Synovitis and bovine syncytial virus isolation in experimentally induced malignant catarrhal fever

Abstract Synovial membranes and fluid from joints of cattle with experimentally induced and naturally-occurring malignant catarrhal fever (MCF) were examined grossly and by histological, ultrastructural, immunological and virological means. Synovitis, characterized by focally roughened synovial membranes and cloudy synovial fluid with occasional fibrin strands, was frequently found, particularly in tibiotarsal joints. Histologically, there was variable mononuclear cell infiltration of synovial tissues with necrosis, and fibrin exudation in severe cases. Ultrastructurally, most synoviocytes were intermediate between Types A and B. Variably severe degenerative changes were observed in synoviocytes and lymphoid cells. Lymphoid vasculitis was common in severely involved joints. Synovial fluid contained markedly increased numbers of mononuclear cells and elevated total protein, but no significant elevation in serum titres of rheumatoid factor was associated with the development of MCF in the experimental disease. Bovine syncytial virus was isolated from cells of synovial fluid of 9 of 10 cattle with experimental MCF and from other tissues of 8 of 9 cattle with MCF. The significance of this finding is unclear.