Dietary microbial levan enhances cellular non-specific immunity and survival of common carp (Cyprinus carpio) juveniles

A preliminary study with a 75 days feeding trial was conducted to study the immunomodulatory effect of microbial levan on Cyprinus carpio juveniles. Five purified isonitrogenous and isocaloric diets with graded levels of levan, namely (T1) 0.1% levan, (T2) 0.2% levan, (T3) 0.5% levan, (T4) 1.0% levan, and a control group without levan were fed to five groups of fishes in triplicate. The total erythrocyte count and haemoglobin content was significantly (p < 0.05) enhanced in the T3 group, though the total leucocyte counts, total protein and A/G ratio did not show any significant difference (p > 0.05) when compared with the control group. The respiratory burst activity (NBT) of blood phagocytes and lysozyme activity was also highest in T3 group. The relative survival percentage after challenge with Aeromonas hydrophila was highest (100%) in the T3 group followed by 83.33% and 66.67% in the T2 and T4 groups, respectively. This suggests that microbial levan at 0.5% can be used as a dietary immunostimulant for C. carpio juveniles. This is the first report on microbial levan having an immunomodulatory effect on C. carpio.