Identification of a reassortant infectious bursal disease virus (genotype A3B1) in the United Kingdom: pathogenicity and immune complex vaccine efficacy.

The immunosuppressive effects of the infectious bursal disease virus (IBDV), together with its ongoing genome evolution, still represent a major challenge for poultry farmers. This study characterized a novel reassortant IBDV strain, named UK2020, which was isolated in broiler farms from the United Kingdom in 2020 and has now spread across Europe. The phylogenetic analysis revealed that UK2020 belongs to the genotype A3B1, with segment A derived from a very virulent strain (vvIBDV) and segment B from a classical attenuated virus. The pathogenicity studies conducted in both specific-pathogen-free and commercial broiler chickens, with and without maternally-derived antibodies (MDAs), demonstrated that UK2020 caused subclinical infections. These infections resulted in splenomegaly, moderate inflammation of the bursa with histopathological changes, such as lymphoid depletion, inflammatory cell infiltration and plical oedema, and a significant humoral response, despite the absence of clinical signs. The efficacy of a commercial IBD immune complex (Icx) vaccine was also evaluated. Vaccinated birds showed significantly higher antibody titres and reduced lymphoid organ damage compared to mock-vaccinated controls. These results highlight the need for continuous field surveillance to monitor the circulation and impact of reassortant IBDV strains under the current control programmes. The complete sequencing of segments A and B is essential for accurate detection and classification of such strains. The use of Icx vaccines may contribute to mitigating bursal and splenic lesions associated with A3B1 infections like UK2020, especially in flocks with variable MDA levels.