HUMORAL ANTIBODY-RESPONSE OF BROWN TROUT SALMO-TRUTTA VACCINATED AGAINST FURUNCULOSIS

Two year old brown trout Salmo trutta L. in a freshwater farm with a history of enzootic Aeromonas salmonicida infections were vaccinated with a commercial vaccine against furunculosis by intraperitoneal injection either with or without a booster. Antibody levels to 3 cellular antigen preparations from A. salmonicida, i.e. whole cells of A. salmonicida, A-layer protein, and lipopolysaccharide (LPS) isolated from the bacterium, were measured using an ELISA technique. Antibodies to whole cell A. salmonicida and LPS were demonstrated in both vaccinated and non-vaccinated fish. Antibodies to A-layer protein were only detected in very low quantities in non-vaccinated fish, In vaccinated fish, the levels of these antibodies were significantly higher in boosted fish than in fish vaccinated only once. Western blot analyses of fish serum also revealed antibodies to LPS and A-layer protein. Antibody activities to extracellular products from A. salmonicida were mainly against a 70 kDa component and LPS. Prior to a challenge test in which furunculosis was induced by increasing the water temperature, all fish were bled and individually marked. Only boosted fish showed significant protection in this challenge, and no mortalities were recorded in this group. The groups of fish which had received only 1 immunization were not protected against furunculosis. In these groups, pre-challenge antibody levels to cell membrane antigens, as measured by ELISA, did not differ between survivors and non-survivors. This indicates that high levels of specific antibodies to cellular antigens do not correlate with survival of the individual fish.