الفهرس 
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Abstract
In this study,a total of (342) marine fishes of five different species that were represented as: seabass, seabream ,Tilapia zillii , Mugil capitus and Mugil seheli randomly collected from Lake Temsah at Ismailia governorate from August 2015to july 2016. The clinical picture of the diseased fishes, was recorded as 66 fishes were found to be infected with P. damselae subsp. piscicida, as gram negative, non-motile cocobacili short rods with bipolar staining bacteria, seabass was the most infected fish spp (23), followed by seabream (18), then Mugil capitus (10), T. zillii (9) while M. seheli was the lowest infection (6) and 128 fishes were found to be infected with P. damselae subsp. damselae as gram negative, short rod and motile bacteria , seabass was the most infected fish spp (36), followed by seabream (32), then Mugil capitus (23), T. zillii (22) while M. seheli was the lowest infection (15). The results indicated that, the total prevalence of P. damselae subsp. piscicida in all examined species (19.29 %), the highest seasonal prevalence was recorded in summer season (31.313 %) followed by autumn (20%) then spring (14.61%) and winter (7.24%). On the other hand, the total prevalence of P. damselae subsp. damselae in all examined species (37.42%), the highest seasonal prevalence was recorded in summer season (60.60%) followed by autumn (42.35%) then spring (24.72%) and winter (14.49%). The histopathological studies of naturally infected marine fishes showed severe inflammatory reactions in different organs with accumulation of melanomacrophages and necrosis. The pathogenicity of P. damselae sub. damselae appeared through 7 days after (I/P) injection and the mortality reached to 63% in infected with no addition groups while in control groups and infected with supplementation groups showed no death . Clinical signs and P.M lesion were similar to that in naturally infected fishes. The hematological assay of infected fish revealed blood hemolysis showed in decrease in RBCS numbers, Hb concentration and Hct, increase in total WBCS (neutrophils, monocytes) with lymphocytopenia. Total plasma proteins amounts, albumin and globulins were decreased. With addition of Dosto oregano powder 10% by amount of 0.25g/ k.g in basal diet , it lead to enhance the hematological parameters of fishes before infection as there were increasing in HB, RBCs, HCt, WBCs, Total protein, albumin and globulins and after infection as there were increasing in RBCs, WBCs, Total protein, albumin and globulins. These results indicated that oregano act as immunostimulant and antimicrobial.