الفهرس 
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Abstract
A total number of 940 apparently healthy a live O. niloticus obtained from Abbassa fish hatcheries at Sharkia province were used in our study. Out of them, 640 fish with an average body weight (45 ± 5 g) were used for studying the effect of some environmental parameters as (dissolved oxygen, water temperature, water salinity and unionized ammonia) on virulence of C. perfringens type A infecting O. niloticus. Besides, 180 fish with an average body weight (16.71 ± 0.01 g) were used for studying the effect of B. amyloliquefaciens as a probiotic on growth performance of O. niloticus, in addition to its role as a prophylactic agent against C. perfringens type A infection in O. niloticus. Also, 120 fish with an average body weight (40 ± 5 g) were used for studying the effect of B. amyloliquefaciens as a biotherapeutic agent. A total number of 32 adult male albino rats of (95-110 g) body weight were used for studying effect of C. perfringens type A on albino rats (Rattus norvegicus) as fish eaters. The mortality rate, clinical signs, postmortem findings and histopathological changes were recorded at the end of each experiment. Moreover, blood samples were collected and the serum was separated either from the fish or rats at the end of each experiment to determine some biochemical and immunological parameters.