Pathogenicity of the Egyptian A/H5N1 avian influenza viruses in chickens

Long-term circulation of highly pathogenic avian influenza H5N1 viruses of clade 2.2.1 in Egyptian poultry since February 2006 resulted in the evolution of two distinct clades: 2.2.1.1 represents antigenic-drift variants isolated from vaccinated poultry and 2.2.1.2 that caused the newest upsurge in birds and humans in 2014/2015. In the present study, nine isolates were collected from chickens, ducks and turkeys representing the commercial and backyard sectors during the period 2009-2015. The subtyping was confirmed by hemagglutination inhibition (HI) test, RT-qPCR and sequence analysis. The Mean Death Time (MDT) and Intravenous Pathogenicity Index (IVPI) for all isolates were determined. Sequence analysis of the HA gene sequences of these viruses revealed that two viruses belonged to clade 2.2.1.1 and the rest were clade 2.2.1.2. Antigenic characterisation of the viruses supported the results of the phylogenetic analysis. The MDT of the isolates ranged from 18 to 72 h and the IVPI values ranged from 2.3 to 2.9; viruses of the 2.2.1.1 clade were less virulent than those of the 2.2.1.2 clade. In addition, clade-specific polymorphism in the HA cleavage site was observed. These findings indicate the high and variable pathogenicity of H5N1 viruses of different clades and host-origin in Egypt. The upsurge of outbreaks in poultry in 2014/2015 was probably not due to a shift in virulence from earlier viruses.