| Edwardsiella tarda is a species of opportunistic pathogen that threats aquaculture seriously. Since it was first reported by Hoshina (1962), it has caused diseases in tens of species of fishes, and caused serious losses to the industry. In fact, the hosts of this pathogen are very wide including fish, amphibians, reptiles, and mammals, even in human, many cases caused by this pathogen have also been reported. Therefore, it has attracted much attention of many scientists. At the beginning of this thesis the studies on this pathogen were reviewed in details, including its physiological and pathological characters and pathogenesis. Treatments against it were also introduced.Before the main experiments primary study on this pathogen was done. When the bacteria were incubated at 25 , the growth curves in different conditions showed that the lag phase was about 4h, and after the logarithmic phase, from 16th to 40th h it was the stationary phase, and then the numbers began to decline. In the medium containing better nutrition components the quantity of each phase was higher and in higher salinity condition the quantity was lower. Infection test showed that 3-day LDso of the pathogen to Japanese flounder, Paralichthys olivaceus (with mean weight: 10 g) was 2.0 108 CFU/mL. Intracellular substances of the pathogen had lethal effect on the fish and extracellular productions had no lethal effect.To evaluate the effect of temperature on the virulence of E. tarda to Japanese flounder, P. olivaceus, experiments were designed, using the testsof both LD50 and isolation, a two-factor design was conducted to evaluate the effects of both pathogen incubation temperature and fish cultivation temperature on the pathogen virulence. E. tarda was incubated at 15, 20, 25 and 30 1 , and the fish (mean weight: 10g) were reared at 15, 20 and 25 1 respectively. Then, in LD50stest after the fish were intraperitoneally injected with suspensions of E. tarda 4-day LDsos were calculated. In isolation test, after the fish were intramuscularly injected, every 24h, six living fish were picked out randomly from each group to detect the proliferation of the pathogen in their livers. Two-way ANOVA analysis of the results of 4-d LD50 test showed that both temperatures significantly affected the virulence of E. tarda (P<0.01) and the interaction between them was also significant (P<0.01). At 15 of fish rearing temperature, the virulence of E. tarda was the highest at 25 of pathogen incubation, followed by 20, 15 and 30 . When fish rearing temperature raised to 20 and 25 , the virulence of E. tarda incubated at all temperatures became stronger. The isolation test demonstrated similar results to those of LD50 test. ANCOVA analysis showed that the pathogen incubated at 20 had the highest proliferation rate, and that at 15 the lowest. The higher rearing temperature increased the proliferation rate of the pathogen in fish.To evaluate the effect of salinity on the virulence of E. tarda to Japanese flounder, P. olivaceus, the incubation salinity of E. tarda was at 0, 10, 20 and 30g/l respectively, and the fish with mean weight of 50g was cultured in the natural seawater of 30g/l. The results of one-way ANOVA in 4-d LD50 test showed that the incubation salinity affected the virulence significantly (P<0.05). The virulence was lower when the salinity of incubation medium was at 0 and 30g/l, higher at 10 and 20g/l. The results of isolation test were accordant with those of LD50 test. At 20g/l E. tarda had a faster proliferation rate than that at 10g/l.To evaluate the effect of salinity on the anti-edwardsiellosis ability ofJapanese flounders, healthy juvenile Japanese flounders with mean weight of 50 g were cultured at different salinities: 16g/l, 32g/l and 40g/l. The fish were acclimated at the aimed salinity for a week before infection. In LD50 test, after the fish were intraperitoneally injected with suspensions of E. tarda the 5-day LD50s were calculated. In isolation test after the fish were intramuscularly injected, every 24h, f...
|
PDF Full Text Request