A Very Virulent Genotype of Infectious Bursal Disease Virus Predominantly Associated with Recurrent Infectious Bursal Disease Outbreaks in Tunisian Vaccinated Flocks

Abstract

Outbreaks of infectious bursal disease (IBD) still continue to afflict the Tunisian poultry industry even in those flocks where the vaccination program is strictly applied. To characterize the viruses that circumvent protection provided by vaccination, field isolates of infectious bursal disease virus (IBDV) obtained from vaccinated flocks that have repeatedly experienced IBDV outbreak episodes were analyzed from bursal samples by reverse transcription coupled with polymerase chain reaction and dideoxynucleotide sequencing of the VP2 hypervariable region. Although sequence data were obtained from samples collected from three distinct flocks over a period of 3 years, only limited sequence variation has been observed. The few nucleotide changes were silent and the deduced amino acid sequences were identical. Thus, the virus population that predominates in the field seems to represent a homogeneous antigenic pool. Compared with the VP2 sequences of several IBDV strains, this predominant pool was found to be closely related to the very virulent (vv) IBDV viruses described in Europe and Asia. Sequence and phylogenetic analysis of the precursor polyprotein coding sequence of a representative Tunisian isolate further confirmed its assignment to the vv genotype. The deduced amino acid sequence of the whole polyprotein of the Tunisian isolate was found to be identical to a South Korean IBDV strain. Alignment of the polyprotein amino acid sequence of 35 IBDV strains identified additional mutations outside the VP2 variable domain and which occur frequently in vv strains. Based on this comparative analysis, the set of amino acid residues that should represent a typical vv profile involves Ala222, Ile242, Ile256, Ile294, Leu451, Tyr680, N685, Ser715, Asp751, Val990, and Ala1005. Such a combination of amino acid changes was observed for the majority of vvIBDV strains that define a distinct phylogroup.

Phylogenetic evidence for homologous recombination within the family Birnaviridae

“…The two Korean potential recombinant strains, KK1 and KSH, were previously identified as divergent vvIBDV strains (Kwon et al, 2000;Mardassi et al, 2004). Breakpoints of these two potential recombinants were estimated at almost identical locations ( Table 2), suggesting that they were likely to have originated from the same recombination events.…”

Simultaneous alteration of residues 279 and 284 of the VP2 major capsid protein of a very virulent Infectious Bursal Disease Virus (vvIBDV) strain did not lead to attenuation in chickens

“…Using as recipient hosts infectious clones pVAXSA.Rib and pVAXSB.Rib of the cell culture-adapted and attenuated IBDV P2 strain, we attempted to rescue in CEF cells IBDV virus specifying the polyprotein sequence of the Tunisian bursal-derived vvIBDV PO7 strain [ 35 ]. The VP2 amino acid residues at positions 253 (Q), 279 (D), and 284 (A) of the wild-type PO7 strain corresponded to those of non culturable vvIBDV strains.…”

“…The full-length coding sequence of the polyprotein and the 3′ non-coding region of PO7, a Tunisian bursal-derived field isolate of vvIBDV [ 35 ], was amplified by PCR using the primer pair IB2SP1/SAR1 (Table 1 ) and the Expand High Fidelity Taq polymerase mix (Roche Applied Science, Germany). The resulting amplicon was cloned into kpn I/ Eco RI-restricted pCR2.1 plasmid vector (Invitrogen), yielding the construct pCR.PO7poly.wt.…”

Molecular characteristic of VP2 gene of infectious bursal disease viruses isolated from a farm in two decades

“…(The virulence of a strain is primarily defined by the ability to cause mortality in susceptible chickens). Since in vivo studies are expensive, time consuming, and sometimes not possible, genetic characteristics that define the vvIBDV phylogenetic group could provide valuable information on virulence [9,17]. Using a combination of molecular assays and pathogenicity studies, Ignjatovic [18] demonstrated that an IBDV isolate associated with high mortality in the field was not a vvIBDV strain.…”