Studies on naturally occurring infectious bursal disease viruses suggest that a single amino acid substitution at position 253 in VP2 increases pathogenicity. 
Three classic IBDV strains were previously isolated from commercial layer chicken flocks and shown to be phylogenetically related to vaccine strains but pathogenic in susceptible chickens.  In this study, their viral genomes were sequenced and compared to sequences of vaccines being used in those flocks.  The vaccine strains examined were sequenced directly from the manufacturer and had identical genome segment B sequences.  Compared to these vaccines, the GA-1, H-30 and CS-2-35 isolates each had one silent mutation in the gene that encodes VP1.  Compared to the two vaccines used at the time CS-2-35 was isolated, the segment A sequence of CS-2-35 contained numerous nucleotide and amino acid mutations suggesting the CS-2-35 virus was not closely related to these vaccines.  This virus however did have amino acid mutations in VP2 that are reported to be necessary for replication in cell culture and lacked two of the three amino acid mutations previously shown to be necessary for virulence.  These data suggest that CS-2-35 was a descendant from an attenuated strain of IBDV.  When the segment A genomic sequences of the GA-1 and H-30 viruses were compared to the vaccines being used in those flocks they were most closely related to the attenuated D78 vaccine strain.  In genome segment A, three nucleotide mutations in GA-1 and four in H-30 were observed compared to the D78 classic vaccine.  These nucleotide mutations caused one amino acid (H253N) change in the GA-1 virus and two amino acids (H253Q and G259D) were different in the H-30 virus.  In addition, both the GA-1 and H-30 viruses had the amino acid G76 in VP2 that appears to be unique to the vaccine D78.  The data suggest that GA-1 and H-30 are genetically related and have a common ancestor even though they were isolated from geographically distant flocks.  The evidence also suggests that GA-1, H-30 and CS-2-35 could be reversions from attenuated vaccine viruses or by coincidence genetically resemble classic IBDV vaccines.  It should be noted that some of the classic virus vaccines were not being used according to the manufacturer's recommendations at the time the GA-1, H-30 and CS-2-35 strains were isolated.  Together, the molecular and pathogenicity data indicate that a single amino acid mutation from Histidine (H) to Glutamine (Q) or Asparagine (N) at position 253 in VP2 will markedly increase the virulence of an attenuated IBDV.