Background: Prebiotics contain non-digestible ingredients, which beneficially affect to the host by selectively stimulating the growth of one or a limited number of bacteria at the colon. Inulin and oligofructose are the most commonly used as prebiotic food ingredients in aquaculture. Dietary supplemented of inulin has shown an enhancement on growth and health of fish.  Aim of the Study: The current study aimed to assess the effects of varying doses of inulin as a natural prebiotic on the growth, nutrient utilization, liver histology, immune responses and ecotoparasitic infection of fingerlings Nile tilapia (Oreochromis niloticus).  Materials and Methods: Eighteen full glass aquaria measuring (75×40×35cm) were distributed into six treatments and each treatment was represented in three replicates (25 fish in each aquarium with an initial body weight of 9.23±0.25g). Six diets were formulated by using two levels of sorghum (15 and 30%) and the inulin, which produced and imported from China was used in each level with three doses 2.5,5.0 and10.0g/kg diet. Fish samples of (blood, histological examination of liver and spleen, ectoparasitic infection and proximate composition of fish) were detected.  Results: The results demonstrated that the highest growth performance and feed efficiency were obtained with dietary low inulin ratio (2.5 g/kg diet). White Blood Cell (WBC), Alternative Complement Pathway (ACP) and Superoxide Dismutase Activity (SOD) were affected significantly (P˂0.05) with increased dietary inulin doses. Results of histopathological examinations showed that the fish fed in low dose of inulin improve liver and spleen structures. Also, the ecoparasitic infection showed enhancement in resistance for ectoparasatic with all doses of inulin.
Conclusion: The present results showed that inulin supplementation at low dose (2.5g/kg diet) in the two levels of sorghum inclusion (15 & 30%) in dietary Nile Tilapia, enhancing growth performance, immuno-haematological indices, parasitic infection and each of liver and spleen structure of Nile tilapia fingerlings.