Comparison of the adjuvant effect of propolis and Freund on the efficacy of Aeromonas hydrophila vaccine in common carp (Cyprinus carpio)

The success of many vaccines relies on their association with selected adjuvants in order to increase their immunogenicity and ensure long-term protection. Propolis is a natural compound, mostly known for its immunostimulatory properties. In this study the adjuvant effects of propolis in combination with formalin-killed Aeromonas hydrophila vaccine in Common carp (Cyprinus carpio) were evaluated and compared with Freund’s adjuvant. Three hundred juvenile carp (weighing 50.4±3.2 g) were divided randomly into four groups in triplicates. The fish were intraperitonealy injected with A. hydrophila bacterin (A.h) in combination with normal saline (Formalin Killed Bacterin, FKB group), Freund (FKB +F group), and Propolis (FKB +P group) respectively. The control group was injected with normal saline.  Serum samples were taken from fish in each group every other week (days 0, 14, 28 and 42) of the experiment and immunological parameters including anti A. hydrophila antibody titer, serum lysozyme and antibacterial activity, complement activity, Nitro blue tetrazolium activity and serum total protein and globulin were compared among the groups. At the end of study the remaining fish in each group were challenged with virulent A. hydrophila and mortality was recorded for 10 days and Relative Percentage Survival (RPS) was calculated and compared among the groups. Results showed that although antibody titer and most of none specific immune responses increased in groups 2 and 3 compare to control group (p<0.05), no significant difference were seen in group 1 and 3 (p>0.05). Besides no significant change was observed in mortality rate following the challenge in the propolis adjuvanted group compared with the FKB group (p>0.05). According to results, it can be concluded that although propolis as an adjuvant can promote some immune responses of common carp, it can't affect efficacy of vaccine and Ab titer of injected antigens, so it seems more work is needed to present propolis as a proper candidate for the development of a natural adjuvant in fish vaccines.