Recent advances in fish parasitology in Japan

Abstract

Recent advances in fish parasitological studies in Japan focus mainly on myxosporean, microsporean and monogenean infections. <b>Myxosporeans:</b> The two-host life cycle has been elucidated for several freshwater species in Japan. Actinosporean stages were studied, not only their morphology, but also their biology, viz. their viability in water, response to fish mucus, and portals of entry to fish. Myxobolus artus and Thelohanellus hovorkai, both parasites of carp Cyprinus carpio, cause severe damage when spores leave the host. Kudoa amamiensis infection of yellowtail, Seriola quinqueradiata, had a patchy geographical distribution in Okinawa, perhaps reflecting the distribution of its unknown alternate invertebrate host. With the development of PCR probes, the sensitivity of detection of infection in fish has dramatically increased. <b>Microsporeans:</b> Little is known about their mechanism of infection. Macrophages of ayu, Plecoglossus altivelis, recognized glycoproteins on the surface of Glugea plecoglossi spores, and produced much more H2O2 than O2 against phagocytized spores. It may be that G. plecoglossi utilizes this host defense mechanism to establish the infection in ayu. With no chemotherapeutants commercially available, Microsporidium seriolae infection of yellowtail was prevented by using filtered water to maintain fry in tanks, and Glugea-infected ayu was treated by raising the temperature of rearing water. Both PCR and a fluorescent dye, Uvitex 2B, were very sensitive tools to detect infection. <b>Monogeneans:</b> Egg entanglement on the culture net mesh was quantified in Heterobothrium okamotoi infection of tiger puffer, Takifugu rubripes. In vitro attachment of several species of oncomiracidia on cell wells coated with lyophilized extracts of skin epithelia of different fish species suggests a complex nature of their host specificity. Japanese flounder, Paralichthys olivaceus, and tiger puffer infected with Neobenedenia girellae and H. okamotoi, respectively, acquired immunity against reinfection. Tiger puffer develops antibodies against H. okamotoi, but the mechanism of immunity is not yet completely understood. Freshwater or H2O2 bath treatment of B. seriolae-infected yellowtail is effective, but laborious. Experiments showed that, as an alternative method, oral administration of praziquantel was easier and as effective as bath treatment. Most studies introduced here are related to parasite biology. This is because we aim to develop control measures against parasitic diseases with minimal reliance on chemotherapeutic agents.