Appraisal of Three Commercial Inactivated Avian Influenza Vaccines Efficacy Against Challenge with Egyptian Duck-Origin Clade 2.3.4.4b H5N8 in Commercial Broiler Chickens

Abstract

The highly pathogenic avian influenza (HPAI) H5N8 virus belonging to clade 2.3.4.4bwas detected in Egypt during 2016, which is one of the few countries depends onvaccination as a main control strategy. This study was conducted to assess the efficacy ofthree different commercial inactivated AI vaccines of different clades (clade 1, clade2.3.4.4b and (Re-6 & Re-8)) in commercial broiler chickens against HPAI H5N8 clade2.3.4.4b.The clinical protection percent was 67%, 80% and 87% in G1 (clade 1), G2 (clade2.3.4.4b), and G3 (Re-6 & Re-8), respectively. The lowest mean antibody titer was recordedin G1 which exhibited unsatisfactory level of immune response. Also, Antisera raisedagainst clade 1 vaccine showed minimal reactivity with clade 2.3.4.4b and Re-6&Re8antigens suggesting poor cross-protection. This was reflected on the antigenic relatedness(R- value) which was low (11.3%) indicating major antigenic difference. Samples werecollected for examination of histopathological changes from thymus, trachea, spleen, bursaof Fabricius, lung, and cerebrum. Only mild cerebrum lesions were found in all challengedgroups indicating reduced neurotropism. In lymphoid tissues, moderate to severe lesionscore were detected. This impairment effect on some bird immune organs highlight theimmunosuppressive effect of the virus, which can appear in the vaccinated exposed birdsto HPAI virus. It is recommended to update the vaccine seed strains to be closely related tothe circulating field strains to obtain better protection levels. Also, continuous evaluation ofthe validated AI vaccines against recent field strains is substantial.