The lungs are purple or black and contain large areas of necrosis (Fig. 24.5). Microscopically, there are hemorrhages, some suppuration, large areas of necrosis, and enormous numbers of bacilli in most of the alveoli (Fig. 24.6). Plague spreads rapidly in the lungs and causes acute pulmonary insufficiency, sepsis, and toxemic shock.

Fig 24.5 Lung in primary pneumonic plague. There are necrotic nodules with intense hyperemia and hemorrhage in the lower lobe, while the upper lobe contains only necrotic nodules. Intervening normal pulmonary parenchyma shows foci of compensatory emphysema. AFIP 40657. (From Connor DH, Chandler FW (eds): Pathology of Infectious Diseases, Appleton & Lange, Stamford, Conn., 1997).

Fig. 24.6 Pneumonic plague with huge masses of plague bacilli (Y. pestis) and macrophages filling alveoli. No intact pulmonary parenchyma is apparent. Large necrotic cavities are present.