Vibriosis of Ayu

Abstract

Two hundred fifty nine strains of Vibrio anguillarum were isolated from cultured ayu of various districts in Japan from 1974 to 1977. These strains were tested for their susceptibility to 6 chemotherapeutic agents : sulfamonomethoxine, chloramphenicol, tetracycline, nalidixic acid, nitrofurazone and trimethoprim. Nine of the 259 strains were sensitive to all drugs tested. The remaing 250 strains were resistant to various combinations of the six drugs tested. Transferable R plasmids were detected in 165 out of the 250 resistant strains, and strains from all districts turned out to have some transferable R plasmids. This high incidence of drug resistant strains of V. anguillarum was assumed to be due to the selective pressure exerted by chemotherapeutic agents used during culturing the fish. These strains fall into at least three different O sero-groups.
The efficacy of hyperosmotic infiltration technique (HI) in controlling vibriosis of ayu was tested. Experimental fish were first placed in a solution containing 5.32% NaCl for 2 min. and then placed in a vaccine solution of V. anguillarum (5.0 × 10⁸/ml) bacterin for 3 min. A month after immunization, the immunized fish were found to be resistant to V. anguillarum to intraperitoneally injected.
In the second experiment, formalin-added culture of V. anguillarum was centrifuged and the precipitated bacterial cells were resuspended in saline solution of the same volume as the original culture. As in the first experiment, the fish were held in a solution of 5.32% NaCl for 2 min, and then placed either in the above suspension or the medium supernatetant for 3 min. Immersion of fish in the suspension provided adequate protection but immersion of fish in the supernatant was not effective to protect against intraperitoneally injected V. anguillarum a month after vaccination. Treatment with the supernatant caused high mortality to ayu presumably due to the exotoxin from bacteria but immersion in the Vibrio cell suspension did not cause higher mortality than a control group immersed only in 5.32% NaCl solution during are one month.
In the third experiment, the fish were immersed in 5.32% NaCl solution containing lyophilized whole cells of V. anguillarum for 3 min. The vaccinated fish turned out to be well protected against V. anguillarum intraperitoneally injected a month after treatment. The HI method could be used most effectively to immunize ayu against vibriosis.