Humoral and non-specific immune responses in rainbow trout(Oncorhynchus mykiss) naturally exposed to and immunizedwith 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) weremeasured in kidney macrophages. Fish were allocated to three experimental g roups (n = 50 per group): survivors of natural S. iniaeinfections, non-exposed healthy fish immunized by immersion with formalin-killed S. iniae bacterin and a control group of non-exposedhealthy 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 of0.082-0.091, 0.089-0.091, 53-55 and 1.8-1.9 respectively. In the immunized non-exposed group, the ASApeak antibody level occurredat day 15 (mean OD=0.122) and declined significantly by day 65 (p<0.05). Similarly, the mean PAand P index values of macrophagespeaked 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

Keywords:

Immune response , rainbow trout , naturally exposed , Streptococcus iniae , immersion vaccination