First report on genetic characterization of egg drop syndrome 1976 virus in Egypt

Since its first description in 1991 in Egypt, egg drop syndrome 1976 (EDS-76) virus has received a little attention as a potential cause for the drop in egg production as well as the reduction in egg quality. To date, no studies have been carried out to describe the genetic characteristics of the circulating field EDS-76 virus strains. Thus, the present study was attempted to estimate the emergence of EDS-76 virus in layer flocks and to determine the genetic diversity between the field strains and the vaccine strain 127. During 2022, a total of 5 apparently healthy backyard layer flocks were investigated for the presence of EDS-76 virus infection following complaints of sudden drop in egg production (25-30%), accompanied by high incidence of eggshell defects. EDS-76 virus DNA was detected in the oviduct samples of 4 (80%) flocks by polymerase chain reaction (PCR) assay targeting the hexon gene of the viral capsid. Attempts of viral isolation in duck embryo revealed no embryonic mortality, however, the allantoic fluids of inoculated eggs exhibited a sustained increase in the hemagglutinating (HA) activity throughout three consecutive passages. The obtained strain, designated BH-1, was characterized on the basis of partial hexon gene sequence analysis (GenBank accession number OR531368). The BH-1 strain displayed 99.6% nucleotide identity with the vaccine strain 127. However, amino acid alignments with the vaccine strain 127 revealed that the BH-1 strain carried 5 non-synonymous mutations. In addition, two of these mutations were incorporated into the hexon hypervariable regions (HVRs), which are strictly responsible for eliciting serotype-specific neutralizing antibodies. In conclusion, the present study represents a starting point for genetic characterization of EDS-76 virus in Egypt and highlights the importance for continuous monitoring and characterization of the circulating field EDS-76 virus strains, in order to determine the proper control strategy.