Elevated Water Temperature Treatment of Ayu Infected with Glugea plecoglossi : Apparent Lack of Involvement of Antibody in the Effect of the Treatment

The possibility of antibody-mediated effect of elevated water temperature treatment of Glugea-infected ayu was investigated by challenge test and ELISA. The treatment effectively reduced the prevalence and intensity of infection. In naturally infected fish, the prevalence of infection was significantly decreased by elevated water temperature treatment. The treatment was more effective in naturally infected fish treated thrice than those treated twice, as demonstrated by the smaller number of cysts in the former group of fish than in the latter. The antibody levels were also significantly increased by the treatment in naturally infected fish. However, in the experiment using unifected fish, no significant increase in the antibody levels by the treatment was seen, though the prevalence of infection was significantly decreased by the treatment. Through the challenge test, it was clear that the effect was temporary, and protective immunity was not confirmed by the treatment. These results suggest that the effect may not be antibody-mediated. Thus it is recommended that the elevated water temperature treatment be repeated and applied with continuous monitoring of reinfection.