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 t o 3 successive generations were reared in a fish farm and blood sample s were periodically collected. Pooled and individual sera were screene d for the effectiveness of two kinds of activity generally assigned to the alternative complement pathway: bactericidal activity was assesse d against suspensions of an avirulent form of Aeromonas salmonicida an d spontaneous haemolytic activity (SH50) was titrated using rabbit ery throcytes 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 o ccur. 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 cou ld be observed, it was more difficult to demonstrate between the bacte ricidal activity and the alternative complement pathway. It is conclud ed that the alternative complement activity, although significant in n atural defences, accounts only partly for furunculosis resistance and that bactericidal activity of serum is a more reliable marker of this resistance in rainbow trout. (C) 1995 Academic Press Limited