DETECTION OF A DISTINCT GILL-SURFACE ANTIBODY-RESPONSE FOLLOWING HORIZONTAL INFECTION AND BATH CHALLENGE OF BROOK TROUT SALVELINUS-FONTINALIS WITH FLAVOBACTERIUM-BRANCHIOPHILUM, THE CAUSATIVE AGENT OF BACTERIAL GILL DISEASE

Gill mucus and serum were collected from 2-yr-old brook trout throughout the course of an experimental infection with bacterial gill disease (BGD). This was produced initially by horizontal transmission from sick rainbow trout fry, and second by bath challenge using a pure culture of the bacterium Flavobacterium branchiophilum. Increased serum and gill antibody levels were detected (in an indirect enzyme immunoassay to F. branchiophilum soluble antigen) following both horizontal transmission and bath challenge, although only the increase in gill antibody levels following bath challenge was statistically significant (p < 0.01). Maximum gill antibody levels took longer to reach (57 vs 16 d) during the secondary antibody response. Regression of individual gill antibody titres was poorly correlated with serum antibody titres in the same individuals (r2 = 0.012).