Immunoperoxidase evaluation of pneumonic lesions induced by Pasteurella haemolytica in calves.

Calves inoculated with Pasteurella haemolytica serovar 1 developed lesions of coagulation necrosis in the lungs that were sharply demarcated by leukocytes. The P haemolytica antigen was detected in the area of coagulation necrosis in histologic sections, using an immunoperoxidase technique. In the central area of the necrotic tissue, the bacterial antigen was diffusely presented in the necrotic alveolar wall, fibrin, serous exudate, and degenerated leukocyte. The bacterial antigen also was found in some groups of degenerating leukocytes around the necrotic tissue. The bacterial colonies among these leukocytes had strong specific reactions against P haemolytica. The bacterial antigen was observed in the cytoplasm of macrophages in alveoli around the necrotic lesion. These findings confirmed that coagulation necrosis is an important lesion in calves with pneumonia caused by P haemolytica.