Genetics and Pathogenicity of Natural Reassortant of Infectious Bursal Disease Virus Emerging in Latvia

Abstract

Infectious bursal disease virus is an immunosuppressive pathogen that, despite applied vaccination, is affecting the poultry industry worldwide. This report presents the genetic and pathotypic characterization of a natural reassortant emerging in Europe (Latvia). Genetic characterization showed that strain 25/11/Latvia/2011 represents genotype A3B1, whose segment A is derived from very virulent strains, while segment B is from the classic-like genogroup. Phylogenetic maximum likelihood inference of the B-segment sequence clustered the reassortant strain together with the US antigenic variant E strain. However, the obtained full-length sequence of 25/11/Latvia/2011 revealed that not only reassortment but also dozens of mutations shaped the unique genetic makeup. Phenotypic characterization showed no mortality and no clinical signs of disease but a severe bursa of Fabricius atrophy and splenomegaly in the convalescent birds at 10 days post infection. The results obtained indicate that the acquired genetic constellation contributed to a decrease in virulence; nevertheless, the infection causes severe damage to lymphoid organs, which can lead to impaired immune responses.

Keywords: infectious bursal disease virus; reassortment; viral evolution; virulence.

Figure 1

Phylogenetic tree based on full coding sequence of IBDV for viral segment A (3076nt) using 60 reference strains retrieved from GenBank and maximum likelihood (ML) method, GTR+F+I+G4 model. The main IBDV genogroups according to classification of Islam et al. [18] are denoted (A1: green, A2: blue, A3: red, A4: purple, A6: white, A8: yellow, A0: brown).

Figure 2

Phylogenetic tree based on full coding sequence of IBDV for viral segment B (2640nt) using 59 reference strains retrieved from GenBank and maximum likelihood (ML) method, GTR+F+I+G4 model. The main IBDV genogroups according to classification of Islam et al. [18] are denoted (B1: green, B2: red, B3: yellow, B4: blue).

Figure 3

Mean bursa (A) and spleen (B) to body weight (BW) ratios and mean body weight of experimental groups. Each bar indicates the mean ± SD and statistical significance (* p < 0.05, ** p < 0.01, *** p < 0.001).