Characterization of infectious bursal disease viruses from four layer flocks in the United States.

Twenty bursal samples were obtained from four infectious bursal disease virus (IBDV)-vaccinated layer flocks experiencing problems with immune suppression that was thought to be infectious bursal disease. All the samples were found to be positive for IBDV by reverse transcriptase polymerase chain reaction (RT-PCR). Restriction fragment length polymorphism analysis of the samples identified them as classic molecular group 3 and group 4 viruses. Two samples from each of the four flocks were sequenced, and within a flock, these sequences were identical; however, between flocks, some differences were observed. One virus from each of the four flocks was selected for further analysis. The VP2 hypervariable sequence region of samples GA-1, H-30, and CS-2-35 had nucleotide and amino acid similarities with the D78 and Vi Bursa G classic vaccines that were used in those flocks. The sequence of HPR-2 was similar to the Bursa Vac 4 vaccine used in that flock and the STC virulent classic IBDV strain. The deduced amino acid sequence of these isolates revealed that all the isolates had proline at position 222, which is characteristic of U.S. classic viruses. The phylogenetic analysis of these isolates on the basis of the VP2 hypervariable amino acid sequence clustered GA-1, H-30, and CS-2-35 with the D78 vaccine and HPR-2 with STC. The pathogenicity of these isolates was tested in specific-pathogen-free chickens. Bursa-body weight (B-BW) ratios and histopathologic lesion scores in the bursa were determined. Gross lesions were observed in the bursa, and the B-BW ratios of the birds infected with all four wild-type viruses were significantly different compared with the D78 vaccine and uninoculated control groups. Histopathology of the bursa from groups infected with GA-1, H-30, CS-2-35, and HPR-2 showed different degrees of follicular depletion and necrosis. A very slight lymphoid depletion was observed in the D78-infected group at 5 days postinoculation, and no microscopic lesions were observed in this group at 8 days postinoculation or at any time in the uninoculated control group. The bursa collected from the field virus and D78-infected birds at necropsy revealed the presence of IBDV via RT-PCR, and the VP-2 hypervariable nucleotide sequences of the GA-1, H-30, CS-2-35, HPR-2, and D78 samples were identical to the original viral isolates and vaccine, respectively.