الفهرس 
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Abstract
The present study aimed to investigate the innate immunity and host-pathogen interactions in fresh water fish, O.niloticus by assessing of cytokines transcripts elaborated by Edwardsiella tarda and comparing the levels of transcription of different cytokines (IL-1ß– IL-8– TNFα) by real time PCR. Also this study aimed to characterize the nature of histopathologic alterations and to evaluate the possible protective effect of dietary supplementation of MOS against Edwardsiellosis in adult O.niloticus.
Therefore, 120 healthy adult Nile tilapia with body weight80 ±5g were raised in aquaria 90×40×40 Cm3with capacity of 150 L. The fish were fed manually using commercial rations containing 30% crude protein. Fish were acclimized for 3 weeks divided into 2 groups, group fed on ration only and group fed on ration and MOS, After acclimatization, Nile tilapia fish were divided into four groups, one served as control and injected with 10 μl phosphate buffer saline, group two exposed to infection with Edwardsiella tarda (105 CFU) and supplemented with MOS (500 mg /kg diet daily), group three fed on MOS only (500 mg /kg diet daily) and group four exposed to infection with Edwardsiella tarda (105 CFU) only. The period of the experiment was 6 weeks in a static renewal bioassay with continuous aeration. Randomly, 3 fishes were collected from each group from which tissue samples (head kidney) were collected at 2, 4, 8, 24 hours post infection with Edwardsiella tarda, to determine mRNA gene expression of cytokines (IL-1ß – IL-8 – TNFα), blood samples were collected at 2, 4, 8, 24, 48, 72 hours post infection to assay some immunological parameters (Bactericidal activity, Lymphocyte transformation test and Lysozomal activity). Histopathological alterations also was determined in liver, muscle and spleen of Nile tilapia fish.
The results revealed that:
• There were some abnormal behavioral changes and clinical signs observed like, fish appeared lethargic and showed signs of organo-megaly, ocular disease and also sever signs of septicemia, while control fish exhibited no abnormal behaviors or lesions.
• Fish showed average cumulative mortality percent of 16% in group 4 infected with Edwardsiella tarda and 6.5% in group 2 which was infected with Edwardsiella tarda and fed on MOS, while the mortalities in control group and group 3 fed on MOS only were 0%.
• mRNA gene expression of IL-1ß in different groups, fish showed a rapid induction of IL-1ß at 2 hours post infection and expression reach its highest levels at 4 hours and returned near to control levels at 24 hours post infection.
• mRNA gene expression of IL-8in different groups began to increase at 4 hrs post infection and reached its highest levels at 8 hrs and then began to decline at 24 hrs post infection.
• mRNA gene expression of TNFα in different groups, fish showed a rapid induction of TNFα at 2 hrs post infection. Expression showed its highest levels at 2 hrs post infection, then began to decline at 4 hrs and returned near to control at 8 hrs post infection.
• group 2 which was infected with Edwardsiella tarda and fed on MOS showed the highest levels of expression of mRNA genes of IL-1ß,IL-8 and TNFα, followed by group 4 which was infected with Edwardsiella tarda only, while the lowest levels were observed in group 3 which fed on MOS only.
• There was increasing in the level of some immunological parameters (Bactericidal activity, Lymphocyte transformation test and Lysozomal activity). The higher levels of immune response were observed in group 2, followed by group 4, and group 3, and the lower levels were observed in group1(control).
• The histopathological examination revealed lesions in the spleen, liver and muscle of fish infected with Edwardsiella tarda. The spleen showed moderate to sever activation of melano-macrophage centers (MMCS). Liver showed necrotic and individuality of hepatocytes, sever fatty changes characterized by presence of sharply outlined vacuoles, congestion of pancreatic acini and activation of MMCS. The noticeable lesions of muscle was sever necrotic myocytes with intense infiltration of phagocytic cells. MOS supplementation reduced these lesions into mild activation of MMCS in spleen and mild to moderate necrotic myocytes with mild infiltratio.