Immunomodulatory effect of Aegle marmelos leaf extract on freshwater fish Cyprinus carpio infected by bacterial pathogen Aeromonas hydrophila

Context: Aquatic organisms (especially fish) require potent defense mechanisms to protect themselves against pathogen invasion and disease formation. The use of immunostimulants in fish culture can prevent the diseases through augmentation of both specific and non-specific immunity. Objective: A study was conducted to investigate the efficacy of different dietary doses of Aegle marmelos (Linn.) Corr. Serr. (Rutaceae) leaf extract for the immune response and the disease resistance of the freshwater fish, Cyprinus carpio Linn. (Cyprinidae) infected by Aeromonas hydrophila Chester (Aeromonadaceae). Materials and Methodology: Hematological, specific immune response, non-specific immune response and enzyme assay studies were performed on fish and were scrutinized after 50 days of feeding trial. Results: Fish were challenged with Aeromonas hydrophila at a dose of 1.5 × 104 cells/mL through intraperitoneal injection, and the hematological changes, the immune response, the enzyme activity and the disease resistance of Cyprinus carpio against the pathogen were also studied for 20 days at 5-day intervals. Discussion: The results obtained from the study demonstrated that the fish fed with leaf extract of Aegle marmelos incorporated into feed significantly enhanced the red blood cell count, white blood cell count, hemoglobin, phagocytic activity, nitroblue tetrazolium chloride assay, lysozyme, pathogen clearance and enzyme activity compared with the control group. The survivability was higher in the fish which consumed leaf extract-incorporated feed, and the fish group fed with 5 g diet showed highest percentage survival of the fish. Conclusion: These results indicate that Aegle marmelos stimulates the immunity and makes the freshwater fish Cyprinus carpio more resistant to Aeromonas hydrophila.