Spontaneous bactericidal and complement activities in serum of rainbow trout (Oncorhynchus mykiss) genetically selected for resistance or susceptibility to furunculosis

Half-sib families of rainbow trout (Oncorhynchus mykiss) selected for heritable susceptibility or resistance to furunculosis and belonging to 3 successive generations were reared in a fish farm and blood samples were periodically collected. Pooled and individual sera were screened for the effectiveness of two kinds of activity generally assigned to the alternative complement pathway: bactericidal activity was assessed against suspensions of an avirulent form ofAeromonas salmonicidaand spontaneous haemolytic activity (SH50) was titrated using rabbit erythrocytes in the presence of EGTA and Mg2+. Significant differences in serum bactericidal properties generally occurred between susceptible and resistant families of trout, although transient variations could occur. Seasonal fluctuations, irrespective of the age of the fish, were also observed. However, whereas correlation between the bactericidal activity or alternative complement pathway and the fish resistance could be observed, it was more difficult to demonstrate between the bactericidal activity and the alternative complement pathway. It is concluded that the alternative complement activity, although significant in natural defences, accounts only partly for furunculosis resistance and that bactericidal activity of serum is a more reliable marker of this resistance in rainbow trout.