Diversity of trematode larvae in Indoplanorbis exustus from Phayao Lake, northern Thailand: Genetic evidence of Schistosoma spindale and a potential novel Schistosomatid associated with swimmer's itch.

Indoplanorbis exustus, a pulmonate snail, is an important component of freshwater ecosystems and serves as the primary intermediate host for the larval stages of several medically and veterinary significant trematodes. However, data on its infection status and associated trematode species in the Phayao Lake area of northern Thailand are currently lacking. This study aimed to investigate the prevalence of infection and identify the trematode species present in snails from this region. A total of 1290 snails were collected from Phayao Lake between November 2024 and February 2025 and examined using cercarial shedding and snail-crushing techniques. Larval trematodes were identified based on morphological characteristics and confirmed by molecular analyses targeting three genetic markers: ribosomal DNA (28S and ITS) and mitochondrial cox1. The overall infection rate was 1.09% (14/1290), with five trematode species identified: Schistosoma spindale, Apharyngostrigea sp., Euclinostomum sp., and two unidentified cercariae belonging to the families Schistosomatidae and Aporocotylidae. One of the unidentified Schistosomatidae cercariae exhibited morphological features and genetic similarity to known dermatitis-causing genera, specifically Dendritobilharzia and Gigantobilharzia. In addition, cox1 sequence analysis of S. spindale revealed two genetically distinct populations, suggesting possible intraspecific variation. These findings highlight the notable diversity and spatial distribution of medically and veterinary important trematodes in the region, particularly the presence of S. spindale and a potential dermatitis-causing species, and provide essential baseline data to support future control and prevention strategies.