Current Insights into the Etiology of Community Pneumonia: Pneumococci Still Leading?

Etiology of Community-Acquired Pneumonia

Community-acquired pneumonias are acute infectious diseases of the lung tissue caused by infectious agents outside of the hospital environment. They represent a heterogeneous group of infectious diseases with variable clinical pictures and a wide spectrum of responsible pathogens, differing based on geographic conditions, age, and exposure. Before the antibiotic era, 90–95% of pneumonia cases were caused by the bacterium Streptococcus pneumoniae. With the availability of antibiotic therapy, vaccination, and modern diagnostic methods, their share in the etiology of community-acquired pneumonias has decreased over the years.

According to recent guidelines, however, empirical antibiotic therapy is still recommended upon diagnosing community-acquired pneumonia, regardless of possible changes in the spectrum of the most common pathogens. In recent years, a possible change in the etiology of community-acquired pneumonias has not been reviewed. 

Analyzed Studies

The aim of the study was to shed light on the most common etiological agents of community-acquired pneumonia during the antibiotic era. Clinical studies published from January 1945 to March 2020 were searched through Medline/PubMed databases for the analysis. Studies were included in which the etiology of the disease was clarified in at least 25% of cases. The analysis was not limited to hospitalized patients, although a large portion was hospitalized for pneumonia.

Studies focusing on a specific pathogen, studies dealing with a particular complication of pneumonia (such as empyema), studies conducted during specific outbreak periods (such as the COVID-19 pandemic), and studies focusing on specific population groups were not included in the analysis, except for older individuals, as they represent a significant portion of patients with community-acquired pneumonia.

Analyzed studies were stratified by microbiological techniques, from classical sputum culture methods to modern PCR methods for diagnosing atypical agents and viral causes of pneumonia. The inclusion criteria for the analysis were met by a total of 146 studies with a total of 82,674 patients (77.3% hospitalized, 20.0% hospitalized or treated on an outpatient basis, 2.7% treated on an outpatient basis). 

Results

Streptococcus pneumoniae was identified as the most common etiological agent of community-acquired pneumonia throughout the period, regardless of the microbiological diagnostic technique. On average, it was identified as the etiological agent in 33–50% of cases with known etiology. However, the proportion of infections caused by pneumococci showed a downward trend over the years. The second most common etiological agent of community-acquired pneumonia was Haemophilus influenzae, in 7–16% of cases. The third place was occupied by Staphylococcus aureus and Enterobacteriaceae, including the genus Klebsiella, with approximately the same incidence of 4–10%. Other identified agents included bacteria of the genus Pseudomonas (0.8–4.5%) and Moraxella (1.2–3.5%), with other bacteria being isolated less frequently. Among atypical bacteria, the most frequently isolated were bacteria of the genus Mycoplasma in 4–11% of cases, followed by bacteria of the genus Legionella in 3–8%, Chlamydophila in 2–7%, and Coxiella in <2% of cases.

Regarding microbiological techniques with routine PCR testing for the presence of viral infections, influenza viruses were identified as the etiological agent in 6.2–13.7% of cases and rhinoviruses in 4.1–11.5% of cases. Respiratory syncytial viruses and human metapneumoviruses were isolated less frequently (0.4–4.7%). In studies with consistent PCR testing, respiratory viruses were isolated in 30–40% of cases, with viral and bacterial coinfection occurring in 25–35% of these cases. These findings were confirmed by a separate study, which found a 40% rate of viral and bacterial coinfection.

On average, no etiological agent was identified in 50.2–67.1% of cases, regardless of the type of microbiological technique used. Thus, even with the advent of more sophisticated diagnostic methods, no trend in increasing the proportion of cases with an identified causative pathogen was observed in the last two decades.

Conclusion

The results of an extensive analysis of the etiological agents of community-acquired pneumonias from 1945 to 2020 indicate that the bacterium Streptococcus pneumoniae still plays a dominant role, followed by Haemophilus influenzae and Staphylococcus aureus along with Enterobacteriaceae. The study results support the 2019 guidelines, which recommend empirical antibiotic therapy with ceftriaxone and azithromycin for hospitalized patients with community-acquired pneumonia, covering the most common pathogens, including atypical bacteria.

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Source: Shoar S., Musher D. M. Etiology of community-acquired pneumonia in adults: a systematic review. Pneumonia (Nathan) 2020; 12: 11, doi: 10.1186/s41479-020-00074-3.