Community-acquired pneumonia: epidemiologic and clinical considerations

An incidence of between 2 and 44 per 1000 population has been reported for community-acquired pneumonia. Epidemiologic studies describe a wide range of causative organisms, including Streptococcus pneumoniae, Haemophilus influenzae, Mycoplasma pneumoniae, Legionella spp., Moraxella catarrhalis, Chlamydia pneumoniae and viruses such as influenza A and B. However, the frequency with which they are reported varies widely. On analysis of these studies, the variation can be explained by a number of factors. The results depend on the definition of pneumonia and the criteria for assigning a causative role to any particular organism. Older studies have not included diagnostic methods for newly described organisms such as C. pneumoniae and Legionella spp. The improved diagnostic methods for these organisms and for Mycoplasma pneumoniae are reflected in more recent studies. Further variation depends on the population studied. As many patients with mild pneumonia are successfully treated in the community, those studies that are hospital-based include patients with more severe pneumonia often in the elderly or in patients with underlying diseases such as chronic obstructive pulmonary disease. The prior use of antibiotics not only contributes to the high percentage of cases for which no etiologic agent is found, but also ensures that treatment failures are selected for hospitalization. This further changes the result, depending on the antibiotic agents used most commonly in the community. The inclusion of nursing home patients or groups where alcoholism is more common will also favor particular organisms. Finally, the timing of the study may be such that an epidemic is included. This has relevance mostly for Mycoplasma pneumoniae, C. pneumoniae, Legionella spp. and influenza. In the assessment of the patient with community-acquired pneumonia, any one of the above organisms can be considered to be responsible. As initial treatment is empirical, other information can be used to ensure that an antibiotic with an adequate spectrum is chosen. Factors of importance are age, underlying illness, severity of disease and any locally recognized epidemics or endemic organisms. Differences in clinical presentation are not sufficiently distinct to allow for accurate prediction of the causative agent. Similarly, chest radiograph changes are not sufficiently specific to discriminate reliably between diverse organisms such as S. pneumoniae, Mycoplasma pneumoniae and Legionella spp. Current recommendations for choice of an empirical antibiotic agent are therefore based, not on the assumption of a single etiologic agent indicated by clinical presentation or radiographic appearances, but on age of the patient, severity of illness, the presence of underlying conditions and the range of possible organisms in that patient group.