Etiological and epidemiological investigation of two independent parrot-to-human transmission events of Chlamydia psittaci

Chlamydia is a genus of Gram-negative bacteria belonging to the family Chlamydiaceae and is characterized by an obligate intracellular lifestyle. It infects a wide variety of hosts, including nearly 500 bird species (e.g., parrots, pigeons, and poultry) and livestock species (e.g., sheep, goats, pigs, cattle, and horses). In humans, C. psittaci causes psittacosis, a disease usually linked to contact with infected birds. Individuals who work closely with birds, such as poultry workers, pet shop employees, and pet bird owners, are at the highest risk. Despite its clinical significance, psittacosis is often overlooked, primarily because C. psittaci is not included in standard diagnostic tests for community-acquired pneumonia (CAP).

In April and August 2022, two patients presenting with flu-like symptoms were diagnosed with CAP in Beijing, China. Both were owners of pet parrots that had shown signs of respiratory illness, and some of the birds died before the onset of the patients’ symptoms. Notably, the two patients lived in different districts with no overlap in residence, workplace, or social activities, and no contact between their families or friends, supporting two independent events rather than a transmission cluster. Importantly, C. psittaci was not included in the standard PCR respiratory pathogen panel, so targeted testing was not triggered at the time of admission.

“We report two independent, epidemiologically unrelated events of CAP in the megacity of Beijing associated with pet parrots, where convergent clinical, epidemiological, histopathological, and genetic evidence supports zoonotic transmission of C. psittaci from parrots to humans,” said Yuhai Bi, Professor at the Institute of Microbiology, Chinese Academy of Sciences.

Metagenomic next-generation sequencing was therefore performed as an unbiased diagnostic approach, revealing C. psittaci reads in both patients and their parrots. Integrating the sequence data from parrots and patients, detailed exposure histories, negative results for other respiratory pathogens, metagenomic and PCR confirmation of C. psittaci, and compatible pathological changes in the parrots, we attribute the pneumonia in both patients primarily to C. psittaci infection likely acquired from their pet parrots. Phylogenetic analysis of the ompA gene showed that our strains grouped together and belonged to lineage II, which includes both avian and human isolates, and clustered closely with a parrot-derived strain from Australia.

These findings highlight an urgent need for the development and widespread adoption of certified diagnostic kits and standardized testing protocols for C. psittaci in clinical practice, both in China and globally. Psittacosis should be consistently included in the differential diagnosis of patients in urban cities presenting with pneumonia of unknown etiology, especially those who have an exposure history to pet birds.

This research was supported by the Major Project of Guangzhou National Laboratory (grant number GZNL2023A01001 to Y.B.), the National Natural Science Foundation of China (NSFC) (grant numbers 32425053 and 82341115 to Y.B.), and the Beijing Research Center for Respiratory Infectious Diseases (grant number BJRID2025-007 to Y.B.). 

About Author: (Briefly outline the corresponding author's academic achievements and research contributions) (One author only)

Dr. Yuhai Bi is a Principal Investigator at the Institute of Microbiology, Chinese Academy of Sciences (IMCAS). He is affiliated with the Key Laboratory of Pathogen Microbiology and Immunology, Center for Influenza Research and Early-warning (CASCIRE), and CAS-TWAS Center of Excellence for Emerging Infectious Diseases (CEEID). His research primarily focuses on the surveillance and early warning mechanisms of cross-species transmission, and comprehensive biosafety control strategies for emerging or novel pathogens. https://im.cas.cn/rcdw/jcqn/202601/t20260109_8101108.html