Global Burden of Clinostomum complanatum in Freshwater Fish: A Systematic Review and Meta-Analysis of Prevalence and Pathological Effects

Background Clinostomum complanatum is a trematode parasite that infects a wide range of freshwater fish hosts, causing significant pathological damage, reduced fish productivity, and economic losses. Despite its considerable economic and health impact, its global burden remains poorly quantified. Objective This study aimed to evaluate the global burden of C. complanatum infection in fish, with particular emphasis on prevalence patterns, habitat associations, and pathological impacts. Methods A systematic review and meta-analysis following PRISMA 2020 guidelines included 30 studies reporting C. complanatum prevalence in 70 freshwater fish species from 18 families. Pooled prevalence was estimated using random-effects models in R. Heterogeneity was assessed with Cochran’s Q and I² statistics, subgroup analyses were performed, and publication bias was evaluated using Egger’s regression test. Results Prevalence of C. complanatum varied widely among freshwater fish species, reaching 100% in Squalidus chankaensis and remaining undetected in several fish species ( Cobitis biwae, Hypomesus transpacificus, Salangichthys microdon , and Silurus asotus ). The pooled global prevalence was 28.7% (95% CI: 22.0–36.5%), with extremely high heterogeneity (I² = 99.0%). Significant differences were observed by fish family, country, continent, and habitat type, with the highest prevalence in Percidae (97%), Osphronemidae (67%), and Gobionidae (65%). Africa (49%) and Europe (46.0%) showed the highest regional prevalence, and aquaculture systems had higher infection rates (45.0%) than natural habitats. No effects of publication year, season, or host sex were detected, and no publication bias was evident. Metacercariae predominantly encysted in muscle, buccal, and branchial tissues, forming yellow-grub cysts associated with muscle degeneration and inflammatory responses. The internal transcribed spacer (ITS) region was the most commonly used molecular marker, although molecular diagnostics were underutilized. Conclusion and Recommendation: In conclusion, C. complanatum represents a widespread but highly heterogeneous threat to freshwater fish health, and aquaculture productivity. Routine incorporation of molecular diagnostics, standardized surveillance protocols, and targeted control strategies particularly in aquaculture systems are strongly recommended to reduce infection burden, economic losses, and zoonotic risk.