Basic and practical studies on the immunological prevention of salmonids against bacterial diseases

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Protective immune mechanisms of salmonids against fish pathogen Vibrio anguillarum and Aeromonas salmonicida were investigated.1. Mechanism of activation of macrophages and the role of the macrophages in protection: Relationship between antibody production in rainbow trout and resistance against bacterial infection was investigated. Though the fish immunized with formalin-killed bacteria resisted the challenge with virulent bacteria, there was no obvious relationships between antibody titer and protection. Living V. anguillarum elicited strong chemiluminescent reaction in peritoneal macrophages of rainbow trout. The activation of macrophages seemed to be related to the resistance of rainbow trout.2. Production of C-reactive protein (CRP) and the role of CRP in protection: CRP levels in rainbow trout sera increased significantly after bacterial infection. Purified CRP activated complement system of rainbow trout, and suppressed in vitro growth of bacteria. CRP levels were also affected by water temperature.3. Mechanism of resistance of rainbow trout against bacterial endotoxin: Rainbow trout serum was found to contain higher levels of endotoxin than mammals. Though the amount of endotoxin in serum increased significantly by the injection of endotoxin, no clinical abnormality was observed in fish, indicating that the rainbow trout possessed high resistance to endotoxin.