الفهرس 
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Abstract
Avian influenza (AI) remains a constant threat to the poultry industry in Egypt despite intensive vaccination programs. Therefore, the continuous monitoring of illness in Egypt is essential to detect the new mutant strain of AIV with related pathological pictures as well as identifying the pathogenesis of the new strain is crucial. The present work represents a surveillance study that was conducted to investigate the frequency of Avian influenza virus (AIV) in domestic ducks in some Egyptian governorates between 2018 and 2020. The clinical signs were variable, including nervous signs, diarrhea and respiratory distress. Out of 100 rRt-PCR examined frozen tissue samples, 66 samples were positive for H5N8 avian influenza virus. For more characterization, the full haemaglutinin (HA) gene was sequenced, followed by phylogenetic analysis. The phylogenetic tree revealed that H5N8 field isolate clustering in clade 2.3.4.4b, with amino acids identity, ranged from (97.8% - 90.6%) between recent Egyptian isolates. The highest incidence was in the Giza governorate with 33 positive samples. In addition, according to the seasonal variation, the winter season was the highest with 28 positive samples. The histopathological hallmarks, in H5N8 naturally infected ducks, showed acute inflammatory lesions, including non-suppurative encephalitis, congestion and edema, of the brain, which explain the nervous manifestation in the affected ducks. Additionally, the lung and kidneys were congested as well as the spleen showed congestion with marked diffuse splenic depletion. Necrotizing pancreatitis were also observed. Immunological detection of AIV antigen clarified the widespread presence of viral antigen in different organs that explained its systemic tissue tropism with strong positive peroxidase reaction involving the brain including degeneration of neurons, vascular endothelium, and the Purkinje cells. In addition, influenza antigen was found the trachea, lung, heart, intestine, pancreas, and spleen. The present study confirmed that H5N8 HPAI clade 2.3.4.4b, became a predominant strain during the period 2018-2020, causing severe outbreaks in duck farms in Egypt. The presence of viral antigen in different organs indicates the pantropic nature of the virus.