Primary pneumonic plague begins as a lobular pneumonia (Fig. 24.3) with innumerable bacilli and a proteinaceous effusion in the alveoli (Fig. 24.4): the alveolar walls undergo necrosis followed by hemorrhage. This process spreads and becomes confluent, involving first one lobe and then multiple lobes with hemorrhagic exudate in the alveoli and bronchi. The bronchial and hilar nodes become involved, and hemorrhages and a fibrinopurulent pleuritis may overlie the infected lung.

Fig. 24.3 The lung in pneumonic plague. Note confluent patches of lobular pneumonia involving scattered areas of the upper and lower lobes. There is no significant necrosis in this specimen. The intervening parenchyma shows slight compensatory emphysema. AFIP 40788.

Fig. 24.4 Masses of plague bacilli form the gray background of this pneumonic focus in which there is no intact parenchyma. Fibrin is conspicuously absent. H&E x 230. AFIP 77335. . (From Connor DH, Chandler FW (eds): Pathology of Infectious Diseases, Appleton & Lange, Stamford, Conn., 1997).