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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. Click here for a case report.

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