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.
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.
The effectiveness of vaccination against red sea bream iridoviral disease (RSIVD) was evaluated using two kinds of vaccines in red sea bream (Pagrus major). For one vaccine, RSIV-infected GF cells were inactivated with formalin (1.0% v/v) for 10 days at 4°C and for the other the cell culture supernatant of RSIV-infected GF cells was inactivated with formalin (0.3% v/v) for 10 days at 4°C. Juvenile red sea bream were intraperitoneally injected with vaccines and after 10 days the fish were RSIV-challenged by intraperitoneal injection. Statistical analysis showed significantly high survival rates in the vaccinated groups than that of the respective control groups, indicating the protection is induced by the vaccination. In addition, the expression of the virus specific antigens in the spleen was examined for vaccinated and control fish after RSIV challenge by immunofluorescence test. The result that the expression of antigens was weaker in the vaccinated fish compared with the control fish supported the efficacy of the vaccination.
Co-occurrence of two types of Myxobolus ‘cysts’ was recognized in the gills of cultured common carp (Cyprinus carpio) juvenile; one was a highly harmful large ‘cyst’ type and the other was a harmless small ‘cyst’ type. At a farm investigated, prevalences of large and small-type ‘cysts’ reached a maximum of 25 % in July and 100% in August 1995, respectively. Invasion of the myxosporean forming large ‘cysts’ was presumed to start before mid-June, whereas that of the small-type started after mid-June. Large-type plasmodia developed along with the primary lamellae and finally formed large irregular-shaped ‘cysts’, reaching several mm in size. On the other hand, small-type plasmodia grew in the secondary lamellae and the ‘cyst’ was confined to a single lamella. Morphological and serological comparisons of the Myxobolus spores from the two ‘cyst’ types indicated that both types represented Myxobolus koi Kudo, 1920.
The monogenean Heterobothrium okamotoi, parasitizing, in clusters, the branchial cavity wall of cultured tiger puffer Takifugu rubripes, were collected together with some host tissue, in which the posterior part of the parasite body was embedded, to make in vitro observations of copulation and egg production under the stereomicroscope. Some specimens copulated with other worms within the same cluster by attaching their male copulatory organ to the ventro-lateral or dorso-lateral body of their partners. Self-fertilization was also observed. Eggs were produced at about 2-min intervals. After egg assembly in the ootype, the egg was transferred to the uterus and large numbers of eggs accumulated there. Eggs were connected, at both ends, with previous and successive ones through a continuous filament, forming a long egg string. The length of the filament connecting the eggs corresponded with the length of the duct between the ootype and the uterus. Eggs were extracted from the uterus of freshly collected worms and counted. The highest number of eggs per worm exceeded 1, 500. Based on the measurements of the egg and filament lengths and the number of eggs recorded, the longest egg string was calculated to be over 2.8 m long.