Morphological changes and virus distribution in the ileum of colostrum-deprived calves inoculated with non-cytopathic bovine viral diarrhoea virus genotype-1.

Eight colostrum-deprived calves were inoculated intranasally with a non-cytopathic strain of bovine viral diarrhoea virus (BVDV) genotype-1 and killed in batches of two at 3, 6, 9 and 14 days post-inoculation (dpi). Two non-inoculated animals with similar background served as controls. All infected calves developed mild pyrexia and transient leucopenia due primarily to lymphopenia. Viraemia was correlated with body temperature and inversely related to leucocyte count. Ileal Peyer's patches developed mild follicular lymphoid depletion from 3dpi. This change was accompanied by cellular fragmentation and pyknosis, characteristic of apoptosis, which was most prominent from 6dpi. Lymphocyte apoptosis was confirmed by ultrastructural examination. Stellate cells and macrophages located in the lymphoid follicles were identified as infected by virus from 3dpi and the number of these infected cells increased until 9dpi. Fewer lymphocytes expressed BVDV antigen. Macrophages had morphological features consistent with activation of secretory and phagocytic function from 3dpi. These findings suggest that BVDV is only directly responsible for the destruction of a small number of lymphocytes. Although lymphocyte infection coincided with the onset of apoptosis, the intensity of infection was disproportionate to the marked depletion of gut-associated lymphoid tissue, particularly during the early stages of this process. Characterization of the indirect pathogenic mechanisms involved in the lymphoid depletion associated with BVDV infection will require additional study.