The Pathogenesis of Experimental Bluetongue Virus Infection of Calves

Abstract

Abstract. 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 avidinbiotin 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, especially 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 virus; cattle; viremia. Bluetongue is an insect-transmitted, non-contagious viral disease of domestic and wild r u m i n a n t~.~~~J 1,20, 30 The causative agent is bluetongue virus (BTV), a member of the Reoviridae and prototype virus of the genus orbivirus. The consequences of BTV infection of cattle and sheep frequently are different;...

Experimental reproduction of bluetongue virus serotype 8 clinical disease in calves

“…These results are probably explained by the fugacity of the viral replication in the lymph nodes during the earliest phase of the infection, and by the longer persistence of the virus in the spleen. Similar findings were previously observed in intravenously infected calves (MacLachlan et al, 1990(MacLachlan et al, , 2008.…”

“…The subsequent increase of the viraemia 4 days P.I., was probably to refer to the transit of the virus from the sites of massive replication to the blood stream. In the literature for other BTV serotypes, it is reported that in cattle the release of BTV from the spleen is followed by high titer viraemia which persists for several weeks postinfection (MacLachlan et al, 1990). After 14 days P.I., the virus was detected by RT-qPCR in the spleen and the tongue of both animals, while the tested lymph nodes were negative (data not shown).…”

“…The temporary reduction of the viral RNA load in the blood stream until the third day P.I., was probably to refer to the distribution of the virus to the sites of primary and secondary replication, such as lymph nodes, and spleen. Lymphoid organs are important sides of viral replication in sheep and cattle (MacLachlan et al, 1990). The subsequent increase of the viraemia 4 days P.I., was probably to refer to the transit of the virus from the sites of massive replication to the blood stream.…”

“…Clinical signs commonly occur in sheep, while goats and cattle remain mostly subclinically infected (Osburn, 1994). Although the pathogenesis of bluetongue is similar in sheep and cattle (MacLachlan et al, 1990), it is uncertain as to why BTV infection causes extensive microvascular injury in sheep, but less in cattle. Recently the role of species-specific endothelial cell-derived inflammatory and vasoactive mediators has been shown (DeMaula et al, 2002).…”

Flow cytometric analysis of in vitro bluetongue virus infection of bovine blood mononuclear cells

“…Recent studies have established that mononuclear inflammatory cells might be important to the pathogenesis of bluetongue in cattle. Using a variety of in vitro techniques, Whetter et aL (1989) have identified BTVinfected cells in cultures of adherent bovine peripheral blood mononuclear (PBM) cells, and virus has been transiently isolated from PBM cells during experimental infection of calves (Whetter et al, 1989;MacLachlan et al, 1990). Virus is present in high titre in lymph nodes soon after inoculation of calves (MacLachlan et al, 1990), indicating that lymphoid tissue is an important site for initial virus replication.…”