Distribution of Virus and Viral Antigen in Specific Pathogen-free Calves Following Inoculation with Noncytopathic Bovine Viral Diarrhea Virus

Gross and microscopic lesions and distribution of virus were studied in specific pathogen-free calves (SPF) 10 days post-inoculation (PI) with bovine viral diarrhea virus (BVDV). To investigate possible differences in tissue tropism between BVDV isolates, two clinically and antigenically different noncytopathic (ncp) isolates of BVDV were compared in the study. Four calves were exposed to noncytopathic (ncp) BVDV 7937, and four to ncp-BVDV 126. Two additional calves that were not exposed to virus served as controls. Both ncp-BVDV 7937 and ncp-BVDV 126 induced mild disease characterized by variable fever and anorexia. Lymphoid depletion was evident in Peyer's patch of four calves and the thymus of two calves exposed to BVDV. Differences between these isolates in the distribution of BVDV or BVDV antigen in tissues of inoculated calves were not found. High concentrations of BVDV and BVDV-specific antigen were detected in the thymus, Peyer's patch, and mesenteric lymph node of all exposed calves. BVDV was shown to infect cells of the bone marrow without causing microscopic lesions. High concentrations of BVDV were recovered from the bone marrow of all calves exposed to BVDV and BVDV-specific antigen was demonstrated at this location in six of these calves. Platelet counts of calves exposed to BVDV were significantly reduced during infection, which resulted in thrombocytopenia in one calf. Focal areas of necrosis were observed in squamous epithelial cells of the tonsil and ruminai mucosa. BVDV-specific antigen was found in and adjacent to these foci. Calves exposed to ncp-BVDV 7937 had slightly more severe clinical signs than those exposed to ncp-BVDV 126.