and expose the thread-like tubules

Dermocystidium koi
Dermocystidium koi

This swelling on the front (pectoral) fin has burst, exposing the tubules, which are visible under water

Dermocystidium in a tetra
Dermocystidium in a tetra

Early skin lesions in cardinal tetras are small and often look like a clear blister or are pale and opaque as seen between the eyes of this tetra

Dermocystidium koi
Dermocystidium koi

This lesion on the tail of a koi is starting to burst and expose the thread-like tubules

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