Use of Morinda citrifolia Aqueous Extract as an Immunostimulant and Prophylactic Agent Against Edwardsiella tarda in Tilapia (Oreochromis niloticus)

Edwardsiella tarda is an important fish pathogenic bacteria causing massive mortalities and large economic losses in natural environment and fish farming worldwide. Public concerns on the use of antibiotics to control bacterial infectious diseases have been increasing. The objective of this study was to examine the effectiveness of aqueous extract of Morinda citrifolia leaf on nonspecific immunity and disease resistance against E. tarda in the tilapia (Oreochromis niloticus). Parameters used for assessment of the status of innate immunity were phagocytic activity and respiratory burst activity of leucocytes and lysozyme activity in plasma. Dose-dependent increase of the phagocytic activity was observed at week 3 and week 4 in fish given diets with 0% (Diet 1), 1% (Diet 2), 1.5% (Diet 3) and 2% (Diet 4) of M. citrifolia aqueous extract. Fish receiving diet 2, diet 3 and diet 4 had the increased level of lysozyme activity at week 3 and week 4, compared to the control group, but no significant difference in the activity was found between fish treated with diet 3 and diet 4. Diet 2 did not have any effect on respiratory burst activity, while diet 3 and diet 4 caused the similar degree of increase in the activity at week 3 and week 4. When the fish diets were given to the fish 3 weeks prior to intraperitoneal infection of E. tarda, a dose dependent reduction in mortality of the infected fish was obtained. No mortality was apparent in the fish receiving diet 4. The results of this study suggest that M. citrifolia aqueous extract may be used in fish as an immunostimulant and prophylactic agent against E. tarda.