Comparative effects of Aloe vera gel and Freund’s adjuvant in vaccination of common carp (Cyprinus carpio L.) against Aeromonas hydrophila

Natural compounds of Aloe vera gel are well known for their immunostimulatory properties. In this study, adjuvant effects of Aloe vera in combination with formalin-killed A. hydrophila bacterin in immunization of common carp were evaluated and compared with Freund’s adjuvant. A total of 480 juvenile carp were randomly divided into four groups. Groups 1–4 were injected on days 0 and 14 with normal saline (as control), bacterin, bacterin + Freund’s adjuvant and bacterin + Aloe vera gel, respectively. Blood samples were taken on days 14, 28 and 42. Hematoimmunological parameters including white blood cell (WBC) count, alternative complement activity, serum lysozyme and bactericidal activity, respiratory burst activity (NBT) and antibody titer were evaluated and compared among different groups. At the end of experiment, all groups were challenged with live A. hydrophila and relative percentage survival (RPS) compared among groups. According to the results, administration of both adjuvants caused a positive change in fish immune response and most of the aforementioned parameters significantly improved in vaccinated groups compare with control group (P < 0.05). Moreover, WBC count, alternative complement activity, serum lysozyme activity and NBT activity were significantly higher in Aloe adjuvant group compared with Freund’s adjuvant (P < 0.05). Finally after challenge, groups 3 and 4 showed significantly lesser mortality than groups 1 and 2. Overall, our results indicated that combination of A. hydrophila bacterin with adjuvant can improve the vaccine efficacy and resistance against A. hydrophila infection, and in comparison with traditional adjuvants, Aloe vera gel could be used as a natural adjuvant with similar or even greater positive effects on vaccination of common carp.