Pulmonary lesions induced by a Pasteurella haemolytica cytotoxin preparation in calves.

Intrabronchial instillation of a Pasteurella haemolytica type A1 crude cytotoxin preparation in calves resulted in pulmonary gross and microscopic lesions comparable to spontaneous and experimental pasteurellosis. In the acute stage of the lesion electronmicroscopy revealed intravascular accumulation, degeneration and fragmentation of leukocytes in the interalveolar septa. Secondary thrombus formation and increased vascular permeability resulted in alveolar flooding, fibrin deposition, extravasation of erythrocytes and loss of alveolar epithelium. No cytotoxicity was observed for the tracheal (in vitro) and bronchial epithelium (in vivo). The pathogenesis of the vascular lesions and their significance for the development of the typical lesions of pneumonic pasteurellosis is discussed.