Intermediate insights: tracing trematodes infecting amphibians via their first intermediate snail hosts

Amphibians are a prime example of the global biodiversity crisis, as they represent the most threatened group of vertebrates. Yet, amphibian macroparasites remain one of the most poorly described groups of parasites, with the majority of species lacking comprehensive morphological, molecular, or ecological data. Among these, digenean trematodes constitute a dominant group and feature multi-host life cycles. This study examines the first intermediate hosts of trematodes, aquatic gastropods, to assess the occurrence, prevalence, and seasonality of amphibian trematodes.

A total of 5362 snails from five families (Bithyniidae, Hydrobiidae, Lymnaeidae, Physidae, and Planorbidae) were collected in three stream systems in North Rhine-Westphalia, Germany and investigated for trematode infections. Detailed information on amphibian-infecting trematode cercarial morphology and measurements were provided via light microscopy and scanning electron microscopy (SEM). Comprehensive molecular analyses were conducted and novel sequences generated for multiple genetic markers (28S rDNA, ITS1-5.8S-ITS2, ITS2, cox1, and nad1).

Three trematode taxa infecting amphibians as second intermediate and/or definitive hosts (Lecithopyge, Cephalogonimus, and Opisthioglyphe) were identified exclusively from lymnaeid snail hosts. A total of 79 novel sequences were generated for 21 trematode isolates. Phylogenetic analyses based on 28S and ITS1-5.8S-ITS2 sequences resulted in concordant taxonomies. Distinct seasonal infection patterns allowed for insights into the species' life cycles.

Our findings highlight significant gaps in the knowledge of amphibian macroparasites and underline the value of studying cercariae occurrence in snail intermediate hosts as a method for monitoring amphibian trematode biodiversity without affecting amphibian populations.