DYNAMICS OF VIRAL SPREAD IN BLUETONGUE VIRUS-INFECTED CALVES

The kinetics of viremia and sites of viral replication in bluetongue v irus (BTV) infected calves were characterized by virus isolation, sero logy and immunofluorescence staining procedures. In addition, the role of the regional lymph node and lymphatics draining inoculated skin in the pathogenesis of BTV infection was determined by analyzing efferen t lymph collected from indwelling cannulas. Viremia persisted for 35 t o 42 days after inoculation (DAI) and virus co-circulated with neutral izing antibodies for 23 to 26 days. Virus was first isolated from peri pheral blood mononuclear (PBM) cells at 3 DAI, after stimulation of PB M cells with interleukin 2 and mitogen. BTV was frequently isolated fr om erythrocytes, platelets and stimulated PBM cells but never from gra nulocytes and rarely from plasma during viremia. Virus was consistentl y isolated from erythrocytes late in the course of viremia. Interrupti on of efferent lymph flow by cannulation delayed the onset of viremia to 7 DAI. BTV was infrequently isolated from lymph cells, and few fluo rescence positive cells were observed after lymph and PBM cells were l abelled with a BTV-specific monoclonal antibody. Virus was isolated fr om spleen by 4 DAI and most tissues by 6 DAI, whereas virus was isolat ed from bone marrow only at IO DAI. Virus was not isolated from any ti ssue after termination of viremia. It is concluded that primary viral replication occurred in the local lymph node and BTV then was transpor ted in low titer to secondary sites of replication via infected lymph and PBM cells. We speculate that virus replication in spleen resulted in release of virus into the circulation and non-selective infection o f blood cells which disseminated BTV to other tissues. Virus associati on with erythrocytes likely was responsible for prolonged viremia, alt hough infected erythrocytes eventually were cleared from the circulati on and persistent BTV infection of calves did not occur.