Efficacy of Elevated Water Temperature Treatment of Ayu Infected with the Microsporidian Glugea plecoglossi

Abstract

Preliminary experiments showed that treatment of Glugea-infected ayu at an elevated water temperature of 27-29°C was effective in controlling the disease. Attempts were made to determine the optimal timing of the treatment and to compare the efficacy of single and double treatments (performed once or twice), after infecting fish by feeding fresh spores of G. plecoglossi.
In Experiment 1, fish were treated, 6, 11, 16, 21, 26 or 31 days after the challenged infection, by elevating the water temperature from 20°C to 29°C for a period of 5 days. The best result was obtained for the fish treated after 21 days, showing a relative prevalence of infection (= prevalence of infection in experiment group/prevalence of infection in control group) of 19.0%.
In Experiment 2, the elevated water temperature treatment was conducted once or twice (double treatments with 7 days interval after the first treatment). In experimental groups treated once, no infection was found in fish treated 16 days after the challenged infection, while 87.5 % of control fish (fed with spores, but not treated) were infected. In experimental groups treated twice, no infection was found in all fish except those which received the first treatment 31 days after infection.
It was concluded that the elevated water temperature treatment was more effective to infected fish 10-20 days after infection than when performed at an earlier or later stage of infection, and that the repeated treatments gave even better results and could provide a practical method for the control of ayu glugeosis. It is recommended that the treatment be avoided for the fish with other diseases.