Evaluation of inulin as prebiotic on Rainbow trout (Onchorhynchus mykiss) (Walbaum, 1972) immunity characteristics and resistance to Streptococcus sp infection

Use of dietary supplements that can promote immune system function is one of the strategies in which not only provide essential nutrients to support growth and development of the cultured organisms but also may be one of the most promising means to influence the culture organism''s health and resistance to stress and disease –causing agents. One group of health promoting compounds is referred prebiotics. Inulin a polysaccharide is a prebiotic that its health promoting function has been proved in human and mammals but little is known about its effect on fish. In this study fish were fed with trout commercial food that contained 40% protein، 14% lipid and estimated digestible energy level of 3. 5 kcal/g. Inulin had supplemented in diet at 0. 5 and 2% at weight. Each diet was fed to three replicate groups of Rainbow trout in P. V. C tanks and fish were fed twice daily to apparent satiation. After two month feeding 10 fish selected in each replicate and challenged with Streptococcus sp to test the effect of diet on disease resistance. Diet supplementation significantly stimulated serum lysozyme levels، total، IgM، and WBC (P<0. 05). Also Lymphocyte numbers in fish fed inulin were significantly higher in comparison with control group (P<0. 05). After 15 days of bacterial injection 15-20% of fish fed with 0. 5 and 2% inulin show disease signs (86% survival) but survival of control fish was only 65%. It conclude that inulin as prebiotic has positive effect on immune system in Rainbow trout and increase resistance against pathogenic bacteria Streptococcus sp.