A case report of spherical pneumonia caused by Chlamydia psittaci infection

Psittacosis is a zoonotic infectious disease caused by Chlamydia psittaci, typically transmitted from birds to humans. Its clinical manifestations are often subtle and non-specific, making it prone to being misdiagnosed as common community-acquired pneumonia. Round pneumonia (or spherical pneumonia) refers to a lung infection that appears as a round or oval density on computed tomography (CT). It is uncommon in adults. Spherical pneumonia specifically caused by Chlamydia psittaci infection is particularly uncommon. Here we report a case of a 43-year-old male who was admitted due to a 4-day history of fever. CT imaging revealed a solitary, round high-density shadow approximately 4 cm in diameter in the right lower lung, with clear margins and surrounding ground-glass opacity. The patient reported direct contact with ornamental parrots at a zoo 10 days before the onset of symptoms. The diagnosis of spherical pneumonia caused by Chlamydia psittaci infection was confirmed by targeted next-generation sequencing (tNGS). The patient's symptoms resolved following treatment with minocycline, and a follow-up CT scan 32 days later showed near-complete resolution of the lesion. This case demonstrates that Chlamydia psittaci can also produce a typical spherical lesion, thereby broadening the recognized imaging spectrum of this pathogen. When clinicians encounter a patient with imaging findings suggestive of spherical pneumonia and a history of bird exposure, Chlamydia psittaci infection should be suspected, especially when accompanied by extrapulmonary symptoms such as headache, myalgia, or hepatosplenomegaly, necessitating a comprehensive assessment that integrates epidemiological history and molecular diagnostic methods.