Objective To identify causal factors of a case of severe Chlamydia psittaci pneumonia in Yangpu District and provide a scientific basis for effective prevention and control.

Methods Basic information and epidemiological data of the patient were collected through telephone interviews and field epidemiological surveys. Specimens from the patient， close contacts and the environment were collected for pathogen detection. Metagenomics next-generation sequencing （mNGS） was used to identify unknown pathogens.

Results A 65-year-old male patient with a history of hypertension and diabetes was admitted to the hospital with symptoms of fatigue， poor appetite for a week， fever and cough for four days. A chest computer tomography （CT） scan showed scattered inflammation in the left lung with infiltration of multiple lobes. Blood gas analysis showed type I respiratory failure. The results of mNGS on the bronchoalveolar lavage fluid of the patient indicated that he was infected with Chlamydia psittaci. Epidemiological investigation showed a clear history of avian exposure， with an incubation period of 30 days.

Conclusion This serious pneumonia is a zoonotic disease caused by Chlamydia psittaci. A clear history of avian exposure and the use of mNGS technology can help in the timely diagnosis of this disease.