| Summary: | Centrocestus formosanus is a digenean that requires three host species to complete its life cycle. This study was conducted to observe the damage caused by two life stages of the C. formosanus on its host species. The snail Melanoides tuberculata was induced to shed cercariae by exposing to sunlight and specimens of koi carps were experimentally infected with cercariae. Gills of two infected fish were killed and fixed in Bouin’s solution daily for 21 days. Infected fish were continuously fed to a pond heron (Ardeola ralloides) for three weeks and therafter the bird was killed. Small intestine was resected as duodenum, jejunum, and ileum and fixed in formol saline. Gills and small intestine were prepared to study the histopathological damages. Flared opercula with protruding gills and increased respiratory rate were identified as the primary clinical signs of the fish. Encysted metacercariae were observed in the basal, middle and in the apical portion of the gills’ filaments and gradual distortions and extensive proliferation of the cartilage of the gills resulted in loss of the respiratory epithelium. A progression of fibroblast to chondroblast encapsulation of the parasite was observed in the gill of fish as a host response. The duodenum of the heron was severely infected with adult parasites than jejunum and ileum. Flukes were observed in the villi, mucosae, submucosae, and also in the tunica muscularis of the duodenum. In conclusion, this study revealed that the heavy infection of C. formosanus could cause severe pathological lesions in both koi carps and pond heron. Keywords: Ardeola ralloides, Centrocestus formosanus, Cyprinus carpio, Intestinal pathology, Melanoides tuberculata, Metacercaria
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