Effects of Inactivated Enterococcus faecalis and Mannan Oligosaccharide and Their Combination on Growth, Immunity, and Disease Protection in Rainbow Trout

Abstract We examined the effects of the following seven experimental diets that varied in the concentration of inactivated cells of Enterococcus faecalis (Ef) and mannan oligosaccharides (MOS), on Rainbow Trout Oncorhynchus mykiss: control (C) diet (no Ef and no MOS), diet E0.25% (2.5 g/kg Ef), diet E0.5% (5 g/kg Ef), diet M0.25% (2.5 g/kg MOS), diet M0.5% (5 g/kg MOS), diet EM0.25% (2.5 g/kg Ef + 2.5 g/kg MOS) and diet EM0.5% (5 g/kg Ef + 5 g/kg MOS). Rainbow Trout, initially weighing 36.27 ± 0.42 g (mean ± SD) were distributed into fourteen 60-L glass tanks at a stocking density of 35 fish per tank. Each diet was hand-fed to duplicate groups of fish twice daily for a 12-week period. After the feeding test, an intraperitoneal injection challenge test of Aeromonas salmonicida was conducted over 14 d. Resulting data were submitted to a multivariate analysis of variance. Weight gain increased significantly (P 0.05) in E0.25%, M0.25%, and EM0.5% experimental groups compared with the control. Specific growth rate was significantly higher (P 0.05) in fish fed E0.25%, M0.25%, and EM0.5% diets compared with fish fed the C diet. Feed gain ratio and protein efficiency ratio were significantly improved (P 0.05) in fish fed the EM0.5% diet compared with fish fed the C diet. Feed intake, protein efficiency ratio, protein retention, and the apparent digestibility coefficient recorded slight differences within experimental groups. Hematocrit value and phagocytic activity were significantly higher (P 0.05) in fish fed E0.25%, E0.5%, M0.5%, EM0.25%, and EM0.5% diets compared with fish fed the C diet. Except for fish in the E0.25% group, fish in all other experimental groups showed a significantly higher (P 0.05) mucus weight compared with those in the C group. After the challenge test, cumulative mortality and frequency of A. salmonicida were significantly decreased (P 0.05) in all experimental groups compared with the C group. In conclusion, dosage and single or combined supplementation of Ef and MOS are factors that significantly affect fish performance.