Molecular characterization and pathogenicity evaluation of recent infectious bursal disease virus strains: implications for Newcastle disease vaccine efficacy

Infectious bursal disease continues to cause significant economic losses in the Egyptian poultry industry despite intensive vaccination programs. This study was aimed to molecularly characterize the circulating infectious bursal disease virus strains in Egypt and to compare the pathogenic and immunosuppressive effects of very virulent and variant strains. Ten pooled bursal samples from suspected broiler flocks were subjected to reverse transcription polymerase chain reaction for detection and identification. Nine samples tested positive. Sequencing and phylogenetic analysis identified seven samples (D1, D3, D4, D5, D6, D7, and D8) as very virulent-like strains, showing 98.8-99.2% amino acid sequence identity among them, but only 87.8-91% identity with vaccine strains used in Egypt. Two samples (D9 and D10) were identified as variants with 96-96.4% identity with other Egyptian variants. Two isolates (D8 and D10) were selected to study their pathogenicity and immunosuppressive effects in specific pathogen free chickens, which were orally infected with 105 egg infective dose 50% of each isolate and vaccinated against Newcastle disease, 5 days before infection. The variant strain caused earlier and more severe bursal damage without clinical signs or mortality, while the very virulent strain led to typical disease symptoms and 60% mortality. The mean hemagglutination inhibition titers were lower in variant-infected chickens, while protection against Newcastle disease virus was 60% and 40% in very virulent and variant-infected chickens, respectively, compared to 90% in uninfected chickens. These findings indicate that variant strains are more pathogenic and immunosuppressive than very virulent strains, highlighting the need for effective control measures.