Emergence of the novel infectious bursal disease virus variant in vaccinated poultry flocks in Egypt

ABSTRACT

Since the detection of antigenically atypical very virulent Infectious bursal disease viruses (vvIBDV) in Egypt in 1999, the country has been experiencing recurrent outbreaks with high mortality rates and typical gross lesions associated with typical vvIBDV. However, a significant change occurred in 2023, marked by a notable increase in reported subclinical IBDV cases. To evaluate the field situation, samples from 21 farms in 2023 and 18 farms from 2021 and 2022, all of which had experienced IBD outbreaks based on clinical diagnosis, were collected, and subjected to VP2-HVR sequencing. Phylogenetic analysis revealed that all samples collected in 2021 and 2022 clustered with classical virulent strains and vvIBDV. In 2023, one sample clustered with the Egyptian vvIBDV, another with classical virulent IBDV, and the rest with the novel variant IBDV (nVarIBDV) circulating in China. The alignment of deduced amino acid sequences for VP2 showed that all Egyptian classic virulent strains were identical to the Winterfield or Lukert strains, while vvIBDV strains exhibited two out of the three typical residues found in Egyptian vvIBDV, namely Y220F and G254S, but not A321T. Meanwhile, all Egyptian variant strains exhibited typical residues found in nVarIBDV. However, all Egyptian variants showed a mutation at position 321 (321V), which represents the most exposed part of the capsid and is known to have a massive impact on IBDV antigenicity, except for one sample that had 318G instead. This report highlights the emergence of a new variant IBDV in Egypt, clustered with the Chinese new variants, spreading subclinically in broiler farms across a wide geographic area.

RESEARCH HIGHLIGHTS

• New variant IBDV which emerged in Egypt clustered with Chinese nVarIBDV.

• nVarIBDV spread subclinically across a wide geographic area.

• Mutation at 321 represents capsid's most exposed part, a defining feature.

• Antigenically modified vvIBDV still circulating in Egypt with typical lesions.

KEYWORDS:

• Infectious bursal disease virus (IBDV)
• genotype
• new variant
• Egypt
• broiler
• subclinical

Acknowledgements

We would like to thank farm owners and field veterinarians for their cooperation in sampling, reporting history, clinical signs, and lesions. Special thanks for insightful discussions on clinical observations during current and past IBDV outbreaks. The authors express appreciation to Dr. Ahmed M. El Kady (MEVAC) and Eng. Ahmed M. Badawy (VaccineValley) for supporting AHRI in sampling.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The NLQP-AHRI, a governmental institute, receives funding for diagnosis and research from the Agriculture Research Center, Ministry of Agriculture, and Land Reclamation, Egypt. Ahmed Samy is currently supported by the Bill and Melinda Gates Foundation [108704-001] and BBS/E/I/00007038 and the Pirbright Institute fellowship.