Lysis of myelocytes in chickens infected with infectious bursal disease virus

In specific-pathogen-free chickens infected with the highly virulent HPS-2 strain or virulent reference GBF-1 strain of infectious bursal disease virus (IBDV), pathologic changes of the bone marrow were investigated. On histologic examination, bone marrow lesions were prominent in the HPS-2 group but only mild in the GBF-1 group. The bone marrow of the HPS-2 group showed severe lysis and depletion of heterophil myelocytes with pyknotic nuclear alteration 2-3 days after inoculation. On examination with an electron microscope, heterophil myelocytes were characterized by shrinkage of the cytoplasm and peripheral condensation of nuclear chromatin. IBDV particles were not detected in altered myelocytes. A terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick end-labeling method demonstrated a positive reaction in only heterophil myelocytes. In contrast, nucleosomal DNA fragmentation in HPS-2-infected bone marrow cells was indiscernible by agarose gel electrophoresis. These findings indicate that lysis of bone marrow cells is selectively induced in heterophil myelocytes at an early stage after IBDV infection and independent of virus replication.