Comparison of Virus Replication Efficiency in Lymphoid Tissues among Three Infectious Bursal Disease Virus Strains

In order to study replication efficiency of infectious bursal disease virus (IBDV) in lymphoid tissues, both the virus titers and the virus antigen titers in four lymphoid tissues were compared among chickens inoculated with Ehime/91 (about 50% mortality), J1 (no mortality), or K (attenuated) IBDV strains during 1-7 days postinoculation (PI). IBDV antigens in tissue homogenates were detected by an enzyme-linked immunosorbent assay. In the bursa of Fabricius, higher virus titers were maintained for 1-3 days PI in chickens inoculated with Ehime/91 or J1 strains, whereas the virus titers increased gradually and reached to the peak on 3 days PI in chickens inoculated with the K strain. There were no clear differences in both the virus and the virus antigen titers in bursae and thymus between chickens inoculated with Ehime/91 and J1 stains. However, the virus and/or the virus antigen titers in spleen and bone marrow of chickens inoculated with Ehime/91 strain were higher than those of the J1-inoculated chickens. Immunohistochemical analyses indicated that larger numbers of IBDV antigen-positive cells were detected in spleen and bone marrow of the Ehime/91 group than in those of the J1 group. There was almost no detectable virus and virus antigens in thymus, spleen, and bone marrow of the K-inoculated chickens throughout the experiment. These results suggest that efficient replication of IBDV not only in the bursa but also in the spleen and the bone marrow may be required for clinical infectious bursal disease.