[Excerpts of a presentation by Sofia Egana-Labrin and colleagues at the University of Maryland, AviServe LLC, and the Maryland Department of Agriculture, during the 2024 annual conference of the American Association of Avian Pathologists]

The infectious bursal disease virus (IBDV) causes a major immunosuppressive disease in chickens, leading to economic problems in the poultry industry. The Delmarva (DMV) region is a major US poultry-producing area, and while the control of IBDV relies on broiler and breeder vaccination, immunized flocks may harbor subclinical infections with strains that continue to evolve by antigenic drift and/or reassortment, which can lead to immune escape and vaccine failure.

Several approaches were taken to assess IBDV field isolates. First, we characterized field isolates currently circulating on the DMV based on genome sequence data and structural modeling of the hypervariable region (HVR) of the VP2 capsid gene, encoded by segment A, and the VP1 gene, encoded by segment B.

Second, we evaluated the antigenic cross-reactivity of a panel of recombinant chimeric IBDVs engineered by reverse genetics to define the contribution HVR mutations make to immune escape.

Third, we provide up-to-date whole genome sequencing (WGS) information regarding currently circulating strains. Finally, we evaluated the presence of co-infection viruses in the field samples.

Bursal samples were obtained from commercial farms between 2018 and 2024, and, after RNA isolation, the samples were subject to reverse-transcription polymerase chain reaction (RT-PCR), amplifying the HVR and VP1 genes. Sanger sequencing revealed that all the strains belonged to genogroup A2B1, typical of US variant strains, however, there was a unique "genetic signature" in the HVR of some sequences, compared to the prototype strain DE Variant E: S215N, I272V, S317R, G322E, and E323D, suggestive of a novel IBDV variant.

This variant was present in only 16% of the samples in 2007 but increased in prevalence to over 57% during recent years, suggesting it might have a fitness advantage over other strains.

Additionally, we detected mutations in and around a key region of VP1 that correlates with virulence (amino acids 145-147), with 25% of the sequences having mutations at position 145, and 71% having mutations at position 147. One isolate had two mutations: D146N and D147S, that have been partially attributed to very virulent strains, suggesting there could be differences in virulence between the strains.

Furthermore, 7% of the bursal samples were found to be positive for avian reovirus, demonstrating that coinfection with multiple immunosuppressive viruses occurs in the field.