الفهرس 
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Abstract
Abstract
The HPAI H5Nx viruses belonging to clade 2.3.4.4b have gained a global concern regarding their diffuse dispersal worldwide. The first trial in the current study aimed to assess the efficacy of 3 commercial inactivated H5 vaccines of various clades (1 - 2.3.4.4b - (Re-6 & Re-8)) against Egyptian clade 2.3.4.4b H5N8/2019 in commercial broiler chickens. The Clinical protection percent was 67%, 80% and 87% in G1 (clade 1), G2 (clade.2.3.4.4b), and G3 (Re-6 & Re-8), respectively. Antisera raised against clade 1 vaccine showed minimal reactivity in HI test with clade 2.3.4.4b and (Re-6&Re-8) antigens suggesting the poor cross-protection and the antigenic relatedness (R- value) was low (11% - 13%) indicating a major antigenic difference within these clades. Recently, clade 2.3.4.4b H5N1 virus was detected in Egypt at 2022 which complicate the AI situation in the country. The second trial in the current study aimed to Estimate and compare the efficacy of three commercial inactivated H5 vaccines of different clades (2.3.2 – (Re-5) clade 2.3.4 - 2.3.4.4b) against newly emerged clade 2.3.4.4b HPAI H5N1/2022 & H5N8/2019 in commercial broiler chickens. Clade 2.3.2 vaccine failed to protect clinically against both challenge viruses while, clade 2.3.4 (Re-5) showed 87 % and 40% protection against H5N8 and H5N1, respectively. Clade 2.3.4.4b showed 60% protection against H5N8 compared to 93% protection against H5N1. The obtained results demonstrated that clade 2.3.4.4b H5N8 vaccine antigenically matched effectively with the newly emerged clade 2.3.4.4b H5N1 virus & could afford high level of clinical protection and reduction of tracheal and cloacal viral shedding. Histopathological lesions were recorded in cerebrum, trachea, lung, spleen, thymus & bursa of Fabricius. The immunopathological effect of clade 2.3.4.4b H5Nx viruses on the lymphoid tissues was displayed as moderate to severe lesion scores and represented as congestion, necrosis and depletion of lymphocytes. Finally, continuous evaluation of the validated AI H5 vaccines against newly emerging AI strains is substaitional to deal with that intimidating remark. Also, the vaccinal seed strains should be updated regularly in order to obtain utmost protection levels and assure presence of antigenic matching among prevailing strain and vaccinal strains.
Key words: clade 2.3.4.4b, H5Nx, HPAl, Egyptian, vaccine efficacy, immunopathology.