In 2015, cataract was observed in juvenile Japanese dace Tribolodon hakonensis, reared in a hatchery in Nagano Prefecture, Japan. Metacercariae were found in the lens, with the prevalence and mean intensity of infection of dace (0.5–2.4 g in body weight) reaching 100% and 39.7, respectively, in the most heavily infected fish group. Histologically, the lens became partially fragmented around the metacercariae, which occupied a considerable space, surrounded by their own excreta. Among snails sampled from outdoor ponds and waterways, cercariae with a bifurcated tail were found in the lymnaeid, Radix auricularia japonica. ITS1-5.8S-ITS2 and cytochrome oxidase subunit I (COI) sequences of metacercariae from the lens of dace and cercariae from the lymnaeid were nearly identical to each other, indicating that the lymnaeid is the first and dace is the second intermediate host. However, since the obtained COI gene sequences were close but not identical to known Diplostomum spp., this species was not identified to the species level. The morphology of metacercaria was also described. This is the second reported case of worm cataract in fish caused by Diplostomum metacercariae in Japan.
