الفهرس 
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Abstract
Fish contamination with fungi either as saprophytes or parasites has been reported. Fungal species from the genera Aspergillus, Fusarium, have been implicated in fish diseases by multiple authors . In this study a total number of 100 O.niloticus collected from different localities at El-Behera and Kafr El-Sheikh Governorates (50 fish from each). The fish were obtained from their natural sources . The samples were collected during the period from May 2016 to August 2016. The freshly dead fish specimens were subjected to full clinical, postmortem lesions and mycological examinations . During fish sampling the water samples were collected to measure the different water quality . Also, samples from gills, liver and Kidney were collected for histopathological study. In addition to, 210 apparently healthy O. niloticus fish were obtained alive from private fish farm used for determination of lethal dose50 for A. flavus and F. culmorum. The chronic infection of A. flavus; F. culmorum and mixed between A. flavus and F. culmorum in O. niloticus for 4th weeks were carried out on 120 apparently healthy O. niloticus fish, The spores of fungus was intraperitoneally injection by the 1/2 dose of Lethal concentration fifty LC50 then measured the different parameters for 4 weeks. The Sabouraud’s Dextrose Agar media, Potatoes Dextrose Agar media, Czapek’s Dox Agar media used for isolation and identification of mould and yeast. Kits for determination of aspartate amino transferase; alanine amino transferase; urea and creatinine. The results of physical and chemical parameters were higher in Kafr El Sheikh Governorate than El-Behera Governorate with the exception of pH and DO levels was higher in El-Behera Governorate than Kafr El Sheikh Governorate. The non-ionized ammonia, nitrite, nitrate, and hydrogen sulphide were higher than the acceptable range in both Governorates. Water temperature was very high than the stress level where the present work recorded the 30.5 and 32.5°C in both Governorates. Results of clinical signs and postmortem lesion of naturally examined O. niloticus showing severe distension of the abdomen; congestion and sloughing of the tail fins and congestion of the caudal peduncle area in addition to different degree of ascitis. In addition to other naturally infected fish showing severe hemorrhagic exophthalmia; severe hemorrhagic patches on the gills and necrosis of the gill filaments were also reported. Some of naturally examined O. niloticus showing congestion of different degrees on the liver; others show the sever congestion of the kidneys as well as dead eggs filled the egg sacs. The Clinico-biochemicals parameters (AST; ALT; urea and creatinine) in plasma of the apparently healthy O. niloticus and experimentally infected with A. flavus, F. culmorum and both fungi during chronically infected O. niloticus revealed that the AST and ALT were markedly increased in experimentally infected O. niloticus with spore suspension of1/2 dose of LD50 of A. flavus 10-5 and 1/2 dose of LD50 of mixed A. flavus and F. culmorum 10-4 more than the apparently healthy O. niloticus and experimentally infected with F. culmorum . On the other hand the urea and creatinine levels were elevated in both experimentally infected O. niloticus with spore suspension of1/2 dose of LD50 of F. culmorum 10-4 and 1/2 dose of LD50 of mixed A. flavus and F. culmorum 10-4 more than the apparently healthy O. niloticus and experimentally infected with A. flavus 10-5. Liver of experimentally infected O.niloticus by fungus showing severe degeneration and necrosis of hepatocytes in addition to mycotic elements distributed between hepatocytes; severe degeneration and necrosis of hepatocytes in addition to mycotic elements distributed between hepatocytes; severe degeneration and necrosis of hepatocytes in addition to mycotic elements distributed between hepatocytes.