Genetic Stability of the VP2 Hypervariable Region of Four Infectious Bursal Disease Virus Isolates after Serial Passage in Specific-Pathogen-Free Chicken Embryos

Infectious bursal disease virus strains U2, 586, L1, and Q2 were isolated from pooled bursal samples collected from commercially reared broilers. These viruses were propagated in specific-pathogen-free (SPF) embryonated chicken eggs for 24 or 25 passages. Nucleotide sequences of a 743-bp reverse transcription (RT)/polymerase chain reaction (PCR) product containing the VP2 hypervariable region were compared before and after passage of the viruses in embryonated chicken eggs. To determine the genetic stability of the viruses, each isolate was compared with its egg-passed ancestor; virus isolates were not compared with each other. When the restriction enzymes BstNI and MboI were used, no differences were observed in the restriction fragment length polymorphism profiles of the RT/ PCR products after embryo passage. After embryo passage, six nucleotide changes were identified in the viruses. Among the four viruses examined, these nucleotide changes resulted in a total of five amino acid changes. The amino acid changes were S-222-L in virus 586, K-249-N in viruses U2, L1, and Q2, and G-281-V in virus Q2. Three of the five amino acid changes occurred at residue 249. The convergent nature of this residue shift in three of four of the chick embryo-passed viruses suggests the occurrence of a functional, as opposed to random, mutation. The original isolates caused typical signs of infectious bursal disease in 3-wk-old SPF chicks. Their embryo-passed ancestors also produced typical signs of infectious bursal disease in 3-wk-old SPF chicks, suggesting the amino acid mutations observed did not affect virulence of the viruses.