الفهرس 
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Abstract
In this study, a total of 1864 Oreochromis niloticus were used, from which 944 fish of different life stages were obtained alive from Sahl El-Housinia private fish farm and EI- Abbassa fish farm in Sharkia govemorate to study the main bacterial, mycotic and parasitic diseases infecting 0. niloticus during the different seasons of the year.The prevalence, clinical signs and postmortem lesions were recorded. Also 80 fish were used for the pathogencity test of isolates of A. hyDROPhila and Pseudomonas aeruginosa. Another 840 fish were used to determine the effect of some antibiotics (Oxytetracycline and Enrofloxacin) and some disinfectants (sodium chloride, formalin, hydrogen peroxide and potassium permanganate) in control of infection with A. hyDROPhila, Saprolegnia sp. and monogenean
infestation on behaviour, serum biochemical parameters (Cortisol, AST and Creatinine), immunity and histopathological fmdings of liver and kidney of treated 0. niloticus. The results of this investigation revealed that:- Out of 944 examined 0. niloticus, only 600 fish were clinically infected at the rate of 63.55%. The number of diseased fish among the examined Oreochromis niloticus collected from Sahl EIHousinia private fish farm was 255 fish out of 468; at the rate of54.48% while among Oreochromis niloticus collected from El- Abbassa fish farm was 345 fish out of 476; at the rate of 72.47%. 2- Aeromonas hyDROPhila possessed higher infection rate (70%) than Pseudomonas aeruginosa (58%) with highest prevalence during spring season (94.59%) among preadults (82.60%). The clinical