| Haemorrhagic Septicemia of catfish is a threatening disease with high incidence and mortality in fish-farms . At present, no effective measure has been developed to prevent this disease.In this paper, we have demonstrated that Haemorrhagic Septicemia of Southern Catfish is caused by bacterials. Three strains of bacterial pathogen with high virulence have been isolated from liver, kidney and spleen of diseased Southern Catfish with typical Haemorrhagic Septicemia. The three strains of the pathogen are all capable of causing Haemorrhagic Septicemia in healthy naive Southern Catfish and have been identified to be Aeromonas hydrophila with different biological characters and sensitivities to antibiotic drugs.After experimental infection with the isolated pathogens, the morphology of blood cells and hematological parameters of infected fish were monitored. In infected fish, the red blood cell counts were (1.44 + 0.16) X 106mm-3, which were significantly lower than that of control[ (2.28 +0.17) X 106mm-3], indicating that hemolysis was caused. On the contrary, the white blood cell counts were increased from (6.25 + 1.67) X 104mm-3 to (8.375 + 3.25) X 104mm-3 , suggesting that immunological response was induced. At the same time , the erythrocytes and neutrophils were deformed and the lymphocytes were enlarged. The DLCs (differential leukocyte counts) of neutrophils and monocytes were higher in the infected fish, but the levels of lymphocytes and thrombocytes remained unchanged.Obvious pathological changes were observed in liver, kidney and spleen of diseased catfish with a light microscope. The structure of the liver was significantly changed. The arrangement of liver cells was disturbed and the borderlines between cellls were invisible. The majority of liver cells swelled and became larger than that of healthy catfish, and were even degenerated. In kidney, fewer nephrons and more lymphocytes were observed. Renal capsule space became larger. Renal corpuscle swelled and renal tubule epithelial cells had significantly larger size, and some showed necrosis. In spleen, fewer and scattered lymphocytes were observed.The vaccines made of detox icated bacterial were all capable of inducing immunological response in immunized fish. The naked bacterial cell vaccine has some effect, but not so good as that of others. On 2nd day of immunization, a rapid increasing level of the white blood cell counts in peripheral blood was observed. On 4th day, the white blood cell counts were elevated to levels between (79.8 + 9.5472) X103mm-3and (84.2 + 7.3521) X 103mm-3, and remained on such higher levels until 28th day. The DLCs of white blood cells were also changed after immunization. From 2nd day to 14th day, increasing DLCs of monocyte and neutrophile were observed, and on 7th day both reached to peak points(13.35% and 10.06%), which were very siginificantly higher than that of control(3.58% and 5.99%). After 14th day, the DLCs of both were decreased gradually and on 26th day, returned back to the beginning. Although no increasing lymphocyte levels were observed between 2nd and 4th day, lymphocyte level began to be increased on 7th day, and on 21s' day reached to peak point 47.29%. During the whole immunization process, the fhrombocyte level of immunized fish was slightly lower than that of control, but with no statistically significant difference.Phagocyte was significantly activated after immunization. The phagocytosis percentage(PP) and phagocytosis index(PI) in immunized group were 16.00%~48.40% and 1.36-4.82 respectively, which were both significantly higher than that of control(12.48%~19.57%). On 4th day of immunization, both the PP and PI reached to peak points 48.40% and 4.82 respectively. On 21st day, PP and PI began to be decreased, but both were still higher than those of control.Antibody levels were increased after immunization. Increasing antibody levels began to be observed on 14th day. Except the group immunized with naked cell vaccine, rapid increasing antibody levels were observed on 21st day, and reach...
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