Case Report: Severe psittacosis in an elderly patient without avian exposure: diagnosis via metagenomic next-generation sequencing and rapid response to doxycycline.

Chlamydia psittaci causes psittacosis in both birds and humans, typically following avian exposure. We present a case of severe psittacosis in a 73-year-old woman with no documented bird contact. The diagnosis was ultimately achieved through metagenomic next-generation sequencing (mNGS) after initial conventional serologic tests failed to identify a pathogen. The patient presented with fever and pneumonia that were unresponsive to broad-spectrum antibiotics. mNGS performed on a whole-blood sample collected on hospital day 5 detected C. psittaci, albeit with a low number of specific sequence reads (only 14 reads mapping to the C. psittaci genome). Oral doxycycline (100 mg q12h) was initiated promptly, resulting in defervescence within 24 h and resolution of inflammatory markers. Although community pet parrots were identified as a potential source, the patient denied any direct contact. This case highlights the risk of environmental aerosol transmission in the absence of direct avian exposure, demonstrates the critical role of mNGS in diagnosing culture-negative pneumonia, and underscores the efficacy of early doxycycline therapy. Strengthened public health surveillance of avian reservoirs is imperative to mitigate unrecognized transmission.