Pathogenicity of three genetically distinct and highly pathogenic Egyptian H5N8 avian influenza viruses in chickens

Abstract In late 2016, Egypt was subjected to multiple introductions of reassorted highly pathogenic avian influenza viruses (HPAI) subtype H5N8. In a previous study, we reported three distinct genotypes during the first wave of infection represented by CAO285, SS19, and F446 viruses that were isolated from wild birds, backyard, and a commercial farm, respectively. F466 has subsequently become the predominant genotype currently circulating in Egypt and has been implicated in the emerging of the H5N2 virus. In the present study, we investigated the difference in the pathogenicity and transmissibility of the three genotypes. The intravenous pathogenicity index (IVPI) ranged from 2.68 to 2.9. With the natural route of infection, all these strains took longer to cause mortality in comparison to S75 (HPAI-H5N1). When compared the H5N8 viruses to each other showed that F446 had high mortality rate after inoculation from original concentration of 106 and 104 EID50 of virus. Chickens inoculated with F446 showed the highest viral titer with significant differnt in all tested samples (H5N8 and H5N1 viruses) in experimental and sentinel contact chicken with more efficient transmission to sentinel contact birds and spread from contact to other birds. Histopathological findings revealed the H5N1 and H5N8 viruses affect all organs examined (lung, trachea, brain, spleen) with relatively different affect the S75 cauterized by early marked respiratory adverse effect and F446 & SS19 were characterized by early systematic pathological alteration with mild respiratory pathological changes. The efficient viral replication and transmissibility in the main bird species, like chicken in case of Egypt, represent a key element for the spread and maintains of certain influenza genotypes of H5N8 virus and decrease the incidence of H5N1.