Epidemiology of Severe Pneumonia Caused by

Background. Legionella species, Mycoplasma pneumoniae, and Chlamydia pneumoniae are recognized as im-portant causes of pneumonia in high-income countries, but their significance in middle-income countries, such as Thailand, is unknown. Methods. Population-based surveillance identified inpatient 3489 cases of clinically-defined pneumonia in a rural Thai province for 1 year. Patients who had a chest radiograph performed (for 2059 cases of pneumonia) were enrolled in an etiology study (which included 755 cases of pneumonia among 738 patients). Paired serum, nasopharyngeal swab, and urine specimens were obtained for diagnostic immunologic and molecular tests. Patients aged!18 years were not systematically tested for Legionella species. We report a lower limit of incidence (observed incidence) and an upper limit extrapolated to persons not tested or not enrolled in the study. Results. The incidence of pneumonia due to Legionella longbeachae requiring hospitalization was 5–29 cases per 100,000 population. No case of Legionella pneumophila pneumonia was observed. The definite C. pneumoniae pneumonia incidence was 3–23 cases per 100,000 population; rates were highest among patients aged!1 year (18– 166 cases per 100,000 population) and those aged70 years (23–201 cases per 100,000 population). M. pneumoniae pneumonia had a similar age distribution, with an overall incidence of 6–44 cases per 100,000 population. Thes