Molecular epidemiology of infectious bursal disease viruses: distribution and genetic analysis of newly emerging viruses in the United States.

Genetic mutations in the genome of infectious bursal disease virus (IBDV) have resulted in antigenic and pathogenic variants that continue to cause disease in commercially reared chickens. The extent of the genetic diversity among IBDV strains circulating in the United States is unknown. This study was designed to identify newly emerging viruses infecting chickens on poultry farms experiencing immune suppression-related problems. Fifty IBDV-positive samples were identified from 273 bursa samples using a real-time reverse transcription-polymerase chain reaction (RT-PCR) assay. Mutation probes were designed to the hydrophilic B coding region of the VP2 gene. Six mutation probes used in this study were based on the nucleotide sequences of the Del-E, Bursine 2, D-78, STC, G6, and T1 IBDV strains. Following real-time RT-PCR, these mutation probes identified 11 of the 50 viruses in the melting temperature (Tm) analysis. The results indicated that the remaining 39 viruses had one or more nucleotide mutations compared with the six mutation probes in this region of the VP2 gene. Thirty-eight viruses were chosen for nucleotide sequence analysis across the hypervariable region of the VP2 gene. Within this group of 38 viruses, four were identified by the mutation probes and their nucleotide sequences confirmed that real-time RT-PCR data. In the remaining 34 viruses, nucleotide mutations were observed in as many as 8 of 23 nucleotides across the hydrophilic B epitope coding region. Furthermore, every amino acid position except one between 316 and 324 had at least one substitution mutation. Phylogenic analysis placed two of the 38 viruses sequenced on branches with classic viruses and the remaining 36 viruses were placed on four distinct branches. Branches 1 and 2 contained a majority of the viruses, which were distributed across most of the major poultry-producing states in the United States. These branches contained previously characterized variant IBDV strains. Viruses in branches 3 and 4 were confined to three states and did not contain any previously characterized IBDV strains.