LESIONS AND DISTRIBUTION OF VIRAL-ANTIGEN FOLLOWING AN EXPERIMENTAL-INFECTION OF YOUNG SERONEGATIVE CALVES WITH VIRULENT BOVINE VIRUS DIARRHEA VIRUS-TYPE-II

During the past several years, acute infections with bovine viral diar rhea virus (BVDV) have been causally linked to hemorrhagic and acute m ucosal disease-like syndromes with high mortality. The majority of BVD Vs isolated in such cases have been classified as type II on the basis of genetic and antigenic characteristics. It was our objective to exa mine clinical disease, lesions and potential sites of viral replicatio n, following experimental BVDV type II infection in young calves. On a pproximately day 35 after birth, calves that had received BVDV-antibod y-negative colostrum were infected by intranasal inoculation of 5 x 10 (5) TCID50 of BVDV type II isolate 24515 in 5 mt of tissue culture flu id (2.5 mL/nostril). Calves were monitored twice daily for signs of cl inical disease. Approximately 48-72 h after infection, all calves deve loped transient pyrexia (39.4-0.5 degrees C) and leukopenia. Beginning on approximately day 7 after infection, all calves developed watery d iarrhea, pyrexia (40.5-41.6 degrees C), marked leukopenia (greater tha n or equal to 75% drop from preinoculation values), variable thrombocy topenia, and moderate to severe depression. Calves were euthanized on days 10, 11, or 12 after infection due to severe disease. Gross and hi stological lesions consisted of multifocal bronchointerstitial pneumon ia (involving 10%-25% of affected lungs), bone marrow hypoplasia and n ecrosis, and minimal erosive lesions in the alimentary tract. Immunohi stochemical staining for BVDV revealed widespread viral antigen usuall y within epithelial cells, smooth muscle cells and mononuclear phagocy tes in multiple organs, including lung, Peyer's patches, gastric mucos a, thymus, adrenal gland, spleen, lymph nodes, bone marrow, and skin. This BVDV type II isolate caused rapidly progressive, severe multisyst emic disease in seronegative calves that was associated with widesprea d distribution of viral antigen and few gross or histological inflamma tory lesions.