New Data on the Larval Stages of (Digenea, Plagiorchiata).

(1) Background: , an esophageal parasite that primarily affects water snakes of the genus , has a known life cycle that involves snail and amphibian hosts. However, the biological aspects, chaetotaxic patterns, and pathogeny of this parasite in its hosts have not been fully elucidated. (2) Methods: Snails () were collected in Spain and examined for cercaria emergence. The larvae were used to experimentally infect , and metacercariae were isolated. Their chaetotaxy was studied using microscopy and scanning electron microscopy. The eye histology was also examined. (3) Results: The cercariae displayed distinctive morphological characteristics. The results of this study revealed three types of ciliated sensory papillae on the cercarial teguments, suggesting an adaptation for host detection and orientation. The metacercariae isolated from subcutaneous tissues showed oval bodies covered in spines. The chaetotaxy patterns matched those of Leptophallinae species. This is the first report of the presence of in the snail . Additionally, this study detected metacercariae in the eyes of , emphasizing the need for further research on trematode infections in amphibian eyes. (4) Conclusions: Members of the genus can serve as secondary intermediate hosts for , and the presence of metacercariae in amphibian eyes may have implications for the survival and habitat management of these amphibians. Understanding this parasite's prevalence, transmission dynamics, and impacts on host populations is crucial for conservation strategies.