In the Shadow of an Invasive and Harmful Fish Parasite, Schyzocotyle (Bothriocephalus) acheilognathi: An Updated Invasion Revision with First Record and an Assessment of its Possible Threats to Host Fishes and Public Health

Schyzocotyle acheilognathi (formerly Bothriocephalus acheilognathi), the Asian fish tapeworm of Asian origin, infects a wide range of freshwater fishes, particularly Cyprinidae, and poses a potential threat to native and endemic species in Türkiye. This study documents the first global record of S. acheilognathi in the endemic freshwater fish Squalius pursakensis (Hankó, 1925), while also providing an updated overview of its invasion and potential risks to host fishes and public health. A total of 98 specimens of S. pursakensis were collected seasonally from Sarısu Stream (Eskişehir, Türkiye) between February and November 2023. Of these, six were infected, yielding 25 parasites. The overall prevalence and mean intensity were 6.12 % and 4.87, respectively. Infection parameters were further analyzed according to season, host size, and sex. Literature review indicated that S. acheilognathi currently parasitizes 28 freshwater fish species in Türkiye, though only 106 of 427 fish species have been studied ichthyo-helminthologically. This highlights significant gaps in knowledge and the limited scope of current surveys. Histopathological effects, ecological consequences, and potential public health risks of the parasite warrant attention, given its capacity to spread rapidly across freshwater ecosystems. By reporting a new host and locality record for S. acheilognathi, this study expands the known geographical and host distribution of the parasite. Furthermore, the findings emphasize the need for continuous monitoring, comprehensive parasitological surveys, and risk assessments to safeguard aquatic biodiversity and reduce possible threats to fish populations and human health.