The Pathogenesis of Experimental Bluetongue Virus Infection of Calves

Eleven seronegative calves were intravenously inoculated with bluetongue virus (BTV) serotype 10, and two calves were inoculated with a placebo. Cellular association of BTV during viremia was investigated in three of the calves by titrating virus present in plasma and different blood cell fractions at weekly intervals after infection. Viremia persisted 35 to 49 days in individual calves. Virus was transiently isolated from blood mononuclear cells and plasma collected from two of the calves but was consistently isolated from erythrocytes throughout infection of all three animals. Titers of BTV present in the erythrocyte fraction were comparable to those of the unseparated blood cell fraction. Tissue tropism of BTV was determined by viral isolation from tissues collected from calves euthanatized at 1, 2, 3, 4, 5, 7, 14, 28, 42, and 56 (2 calves) days after inoculation with BTV. Tropism was also determined by immunohistochemical staining of selected tissues with an avidin- biotin complex immunoperoxidase staining procedure using three BTV-specific monoclonal antibodies. The BTV infected calves remained healthy throughout the study. Virus was isolated from at least one tissue collected from calves euthanatized at 1 through 28 days after inoculation, but not thereafter. High titers of BTV were present in the lungs, prescapular and mesenteric lymph nodes, thymus, and spleen of calves euthanatized at 1 to 4 days after inoculation, whereas BTV was either not isolated or isolated in low titer from bone marrow collected from these animals. Very low numbers of infected cells were detected by immunohistochemical staining of sections of spleen (days 1 and 2 after inoculation), prescapular lymph nodes (days 1, 2, 3, and 4), lung (day 3), thymus (day 4), lip (day 5), and trapezius muscle (day 7). The morphology and anatomic location of at least some of the infected cells in sections of lymph node, spleen, and thymus were most consistent with their being mononuclear phagocytes, whereas the infected cells in the sections of lip and trapezius muscle were located within or immediately adjacent to arterioles and probably were endothelial cells. It is proposed that after intravenous inoculation of calves with BTV, viral replication initially occurs in mononuclear phagocytic cells and subsequently in vascular endothelium. The intimate association of BTV with circulating blood cells, es- pecially erythrocytes, apparently impedes prompt clearance resulting in prolonged viremia. The data suggest that although BTV infection of cattle is characterized by prolonged viremia, viral replication in tissues is transient, and a truly persistent infection does not occur in BTV infected calves. Bluetongue is an insect-transmitted, non-contagious viral disease of domestic and wild ruminant~.~~~J 1,20,30 The causative agent is bluetongue virus (BTV), a mem- ber of the Reoviridae and prototype virus of the genus orbivirus. The consequences of BTV infection of cattle and sheep frequently are different;I2 whereas sheep in- fected with BTV may exhibit signs of bluetongue dis- ease, which is presumed to result from virus-mediated destruction of vascular endotheli~m,~J~~~~~~*~~~ BTV in- fection of cattle invariably is a~yrnptomatic.~J ~J~~~~~~~~~~