Aeromonas hydrophila is the main reason of epidemic septicaemia for freshwater fish. In the present study, the effect of Aeromonas hydrophila infection on the non-specific immunity of blunt snout bream (Megalobrama amblycephala) was studied. After Aeromonas hydrophila challenge, lysozyme activity was significantly increased at 4 h, 1 d, 3 d, 5 d, 14 d and 21 d. An increased level of lysozyme activity indicated a natural protective mechanism in fish. The significant increases of superoxide dismutase activity and catalase activity in treatment group were detected at 4 h, 1 d, 3 d, 5 d, 14 d and 21 d after Aeromonas hydrophila challenge. Increase in the superoxide anion and H2O2 is considered to be beneficial for self-protection from disease. Acid phosphatase activity increased significantly at 1 d, 3 d and 5 d after Aeromonas hydrophila challenge. Alkaline phosphatase activity in treatment group showed significant increase at 4 h, 1 d, 3 d, 5 d, 14 d and 21 d compared to control group. Increased phosphatase activity indicates higher breakdown of the energy reserve, which is utilized for the growth and survival of fish. These results revealed that the non-specific immunity of fish played an important role in self-protection after pathogens infection.

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