Emaciation disease of cultured tiger puffer Takifugu rubripes

摘要

A new disease characterized by serious emaciation has been spreading among cultured tiger puffer Takifugu rubripes in Japan. Hitherto unknown myxosporeans have been collected from the intestine of emaciated fish and described as Myxidium fugu, Myxidium sp. and Leptotheca fugu. Two hyperparasitic microsporeans were often found in the plasmodia of M. fugu and L. fugu, but their pathogenic effects to host myxosporeans remain to be clarified. These myxosporean infections had some exceptional features.First, they showed no clear seasonality in their development, and mature spores were rarely observed. Secondly, two of them (Myxidium sp. and L. fugu) are histozoic in the intestine of the host. Thirdly, they probably transmit directly from fish to fish.Histologically, M. fugu showed minimal pathogenicity. On the other hand, in Myxidium sp. infection, the epithelium was detached and dead cells accumulated between the epithelium and lamina propria. In L. fugu infection, infiltrated macrophages surrounded plasmodia, and resultant parasite-macrophage aggregates moved to the lamina propria to form macrophage centers. These host responses caused the basement membrane of the epithelium to be discontinuous, leading to epithelial decomposition. When L. fugu was infected with a hyperparasitic microsporean, these pathological changes tended to be more severe. It is evident histologically that Myxidium sp. and L. fugu with or without the hyperparasitic microsporean were highly pathogenic to the host fish, and this strongly suggests that they are the causative agents of the emaciation disease.