Background.Legionella species, Mycoplasma pneumoniae, and Chlamydia pneumoniae are recognized as important 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 aged ?70 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. These pathogens were associated with 15% of all cases of pneumonia. A nonsignificantly higher proportion of patients with pneumonia associated with L. longbeachae, compared with patients with pneumonia associated with M. pneumoniae or C. pneumoniae, required supplemental oxygen or mechanical ventilation (45% vs. 18%; P < .1). Among patients with atypical pneumonia, only 15% received antibiotics with activity against the associated pathogen.

Conclusion.M. pneumoniae, C. pneumoniae, and L. longbeachae, but not L. pneumophila, are frequently associated with severe pneumonia in rural Thailand. Few patients receive antibiotics that cover atypical pathogens.