Heavy Metacercarial Infection in the Abdominal Cavity of Hatchery-reared Japanese Dace Tribolodon hakonensis

抄録

In August 2013, metacercarial infection in the abdominal cavity of Japanese dace Tribolodon hakonensis fingerlings occurred at a hatchery in Nagano Prefecture, Japan. ​All fish examined in this study (n = 45) were infected, and heavily infected fish showed an abnormally distended abdomen and ascites, with the number of metacercariae reaching 66. ​The metacercariae were distinctly bipartite, representing a Neascus type and classified as a member of Diplostomidae, and phylogenetical analyses based on ITS2 and cox1 sequences tentatively classified them in the genus Posthodiplostomum. ​When they were excysted, only a few were alive and intact, suggesting that dace is not a suitable host. ​Histopathologically, cysts containing dead worms and empty and vestigial cysts were in the abdominal cavity, detached from the host visceral organs and surrounded by host inflammatory cells. ​Diplostomid cercarial larvae were not found in the three species of snails (n = 613) sampled from ponds in the station. ​At the end of rearing in September, the survival rate was 0.96% in a pond with a high infection rate, whereas it was 12.5% in another pond where fingerlings showed no clear disease signs. ​Oral administration of praziquantel at 150 mg/kg fish body weight for 3 consecutive days effectively killed metacercariae in the abdominal cavity.