Rapid Spread of Novel Infectious Bursal Disease Virus Variant (Genotype A2dB1b) in the Near East and Persian Gulf Regions: Molecular Surveillance and Phylodynamic Reconstruction

Infectious bursal disease virus (IBDV) is one of the most impactful pathogens of poultry, with disease manifestations ranging from acute forms to subclinical but immunosuppressive infections. This heterogeneity, accompanied by a significant antigenic variability, is sustained by high mutation rates and frequent reassortments between the two genome segments, along with less frequent recombination events. In recent years, the proposal of several classification systems relying on phylogeny contributed to the characterization of several new IBDV genotypes, shedding light on an increasingly diverse epidemiological scenario. One of the most notable examples is the discovery of novel variant IBDVs (nvIBDVs, genotype A2dB1b), which, after emerging in China around 2015, rapidly spread across East and Southeast Asia. More recently, nvIBDVs were also reported in Egypt and Argentina, prompting concern due to their well-established immunosuppressive potential and divergent antigenic features. The detection of A2dB1b strains in Egypt elicited a molecular survey to track their spread within the Middle East. From November 2023 to November 2024, diagnostic samples were collected from 138 flocks in 7 Near East and Persian Gulf countries. The analyses revealed that 55 of them (39.9%) were positive for field strains belonging to 3 genotypes, suggesting a high infectious pressure. Two genotypes, A3B1c and A6B1a, were already reported in the region, although they were found in additional areas. On the other hand, A2dB1b was identified for the first time in Jordan, Lebanon, and the United Arab Emirates, representing a large share of the field viruses detected in these countries. Phylodynamic analyses revealed that this swift spread may have been caused by separate introduction events from Egypt, East Asia, and even South America, highlighting the complexity of IBDV epidemiology. The obtained results will be crucial to better tackle IBDV in the region, guiding monitoring activities and raising awareness toward its proper control.