First report of Centrocestus formosanus infection associated with mass mortality in the critically endangered Chinese Bahaba (Bahaba taipingensis).

Centrocestus formosanus is a pathogenic heterophyid trematode traditionally known to infect freshwater teleosts, but increasingly recognized for its expanding range into brackish and coastal ecosystems.This study documents the first natural spillover of C. formosanus into the critically endangered marine euryhaline fish, Chinese bahaba (Bahaba taipingensis), which naturally inhabits both high-salinity seawater and brackish estuaries. A devastating disease outbreak accompanied by mass mortality occurred between November 2025 and January 2026 at an aquaculture base in Panyu, Guangzhou, China. B. taipingensis was maintained in low-salinity brackish water (3-5‰) as a result of its euryhaline tolerance. Notably, this low salinity (3-5‰) is known to be the most permissive range for cercarial survival and host-seeking behavior, thereby creating an ecological window for trematode infection. Among the 10 infected fish examined, all exhibited excessive gill mucus and dense macroscopic white spots across the gill filaments. Encysted metacercariae were morphologically identified and molecularly confirmed via 28S rRNA gene sequencing, displaying 99.96% sequence identity to reference strains C. formosanus (MG738251.1 and OR671446.1).Parasitological quantification revealed an exceptionally high mean infection intensity (3832 ± 473 parasites per fish). Histopathological evaluation demonstrated catastrophic branchial remodeling, prominently characterized by host-derived granuloma encapsulation, pronounced tissue fibrosis, lamellar epithelial hypertrophy, widespread lamellar atrophy and fusion, and severe localized swelling of the gill filaments. Diagnostic screening ruled out the presence of concomitant bacterial or viral pathogens. This first documentation of C. formosanus in this euryhaline sciaenid underscores the acute vulnerability and extremely low tolerance of B. taipingensis to this heterophyid fluke, highlighting a critical and previously unrecognized parasitic threat to the conservation and population recovery of this endangered species.