الفهرس 
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Abstract
Newcastle disease (ND) remains a constant threat to the poultry industry in Egypt despite intensive vaccination programs. Therefore continuous monitoring of illness in Egypt is essential to detect the new mutant strain of NDV with related pathological pictures as well as identifying the pathogenesis of the isolated new strain is crucial. The present work is divided into two main studies .Firstly; a surveillance study was conducted to investigate the frequency of Newcastle disease virus (NDV) in broiler breeds in different Egyptian governorates between 2016 and 2018. Out of 40 rRT PCR examined tracheal samples, 18 samples were positive for NDV. Phylogenetic analysis of selected 6 positive samples revealed that the isolated strains were belonging to genotype VII. Various clinical and pathological pictures were detected. The clinical signs including depression, greenish diarrhea and neurologic signs, while the pathological alterations were detected in different organ systems including hemorrhagic tracheitis and proventriculitis, lungs congestion, swollen spleen with scattered necrotic foci, hemorrhages on cecal tonsils and atrophied bursa as well as brain perivascular cuffing with diffuse gliosis and nephritis. Immunological detection of NDV antigen clarified the wide distribution of the virus in almost all examined tissues. The second study was an experimental study that was conducted on commercial broiler chickens infected by velogenic Newcastle disease virus (vNDV) at different ages. One hundred and twenty five broiler chickens, non-vaccinated against NDV were allocated into five groups (25 birds each); one group served as control non infected group and four infected groups that were inoculated with 106 EID50 of NDV at 10, 15, 20 and 30 day of age. Quantitative real time RT-PCR for virus re-isolation as well as expression of IL-6 and IFN-γ genes estimation were performed on lung tissue collected at 3 dpi and kidney tissue at 7 dpi. Serum samples were used to investigate the uric acid and creatinine levels as well as haemagglutition inhibition (HI) test among different experimental groups. Tissue samples from different organs were collected at the 1st, 3rd, 5th and 7th days post inoculation for histopathological, histochemical and immunohistochemical examinations. RT-PCR for virus revealed increase virus load in the lungs and kidneys of older birds. Cytokines profiling revealed significant up regulation of both IL-6 and IFN-γ genes in the groups received infection at older ages. The severity and distribution of pathological lesions were directly proportioned to the age of infection and the time interval post infection. Immunostaining of the viral antigen in the different tissues confirmed the pantropic nature of the virus.