Feeding-regimen of ß-glucan to enhance innate immunity and disease resistance of Nile tilapia, Oreochromis niloticus Linn., against Aeromonas hydrophila and Flavobacterium columnare.

ß-glucan is one of the most potent immunostimulants enhancing innate immune activity, disease resistance and growth performance of many aquatic organisms. Nevertheless, there are few studies on feeding regimens of ß-glucan that correlate to immune response and disease resistance and are important considerations for practical ß-glucan utilization. Thus, the effect of ß-glucan and feeding duration on innate immunity and disease resistance was investigated to establish an optimal feeding regimen of ß-glucan for Nile tilapia (Oreochromis niloticus Linn.). A variety of ß-glucan feeding regimens were evaluated, including: i) feeding for 2 weeks, ii) feeding for 4 weeks, and iii) feeding every-other-week, with the objective of establishing the optimal feeding regimen that enhanced innate immunity and disease resistance. Innate immunity parameters were determined every week for eight weeks. Alternative complement activity of all ß-glucan groups was significantly (P < 0.05) increased at the end of the first week, and then fluctuated but was not significantly (P > 0.05) different to the control until the end of the trial. Increased lysozyme activity was only detected at the end of the second week in all ß-glucan-treated groups, and then decreased to the control level during most of the sampling periods. Phagocytosis percentage was increased and prolonged by ß-glucan feeding, while the phagocytic index was not. Apart from innate immunity, ß-glucan-fed fish demonstrated enhanced disease resistance against Aeromonas hydrophila and Flavobacterium columnare challenge at only the end of the fourth week of the trial. The growth performance of ß-glucan-fed fish was not significantly (P > 0.05) different among the experimental groups and control. Taken together, the result indicated that all ß-glucan-feeding regimens resulted in quite similar outcomes with respect to innate immunity stimulation, disease resistance and growth performance. This novel result suggests that an every-other-week regimen is the optimal choice for Nile tilapia cultivation as an economic cost saving benefit. This is the first study to determine the optimal feeding-regimen of ß-glucan to enhance innate immunity and increase resistance to infection by pathogenic bacteria in Nile tilapia.