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

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.