This disease occurs in fish, when cercariae of the genera Diplostomum, Tylodelphys, Posthodiplostomum and eight others enter fist eyes and introduce metacercaria formation. Although these metacercariae measure only about 1 mm in diameter, the reactions of the fish introduce nodules of several mm in diameter, which protrude the eyes, which become grayish and thus reduced in light reception. The adults of these worms live in birds as final hosts and in water snails as intermediate hosts. Treatment can be done in aquaria by Tremazol® of Fa. Sera, Heinsberg, Germany (containing praziquantel).