Co-occurrence of two types of Myxobolus 'cysts' was recognized in the gills of cultured common carp (Cyprinus carpio) juvenile; one was a highly harmful large 'cyst' type and the other was a harmless small 'cyst' type. At a farm investigated, prevalences of large and small-type `cysts' reached a maximum of 25 % in July and 100% in August 1995, respectively. Invasion of the myxosporean forming large `cysts' was presumed to start before mid-June, whereas that of the small-type started after mid-June. Large-type plasmodia developed along with the primary lamellae and finally formed large irregular-shaped 'cysts', reaching several mm in size. On the other hand, small-type plasmodia grew in the secondary lamellae and the 'cyst' was confined to a single lamella. Morphological and serological comparisons of the Myxobolus spores from the two 'cyst' types indicated that both types represented Myxobolus koi Kudo, 1920.

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