The pathology of influenza virus infections

Abstract

Influenza viruses are significant human respiratory pathogens that cause both seasonal, endemic infections and periodic, unpredictable pandemics. The worst pandemic on record, in 1918, killed approximately 50 million people worldwide. Human infections caused by H5N1 highly pathogenic avian influenza viruses have raised concern about the emergence of another pandemic. The histopathology of fatal influenza virus pneumonias as documented over the past 120 years is reviewed here. Strikingly, the spectrum of pathologic changes described in the 1918 influenza pandemic is not significantly different from the histopathology observed in other less lethal pandemics or even in deaths occurring during seasonal influenza outbreaks.

Figure 1

H&E-stained section of the lung from a 1918 influenza victim showing necrotizing bronchiolitis. There is necrosis of the bronchiolar wall, with submucosal edema and vascular congestion. The epithelial layer is desquamating, and necrotic epithelial cells are present in the lumen. A mixed inflammatory cell infiltrate is present throughout (original magnification 40×).

Figure 2

H&E-stained section of the lung from a 1918 influenza victim showing necrotizing bronchiolitis. There is necrosis of the bronchiolar wall. The epithelial layer is desquamating, and necrotic epithelial cells are present in the lumen. A mixed inflammatory cell infiltrate is present throughout (original magnification 200×).

Figure 3

H&E-stained section of the lung from a 1918 influenza victim showing a massive infiltrate of neutrophils that fills the alveolar air spaces in early bacterial bronchopneumonia. Alveolar capillary congestion is prominent (original magnification 200×).

Figure 4

H&E-stained section of the lung from a 1918 influenza victim showing a pattern of necrotizing alveolitis. The alveolar walls are necrotic, and alveolar air spaces contain edema fluid, desquamated epithelial cells, and inflammatory cells (original magnification 200×).

Figure 5

H&E-stained section of the lung from a 1918 influenza victim showing hyaline membranes lining an alveolar duct and adjacent alveoli. The alveolar air spaces contain edema fluid, strands of fibrin, desquamated epithelial cells, and inflammatory cells (original magnification 200×).

Figure 6

H&E-stained section of the lung from a 1918 influenza victim showing massive pulmonary edema. The alveolar air spaces contain edema fluid. A mild interstitial inflammatory cell infiltrate is also present (original magnification 40×).

Figure 7

H&E-stained section of the lung from a 1918 influenza victim showing massive pulmonary hemorrhage. The alveolar air spaces contain erythrocytes. Interstitial edema and a mild interstitial inflammatory cell infiltrate are also present (original magnification 40×).

Figure 8

H&E-stained section of the lung from a 1957 influenza victim showing massive pulmonary edema and hemorrhage in early bronchopneumonia. The alveolar air spaces contain edema fluid and erythrocytes. A bronchiole shows necrotizing bronchiolitis with epithelial desquamation and necrotic epithelial cells in the bronchiolar lumen (original magnification 20×).

Figure 9

H&E-stained section of the lung from a 1918 influenza victim showing bronchiolitis obliterans. The surrounding alveoli show edema and hemorrhage. There is interstitial capillary congestion, and a peribronchiolar vessel shows a thrombus (original magnification 40×).