Adjuvant effect of Licorice (Glycyrrhiza glabra) extract on the efficacy of lactococcosis vaccine in rainbow trout (Oncorhynchus mykiss)

This study aimed to assess the adjuvant effect of Montanide ™ ISA 763 AVG and Licorice extract (Glycyrrhiza glabra) on the immune responses and disease resistance of rainbow trout (Oncorhynchus mykiss) immunized with Formalin inactivated whole cells of Lactococcus garvieae vaccine. Rainbow trout with an average initial weight of 72±3 g were randomLy divided into four experimental treatments: non-vaccinated fish (C), fish immunized with the vaccine without adjuvant (V), fish immunized with vaccine plus Montanide^TM ISA 763 A VG (VM), and fish immunized with containing Licorice extract (VL). Sixty fish were used for each treatment group in three replicates each with 20 fish. Fish were immunized via intraperitoneal (IP) injection after being anesthetized with clove oil. The results showed that the highest lysozyme activity, antibody titer and hemolytic activity of the alternative complement pathway were obtained in the VM group followed by VL and V groups compared to the control fish at 8 weeks post-vaccination. After challenge through IP route with Lactococcus garvieae, the highest survival rate (96.7%) was observed in VM group, followed by VL (76.7%) and V (76.7%) compared to 20% survival in the control group. In conclusion, the results of this study showed that inclusion of Montanide and lactococcosis vaccine can enhance the potency and efficacy of the vaccine in immunized rainbow trout, but fish immunized with vaccine plus Montanide was superior to fish immunized containing licorice extract.