The pathomorphology of malignant catarrhal fever. I. Generalized lymphoid vasculitis.

A histologic, ultrastructural and immunohistochemical study of the vasculitis in the rete mirabile of 18 calves with experimental and two calves with spontaneous malignant catarrhal fevel showed lesions similar to those reported in graft rejection and some other lymphocyte-associated vasculopathies. The calves were examined at early, mid and late stages of clinical disease. Vascular lesions in the carotid rete were representative of those in other tissues examined. The magnitude of the arterial wall mononuclear cell infiltrate and the degree of arterial wall necrosis and degeneration increased with progression of the clinical disease. The predominant site of mononuclear cell accumulation was the adventitia. There was less medial and intimal involvement. The mononuclear infiltrate was composed of lymphocytes and macrophages with neutrophils and plasma cells rarely present. Endothelial hyperplasia, fibrinoid change and thrombosis were uncommon. Many changes contrasted with those recognized in an active Arthus reaction where adventitial change was minimal and most inflammatory cells were neutrophils or plasma cells. IgG and C3 rarely were detected in vessel walls with fluorescent antibody techniques. Serum immunoglobulins as measured by radial immunodiffusion were not elevated. Viral structures were not seen in tissues examined ultrastructurally.