Classification of infectious bursal disease virus into genogroups

Abstract

Infectious bursal disease virus (IBDV) causes infectious bursal disease (IBD), an immunosuppressive disease of poultry. The current classification scheme of IBDV is confusing because it is based on antigenic types (variant and classical) as well as pathotypes. Many of the amino acid changes differentiating these various classifications are found in a hypervariable region of the capsid protein VP2 (hvVP2), the major host protective antigen. Data from this study were used to propose a new classification scheme for IBDV based solely on genogroups identified from phylogenetic analysis of the hvVP2 of strains worldwide. Seven major genogroups were identified, some of which are geographically restricted and others that have global dispersion, such as genogroup 1. Genogroup 2 viruses are predominately distributed in North America, while genogroup 3 viruses are most often identified on other continents. Additionally, we have identified a population of genogroup 3 vvIBDV isolates that have an amino acid change from alanine to threonine at position 222 while maintaining other residues conserved in this genogroup (I242, I256 and I294). A222T is an important mutation because amino acid 222 is located in the first of four surface loops of hvVP2. A similar shift from proline to threonine at 222 is believed to play a role in the significant antigenic change of the genogroup 2 IBDV strains, suggesting that antigenic drift may be occurring in genogroup 3, possibly in response to antigenic pressure from vaccination.

Fig. 1

Phylogenetic analysis of the nucleotide sequences of hvVP2 of IBDV. The evolutionary history was inferred using the neighbor-joining method with 1000 bootstrap replicates. The optimal tree with the sum of branch length = 1.97063052 is shown. The tree is drawn to scale, with branch lengths in the same units as those of the evolutionary distances used to infer the phylogenetic tree. The evolutionary distances were computed using the maximum composite likelihood method and are in the units of the number of base substitutions per site. The analysis involved 105 nucleotide sequences. All positions containing gaps and missing data were eliminated. There were a total of 366 positions in the final dataset. Reference strains are identified by name and GenBank accession number. The phylogeographic genogroups are identified

Fig. 2

Phylogenetic trees of the hvVP2 of genogroup 3 samples. Reference strains are identified by name and GenBank accession number. The evolutionary history was inferred using the neighbor-joining method with 1000 bootstrap replicates. (a) Nucleotide sequences. The tree is drawn to scale, with branch lengths in the same units as those of the evolutionary distances used to infer the phylogenetic tree. The evolutionary distances were computed using the maximum composite likelihood method and are in the units of the number of base substitutions per site. The analysis involved 45 nucleotide sequences. All positions containing gaps and missing data were eliminated. There were a total of 543 positions in the final dataset. (b) Deduced amino acid sequences. The optimal tree with the sum of branch length = 0.15134382 is shown. The tree is drawn to scale, with branch lengths in the same units as those of the evolutionary distances used to infer the phylogenetic tree. The evolutionary distances were computed using the Poisson correction method and are in the units of the number of amino acid substitutions per site. The analysis involved 45 amino acid sequences. All positions containing gaps and missing data were eliminated. There were a total of 181 positions in the final dataset

Fig. 3

Phylogenetic trees of the deduced amino acid sequences of a portion of VP1 of genogroup 3 samples. Reference strains are identified by name and GenBank accession number. The evolutionary history was inferred using the neighbor-joining method. The optimal tree with the sum of branch length = 0.13646711 is shown. The tree is drawn to scale, with branch lengths in the same units as those of the evolutionary distances used to infer the phylogenetic tree. The evolutionary distances were computed using the Poisson correction method and are in the units of the number of amino acid substitutions per site. The analysis involved 45 amino acid sequences. All positions containing gaps and missing data were eliminated. There were a total of 116 positions in the final dataset