First Fledgling Case Suggesting Re-Establishment of the Transmission Cycle of Chaunocephalus ferox (Digenea: Echinostomatidae) in the Reintroduced Sedentary Oriental Stork (Ciconia boyciana) Population in Japan, with Special Reference to Its Phylogen

The Oriental stork, Ciconia boyciana Swinhoe, 1873 (Ciconiiformes: Ciconiidae), once common as a sedentary stork species across Japan, experienced a marked population decline during the last century, reaching extinction in the country in 1971 as a result of anthropogenic alterations to the natural environment. After the reintroduction of five captive descendants of continental pairs in 2005, the naturalized population of Oriental storks has steadily increased in Japan, reaching >500 wild storks by 2025. We recently encountered a case of apparent intestinal fluke infection with Chaunocephalus ferox (Echinostomata; Echinostomatidae) in a 2-month-old stork fledgling within a few days after leaving the nest on 29 June 2025. The parasite was subjected to morphological and molecular identification based on nucleotide sequences of the small- to large-subunit ribosomal RNA gene and mitochondrial cytochrome c oxidase subunit 1 gene and was identified as C. ferox, a parasite exhibiting high host specificity for ciconiid birds worldwide. This finding suggests the re-establishment of its natural transmission cycle in the field in Japan, accompanied by the dispersal of definitive hosts after a gap of at least 30 years. Following the establishment of a reintroduced sedentary Oriental stork population in Japan, this health-threatening chaunocephalosis may, by chance, have a pronounced impact on infection-susceptible stork chicks cared for and fed by parent birds inhabiting fields with dense C. ferox endemicity (hotspots). Another Oriental stork fledgling from the same nest in June 2025 survived and was observed in March 2026 in a paddy field at a distant locality. Parasites are natural components of healthy ecosystems that contribute to biodiversity and trophic balance; therefore, inappropriate anthropogenic habitat modification may create scenarios in which parasites become overly destructive rather than serving merely as ecological regulators.