Prevalence and identification of metacercarial infections in freshwater fish in Hunan Province

Fish-borne trematodes are prevalent parasites affecting about 80 million people globally. This study aims to identify the common trematode metacercariae species in freshwater fish from Hunan Province, China. We collected 521 fish representing 23 species from seven cities, spanning the major water systems: the Xiangjiang River, Dongting Lake, and the Liuyang River. Metacercariae were detected using compression and artificial digestion methods. Common metacercarial forms were identified using morphological and molecular tools. The internal transcribed spacer (ITS) region was amplified by PCR, and the resulting sequences were used to assess the phylogenetic relationships. Predominant metacercariae were identified as Cyathocotylidae sp. and Heterophyidae sp. based on morphological features, homology analysis, and maximum likelihood (ML) and Bayesian inference (BI) phylogenetic analyses. Nineteen (82.6%) of the 23 examined fish species were infected. Overall metacercarial prevalence was 24.38% using the compression method and 42.15% using the artificial digestion method. Prevalence significantly differed across cities, water sources, and fish species. Cities with the highest prevalence were Xiangtan (41.18-70.59%) and Yueyang (43.28-59.62%). Rivers and lakes represented the high-prevalence water sources, whereas reservoirs showed extremely low prevalence (0% using the compression method and 0-2.7% using the digestion method). Crucian carp (Carassius carassius), grass carp (Ctenopharyngodon idella), and common carp (Cyprinus carpio) were highly susceptible species. These findings support the diagnosis and control of fish-borne trematodes and stress the need for targeted surveillance.