Humoral and non-specific immune responses in rainbow trout (Oncorhynchus mykiss) naturally exposed to and immunized with Streptococcus iniae

In this study, immune responses against Streptococcus iniae infections in rainbow trout (Oncorhynchus mykiss) were analyzed. Anti-streptococcal antibody (ASA) levels, respiratory burst activity (RBA), phagocytic activity (PA) and phagocytic index (P index) were measured in kidney macrophages. Fish were allocated to three experimental groups (n = 50 per group): survivors of natural S. iniae infections, non-exposed healthy fish immunized by immersion with formalin-killed S. iniae bacterin and a control group of non-exposed healthy fish. Blood sera and kidney samples were collected from each group (n = 10 fish) at days 0, 15, 30, 45 and 65 post-immunization (p.i.). Analysis of ASA levels, RBA, PA and P index in survivors of Streptococcus infections revealed optical density (OD) values of 0.082-0.091, 0.089-0.091, 53-55 and 1.8-1.9 respectively. In the immunized non-exposed group, the ASApeak antibody level occurred at day 15 (mean OD=0.122) and declined significantly by day 65 (p<0.05). Similarly, the mean PAand P index values of macrophages peaked at day 15 p.i. (71 and 3.2, respectively) and declined by day 65(p<0.05). Therefore, the acquired immune status of survivors of naturalS. iniae infection was not altered by immunization.