New and Emerging Etiologies for Community-Acquired Pneumonia with Implications for Therapy

Three hundred fifty-nine consecutive patients with community-acquired pneumonia admitted to university, community, and VA hospitals underwent a standardized evaluation, including specialized tests for Legionella spp. and Chlamydia pneumoniae (TWAR). The most common underlying illnesses were immunosuppression (36.3%), chronic obstructive pulmonary disease (32.4%), and malignancy (28.4%). The most frequent etiologic agents were Streptococcus pneumoniae (15.3%) and Hemophilus influenzae (10.9%). Surprisingly, Legionella spp. and C. pneumoniae were the third and fourth most frequent etiologies at 6.7% and 6.1%, respectively. Aerobic gram-negative pneumonias were relatively uncommon causes of pneumonia despite the fact that empiric broad-spectrum combination antibiotic therapy is so often directed at this subgroup. In 32.9%, the etiology was undetermined. Antibiotic administration before admission was significantly associated with undetermined etiology (p = 0.0003). There were no distinctive clinical features found to be diagnostic for any etiologic agent, although high fever occurred more frequently in Legionnaires' disease. Clinical manifestations for C. pneumoniae were generally mild, although 38% of patients had mental status changes. Mortality was highest for Staphylococcus aureus (50%) and lowest for C. pneumoniae (4.5%) and Mycoplasma pneumoniae (0%). We document that specialized laboratory testing for C. pneumoniae and Legionella spp. should be more widely used rather than reserved for cases not responding to standard therapy. Furthermore, realization that C. pneumoniae and Legionella spp. are common etiologies for community-acquired pneumonia should affect empiric antibiotic prescription.