الفهرس 
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Abstract
A total of 200 O. niloticus were collected from El- Abbassa fish farm- Sharkia Governorate. Out of 200 fish, 44 fish in a rate of 22% were suffered from flavobacterial infection which including loss of balance, off-food, loss of scales with excessive mucus all over the body surface and inside the gill pouches, superficial and deep ulcers, progressive fin erosions, fin rot and cotton wool like on the skin. The postmortem lesions were excessive mucus in gill pouches, some gill filaments were congested, while others were pale and in some cases gill filaments showed erosion. Musculature showed superficial ulcers which extended to involve the superficial muscular layer and may extend to deep musculature, Eyes showed cloudiness. Internal examination showed no internal congestion or lesion. The bacteriological examination revealed isolation of different flavobacterial species as Flavobacterium columnare, Flavobacterium psychrophilum and Flavobacterium branchiophilum with a rate of 90.90%, 4.54% and 4.54% respectively. The prevalence of F. columnare infection was the highly one and it was 20%. According to the biochemical characterization of the obtained F. columnare isolates, they were subdivided into 4 groups; one strain from each group was taken for studying the pathogenicity in O. niloticus. The results revealed that the mortality rate were 100% within 7 days in group injected with the highly virulent strain while in the remaining strains, the mortality ranged from 40-80%. F. columnare identification confirmed by PCR, which produced unique and clear PCR bands of the expected size (675 bp), corresponding to the 675 bp internal fragments of the 16S rRNA gene of F. columnare. Antibiogram activity of selected F. columnare strains revealed that all strains were sensitive to Oxytetracycline, Erythromycin, Kanamycin, and Nalidixic acid and 100% resistant to ampicillin, chloramphenicol, Doxycycline Hcl, Neomycin and Sulphamethoxazol. Antibacterial activity of Punica Peel Extract on Flavobacterium columnare showed zone of inhibition at concentration 500 μg. In the present investigation, F. columnare formalin killed bacterin was prepared from the highly virulent strain and the quality control analysis proved that the prepared bacterin was free from any contaminant (aerobic, anaerobic, fungal or mycoplasma). Also, safety test showed that the prepared bacterin did not show any abnormalities or adverse reactions during the observation days among the injected fish. On day zero of the study the treatment group received F. columnare formalin killed bacterin and the negative control group was administered sterile saline. Fourteen days later, all fish received a second administration of bacterin or sterile saline intramuscularly. Antibody titers to F. columnare bacterin were determined using micro-agglutination test which were 960 at 4 weeks post-vaccination. The laboratory challenge study showed that O. niloticus which were injected by F. columnare formalin killed bacterin has protection rate of 76%. Treatment for O. niloticus infected with F.columnare using Oxytetracycline and PPE showed that Oxytetracycline was the most effective treatment for fish infected with F.columnare with survival rate reached to 70%, and PPE was not effective as Oxytetracycline in treatment of fish infected with F.columnare with mortality rate reached to 80%. On the other hand, using PPE was a good choice in the prevention of the occurrence of F.columnare when used in10 g/kg feed ration for month which gave protection rate reached to 60% because it has antibacterial effect and has a good effect on the general health condition of the fish.