Analyses of factors controlling infection and development of fish-infecting myxosporeans

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It has been reported that actinospores, the infective stage of myxosporeans (mainly freshwater species), release the sporoplasms in contact with fish skin mucus and then initiate the invasion into fish hosts. Recently, fish-to-fish transmission of Enteromyxum leei from the intestine of marine fishes has been demonstrated. Therefore, in the present study, I aimed to clarify the characteristics of fish mucus inducing sporoplasm release of freshwater actinospores, and to evaluate the infectivity and the attachment ability of marine myxosporean trophozoites to fish intestinal mucus.Sporoplasm release of actinospores of Myxobiolus areticus infecting salmonid fishes was investigated in relation to skin mucus. Skin mucus from masu salmon (a susceptible fish) stimulated sporoplasm releases dose-dependently ; percentage of sporoplasm release reached 60 % at 1.0 mg-protein/ml. On the other hand, in non-susceptible fishes, percentages of sporoplasm release were 37 % and 20 % in sockeye salmon and goldfish, respectively. It was also shown that several commercial lectins (Con A, etc.) prohibited but some (RCA I, etc.) activated sporoplasm release of actinospores, suggesting the involvement of sugars in chemoreception of actinospores. Further, isolated sporoplasms of actinospores successfully transmitted naive fish by an intrapentoneal injection.Morphological and molecular analyses showed that a myxosporean from the intestine of tiger puffer is identified as E.leei. PCR for detection of E.leei was developed. No difference was observed in attachment to intestinal mucus and susceptibility among marine fish species, indicating that E.leei has a broad host range.It is concluded that fish mucus (from skin or intestine) plays an important role in the invasion of myxosporeans, and that sugar-lectin interaction is one of the factors controlling myxosporean infection.