Molecular Characterization of the Newcastle Disease Virus Currently Circulating Among Broiler Chicken Flocks During 2021 in Sharkia Province, Egypt

Newcastle disease (ND) outbreaks have been occurred in the Egyptian poultry causing high mortalities with severe economic losses. NDV infection of genotype VII has been reported to cause outbreaks in several commercial poultry farms in Egypt. In the present study, NDV was isolated from 6 broiler chicken flocks in Sharkia province suffering from respiratory and/or neurological signs into the allantoic sac of 11-day-old commercial embryonated chicken eggs (ECEs) as well as these isolates were subjected to reverse transcriptase - polymerase chain reaction (RT-PCR). Six isolates (one NDV isolate/each flock) were sequenced to characterize their whole fusion (F) protein gene of NDV. The sequence analysis and phylogenetic study of the six isolates revealed that they were velogenic, belonging to the class II subgenotype NDV VIId with the characteristic amino acid sequences of the F0 protein proteolytic cleavage site motifs (112-RRQKRF-117). As well as the nucleotide blast analysis showed 97.4% - 98.6% nucleotide identity to virulent NDV isolated in Egypt and neighboring countries. On the other hand, the nucleotide blast analysis showed 82.3% - 83% nucleotide identity to reference commercially available vaccine strains. This genetic variation between the commercially available vaccine strains and six studied NDV strains during 2021 may clarify the failure of the currently used vaccines to protect chicken against vNDV subgenotype VIId. Further studies are needed to monitor the protection of the currently used vaccines against recently isolated vNDV strains.