الفهرس 
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Abstract
The aim of the current study to investigate the most common disease problems in Oreochromis niloticus. During this study, total number of 400 fish was randomly collected from Manzala farm at Dakahlia governorate. During the period from October (2015) to November (2016). The collected fish subjected to full clinical, P.M.,bacteriological, parasitological and histopathological examination, and investigation of the preliminary bacterial isolates against some antimicrobial agents as a trial of treatment and consider one of the control methods.
Bacteriological examination revealed the total infection of bacteria (46.75%).
The highest prevalence of bacterial isolates during the whole period of examination of naturally infected O.niloticus was recorded for A.hydrophila (22.66%) followed by V. alginolyticus (19.01%), V.parahemolyticus (13.80%),Streptococcus spp. (12.24%), A.caviae (11.72%),V.cholera (10.16%), Non- chromogenic A.salmonicida (7.55%) however, the lowest prevalence was recorded for Klebsiella oxytoca was percentage of (2.88%). The highest prevalence of isolates was recorded in liver (38.02%), while, the lowest prevalence was obtained by spleen (13.28%).Seasonal prevalence of isolated bacterial strains during the whole period of examination of naturally infected O.niloticus revealed the highest incidence in spring (30.21%), while the lowest prevalence recorded in winter (19.01%).This study also revealed the occurrence of mixed infection in naturally infected O.niloticus that attained (22.75%) as mixed parasitic and bacterial infection.Parasitological examination revealed that the total infestation of (32%). These parasitic infestation in including protozoan as (Trichodina spp.) by (28.13%) and monogenetic trematodes as (dactylogyrus and Cichyidogyrus) by (71.88%).The highest infestation rate during autumn (32.81%), while the lowest prevalence recorded in winter (16.41%). Histopathological findings of the tissue samples which collected from different organs of naturally infected O.niloticus revealed that liver show vaculation of hepatocytes, sever congestion of central vein,Kidney and spleen show degenerative changes and sever hemorrhage,Gills show necrosis .Antibiobiogram susceptilibility testing of some antibiotics on the predominant bacterial isolates as Ciprofloxacin,Nalidixic acid, Colistin sulphate ,Lincomycin ,Oxytetracycline,,Nitrofuration, Erythromycin ,Flumequine .Itrecorded that Ciprofloxacin are the most effective antibiotic against all isolated bacterial strains.