Significant and emerging parasitic diseases of finfish

Fish parasites constitute widely diversified groups of unicellular and multicellular organisms. Recently, less known parasite groups have emerged as serious pathogens. Firstly, infection with dinoflagellates of the genus Ichthyodinium has been found during seed production of leopard coral grouper, Plectoropomus leopardus, in Japan and yellowfin tuna, Thunnus albacares, in Indonesia. Schizonts multiply in host ova, leading to burst of yolk sacs of embryos or hatched larvae. The complete life cycle is unknown, but there is evidence that the infective stage was present in the rearing water. Secondly, heavy infections of endoparasitic turbellarians have been noticed in hatchery-reared stonefish, Inimicus japonicus, fry and cage-cultured sea perch, Lateolabrax sp. and greater amberjack, Seriola dumerili. Furthermore, wild Japanese sea perch, Lateolabrax japonicus, and cultured red sea bream, Pagrus major, were concurrently infected. It is not clear whether these turbellarians, probably belonging to the genus Paravortex, comprise a single species. Hitherto unknown parasitic diseases among wild fish should also be noted: the monogenean, Neoheterobothrium hirame, has been a serious threat to wild olive flounder, Paralichthys olivaceus, since its sudden appearance in the early 1990s. Circumstantial evidence and molecular data strongly suggest its natural host is southern flounder, Paralichthys lethostigma, in North America, and that infected fish was introduced into Japanese waters. Among freshwater fishes, metacercariae of the bucephalid trematodes, Parabucephalopsis parasiluri and Bucephalopsis ozakii, and the leech, Limnotrachelobdella sinensis, have been found in wild cyprinid fishes of Japan. It is suspected that these parasites were introduced through importation of live shellfish and fish, respectively. These cases in the wild are more serious than those occurring in aquaculture, as it is unrealistic to take control measures for the containment of diseases of wild fish.