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 i n specific pathogen-free calves (SPF) 10 days post-inoculation (PI) wi th bovine viral diarrhea virus (BVDV). To investigate possible differe nces in tissue tropism between BVDV isolates, two clinically and antig enically different noncytopathic (ncp) isolates of BVDV were compared in the study. Four calves were exposed to noncytopathic (ncp) BVDV 793 7, and four to ncp-BVDV 126. Two additional calves that were not expos ed to virus served as controls. Both ncp-BVDV 7937 and ncp-BVDV 126 in duced mild disease characterized by variable fever and anorexia. Lymph oid depletion was evident in Peyer's patch of four calves and the thym us of two calves exposed to BVDV. Differences between these isolates i n the distribution of BVDV or BVDV antigen in tissues of inoculated ca lves were not found. High concentrations of BVDV and BVDV-specific ant igen 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 BV DV were recovered from the bone marrow of all calves exposed to BVDV a nd BVDV-specific antigen was demonstrated at this location in six of t hese calves. Platelet counts of calves exposed to BVDV were significan tly reduced during infection, which resulted in thrombocytopenia in on e calf. Focal areas of necrosis were observed in squamous epithelial c ells of the tonsil and ruminal mucosa. BVDV-specific antigen was found in and adjacent to these foci. Calves exposed to ncp-BVDV 7937 had sl ightly more severe clinical signs than those exposed to ncp-BVDV 126.