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Abstract
Flavobacterium columnare is considered one of the most important pathogen nowadays . It causes serious problems in fish industry.
During this study, total number of 1490 of Tilapia spp. and C.gariepinus which varied in life stages showed signs of columnaris disease. These fishes were examined clinically for any gross and postmortem abnormalities. These fish were collected from different sources. These fishes were examined during different seasons of the year. Fish were subjected to bacteriological examination to determine the prevalence of F.columnare.On the other hand, Thirty hundred and fifty two of apparently healthy O.niloticus with average body weight 35±5 gm were used for studying the pathogenecity of F.columnare isolates, effect of environment (temperature and salinity) on the pathogencity of F.columnare and trials for treatment of experimentally infected fish by different antibiotics according to the results of sensitivity test against F.columnare . Also trials were making by using the humic acid for the prevention the occurrence of F.columnare.