Evolving Understanding of the Causes of Pneumonia in Adults, With Special Attention to the Role of Pneumococcus

Abstract

Before 1945, Streptococcus pneumoniae caused more than 90% of cases of pneumonia in adults. After 1950, the proportion of pneumonia caused by pneumococcus began to decline. Pneumococcus has continued to decline; at present, this organism is identified in fewer than fewer10%-15% of cases. This proportion is higher in Europe, a finding likely related to differences in vaccination practices and smoking. Gram-negative bacilli, Staphylococcus aureus, Chlamydia, Mycoplasma, and Legionella are each identified in 2%-5% of patients with pneumonia who require hospitalization. Viruses are found in 25% of patients, up to one-third of these have bacterial coinfection. Recent studies fail to identify a causative organism in more than 50% of cases, which remains the most important challenge to understanding lower respiratory infection. Our findings have important implications for antibiotic stewardship and should be considered as new policies for empiric pneumonia management are developed.

Keywords: Streptococcus pneumoniae; antibiotic stewardship; community-acquired pneumonia; etiology; pneumococcus.

Figure 1.

A, Frequency of Streptococcus pneumoniae as a cause of community-acquired pneumonia, United States/Canada. B, Comparison of the frequency of S. pneumoniae between the United States/Canada (blue) and Europe (red). The area of each circle is proportional to the number of patients in each study. For both figures, the year is the reported (or estimated) midyear of investigation (see Supplementary Material for details).