الفهرس 
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Abstract
The present study aimed to differentiate between avian tumor viruses by using the immunohistochemical techniques and histological examination. A total number of 250 samples from 10 different governorates were used, the samples classified as 190 Layers, 30 breeders, 20 broilers and 10 turkey. Clinical examination of live bird are carried out, and also Necropsy was performed on the dead and sick chickens. Samples were divided into two parts part kept in 10% neutral buffered formalin, and the other frozen For molecular examination. Marek’s disease (MD) is one of the economically important lymphoproliferative viral disease of poultry causing severe economic loss in the poultry industry . Gross lesions, histopathological examination with histochemical stain and molecular detection by Polymerase Chain Reaction (PCR) were applied to identify the type of lymphoma induced by different tumor viruses. At necropsy, large miliary whitish nodular lesions were seen on the organs with enlargement of sciatic nerve and thickening of feather follicle. Microscopically, lymphoproliferative lesions of various extents were seen in different organs by infiltration of pleomorphic lymphocytes were noted leading to extensive damage of the parenchyma. Lymphoid leukosis showed neoplastic cell populations characterized by large lymphoblastic cells, with poorly defined cytoplasmic membrane and basophilic cytoplasm. While in myeloid leukosis induced by subgroup-J of ALV characterized by infiltration of myelocytic cells which characterized by eccentrically located nuclei and had marked nucleolus with clear mitotic figure. The cytoplasm was filled with conspicuous spherical eosinophilic granules. These myelocytic cells showed characteristic brilliant red colored cytoplasmic granules when stained with May- Grünwald Gimsa. REV showed 2 forms : acute reticular form that apperared as infiltration and proliferation of large vesicular cells of the reticuloendothelial system or primitive mesenchymal cells had large vesicular nuclei, which contained one or two nucleoli, with scantly faintly basophilic cytoplasm. The nuclei were large and vesicular and contained one or two nucleoli , and other type is T-cell lymphoma was characterized by a uniform population of large lymphocytes with large vesicular nuclei containing one or two nucleoli with little, faintly staining, basophilic cytoplasm. Immunohistochemical technique against CD3 as a marker against T-cell and IgM as a marker against B-cell that help in differentiate between the type of lymphoma that induced by tumor viral diseases together with the use of PCR as a molecular techniques aid in diagnosis and confirmation of the result .