Abstract:Objective To explore the clinical characteristics of patients with severe pneumonia infected by Chlamydia psittaci and the application of metagenomic next-generation sequencing(mNGS) in pathogenic diagnosis. Methods 9 patients with severe Chlamydia psittaci pneumonia hospitalized in Fujian Provincial Hospital from November 2019 to February 2021 were enrolled, including 5 males, 4 females, with a median age of 67 years. Their medical records were retrospectively analyzed, and their clinical manifestations, laboratory tests, imaging characteristics, mNGS etiology results and treatment process were described. Results Among 9 patients, four had contacted with birds or poultry before the onset of disease. Clinical manifestations were mainly high fever, cough, dyspnea, fatigue and myalgia. The white blood cell count of patients were normal or slightly increased, while the neutrophil proportion and procalcitonin were significantly increased. Some patients had abnormal hepatic functions and renal functions. The main feature of chest CT scan was pathy consolidation of unilateral or bilateral lobes. All patients were tested for mNGS, and the nucleic acid sequence of Chlamydia psittaci were detected. Treatments based on doxycycline monotherapy or doxycycline plus moxifloxacin were initiated, and all patients recovered. Conclusion Chlamydia psittaci pneumonia has a low incidence and is critically ill. mNGS testing should be conducted for suspected cases as soon as possible, and tetracycline-based treatment programs should be initiated in time.