الفهرس 
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Abstract
In the present study, 150 fish specimens of different ten fish species (8 marine fish species and 2 species of freshwater fish) collected from Japan were examined using polymerase chain reaction (PCR) which confirmed 61 isolates of A. hydrophila at the molecular weight (103bp).
For the first time, the PCR used for detection of ten virulence genes of A. hydrophila using specific primers that revealed, the detection of six virulence genes only.
The lethal dose fifty (LD50) experiments were made using six different strains with different dilutions (106-109) in Oreochromis niloticus and revealed that, the dilution (106) cfu caused 50% mortalities in the fish within (12-24h) for the strains carrying (1-5) virulence genes, although the strain (TG-3) free from virulence genes caused 50% mortalities at the dilution (1.8×106) cfu at 36 h.
The histopathological investigation of the LD50 experiments of dead fish with different strains and dilutions of A. hydrophila , revealed that, the liver showed congestion, vacuolation and necrosis of hepatopancreasee and infiltrations of mononuclear cells and heterophils as well as activation of melanomacrophages, while the spleen showed lymphoid depletion, inactivation and depletion of melanomacrophage centers and congestion and the kidney revealed vacuolar degeneration and necrosis of the renal tubular epithelium together with renal casts. In addition congestion, activation of melanomacrophages, interstitial mononuclear cells and heterophils infiltration.