Six hundred and forty Nile tilapia (Oreochromis niloticus) weighing 80-100g were randomly allocated into eight equal groups (80 each). The first group acts as control. Groups S, B and L were fed on a ration supplemented with Saccharomyces cerevisiae, beta-glucans and laminaran, respectively for 21 days. Groups M, MS, MB and ML were subjected throughout the experiment to sublethal concentration of mercuric chloride (0.05 ppm). Gps. MS, MB and ML were fed on a ration containing S. cerevisiae, beta-glucan and laminaran respectively for 21 days. Fish were challenged with Aeromonas hydrophila (0.4 x 10(7) cells mL(-1)) via intra-peritoneal injection and the mortality rate was recorded up to 10 day post-challenge. The non-specific defense mechanisms, cellular and humoral immunity, beside the total and differential leukocytic count were determined. Lymphocyte transformation index, phagocytic activity percent, phagocytic index, total lymphocyte count, serum bactericidal activity and nitric oxide as well as the survival rate were insignificantly changed after 21 day in gps. MS&ML, when compared with mercuric chloride immune depressed group M. These parameters as well as the neutrophil adhesion, serum nitric oxide and survival rate were significantly increased in gp. MB when compared with gp. M. Meanwhile the cellular and humoral immunity beside the survival rate were significantly increased in groups S, B, L when compared with control group. It could be concluded that the whole yeast S. cerevisiae, beta-glucan and laminaran can be used as immunostimulants for the farmed Nile tilapia. The beta-glucans could be used in farmed Nile tilapia, under immune depressive stressful condition to increase their resistance to diseases.
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