Abstract
Ichthyophonus disease broke out among cultured ayu Plecoglossus altivelis, in the summer through spring of 1979. Diseased fish displayed pale body coloration, small open ulcers on the body surface, swollen abdomen due to accumulation of ascitic fluid and the production of small nodular lesions in visceral organs. Histopathological aspects were dissemination of multinucleate spherical bodies of Ichthyophonus and reactions against the spherical bodies by macrophages, multinucleate giant cells and granulomas in the affected visceral organs of diseased fish.
