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The worldwide influenza epidemic in 1918–1919 caused about 50 million deaths. In an effort to understand why the mortality rate was so high, investigators reexamined preserved lung tissue, stained slides, and records from 68 soldiers stationed at U.S. Army training camps who had died from probable influenza between May and October 1918.
The men had a median age of 27 (range, 18–32), and nine of them had died during the 4 months before the pandemic was identified. Pneumonia or bronchopneumonia with or without influenza had been diagnosed in all 59 men for whom medical records were available.
The 68 cases showed a spectrum of histopathological changes, including features of bronchitis (4/4 with available bronchial tissue), bronchiolitis (39/68), primary influenza viral pneumonia with diffuse alveolar damage (36/68), acute edema (41/68), acute hemorrhage (27/68). In addition, all 68 cases had evidence of severe acute bacterial pneumonia, and bacteria — predominantly gram-positive organisms morphologically consistent with Streptococcus pneumoniae, Streptococcus pyogenes, or Staphylococcus aureus — were found in recut tissue specimens from 63 of 67 cases. These results were concordant with bacterial lung-culture results recorded in 1918 for 44 of the cases. Influenza viral antigens were identified in alveolar lining cells, apical cells of bronchial epithelium, and hyaline membrane material, with no difference between prepandemic and pandemic cases. Analysis of influenza viral RNA from a subset of cases showed a shift from mixed “avian-like” and “human-like” glycan binding specificity in prepandemic cases to “human-like” specificity in pandemic cases.
Sheng Z-M et al. Autopsy series of 68 cases dying before and during the 1918 influenza pandemic peak. Proc Natl Acad Sci U S A 2011 Sep 27; 108:16416.
Comment
These data provide several key insights. First, the pandemic virus was circulating in the U.S. for at least 4 months before the pandemic was recognized. Second, the overall characteristics of influenza infection in the cases studied here were quite similar to those seen in fatal 2009 H1N1 influenza cases and were not suggestive of more-severe viral disease. Finally, mortality was universally associated with concurrent severe bacterial pneumonia, which today should be preventable with immunization or treatable with readily available antibiotics.