Use of a genetic marker for wild-type potentially pathogenic infectious bursal disease viruses

An amino acid mutation at residue 284 (Ala to Thr) in the VP2 protein of infectious bursal disease viruses (IBDVs) has been correlated with the ability to replicate in cell culture. In this study, we designed a molecular test for this mutation. The reverse transcriptase/polymerase chain reaction (RT/PCR) was used to amplify a 743-bp region of the VP2 gene that contained the codon for amino acid 284. The restriction endonuclease NgoMIV was selected for this study because the first three nucleotides of its six-base recognition sequence are the codon responsible for the amino acid alanine at residue 284. The RT/PCR products from 10 known pathogenic and 16 vaccine strains of IBDV were examined for the presence or absence of the NgoMIV site. We also examined 189 field strains of IBDV for the NgoMIV site. All 10 known pathogenic IBDV strains contained the NgoMIV site, indicating they contained alanine at residue 284. None of the vaccine strains had the NgoMIV site, suggesting they had threonine or another amino acid at residue 284. The results suggest that the presence of this NgoMIV site can be used as a marker for the identification of wild-type (nonvaccine) IBDV strains. The RT/PCR products from 152 (80.4%) of the field strains had the NgoMIV site and thus have the potential to be wild-type pathogenic viruses. The RT/PCR products from 37 (19.6%) of the field strains were not cleaved by NgoMIV and thus are potentially attenuated vaccine strains. Molecular diagnostic assays have been used to place IBDV strains into genetically related groups. The identification of this genetic marker now makes it possible to identify viruses that are wild-type strains that have the potential to be pathogenic viruses.