Psittacosis in Birds and Humans: Chlamydia psittaci Diagnostics and One Health Transmission

Introduction

Psittacosis, also termed ornithosis when originating from non-psittacine birds, is a bacterial zoonosis caused by the obligate intracellular pathogen Chlamydia psittaci. The disease imposes a significant burden on avian health, particularly in psittacine species such as cockatiels, budgerigars, and macaws, and poses a recognized occupational hazard to avian veterinarians, pet shop employees, and poultry workers [4, 14]. C. psittaci is classified within the order Chlamydiales, family Chlamydiaceae, and exhibits a biphasic developmental cycle alternating between infectious elementary bodies (EBs) and metabolically active reticulate bodies (RBs). The pathogen demonstrates a broad host range that includes over 460 avian species, with feral pigeons and psittacines serving as principal reservoir hosts in urban and domestic environments [3, 13]. Understanding the diagnostic landscape, transmission pathways, and control measures under a One Health framework is essential for reducing interspecies spread.

Pathogen Biology and Host Interactions

The developmental cycle of C. psittaci begins when EBs attach to host epithelial cells, predominantly respiratory and conjunctival mucosa, via heparan sulfate-mediated interactions. Internalization occurs through clathrin-mediated endocytosis, followed by differentiation into RBs within a membrane-bound vacuole termed the inclusion. RBs replicate by binary fission, utilizing host-derived ATP, and redifferentiate into EBs at 48–72 hours post infection, culminating in host cell lysis and release [9]. Recent interactomics studies using Inc/GFP chimera proteins have identified novel inclusion membrane proteins, such as Cps0558, that interact with host cellular trafficking pathways to evade lysosomal fusion and sustain intracellular survival [9]. The type III secretion system (T3SS) of C. psittaci injects effector proteins into the host cytosol, modulating apoptosis and cytokine responses.

In avian hosts, the pathogen preferentially colonizes the respiratory epithelium, air sacs, and conjunctiva, with systemic dissemination to the liver, spleen, and gastrointestinal tract. The incubation period in birds ranges from 3 days to several weeks, influenced by strain virulence and host immune status. The elementary bodies are highly stable in the environment, surviving for weeks in dried feces and feather dust, which facilitates indirect transmission.

Transmission Dynamics in a One Health Context

Transmission of C. psittaci occurs primarily via inhalation of aerosolized fecal dust, respiratory secretions, or feather dander from infected birds. Direct contact with contaminated fomites, such as feed, water, and cages, also contributes to spread. In urban environments, feral pigeons (Columba livia) act as key reservoirs, with spatial clustering analyses indicating a higher prevalence in areas with high human population density [3]. Psittacine birds in the pet trade, especially illegally traded parrots, introduce the pathogen into naive populations [13].

A One Health approach recognizes the interconnectedness of avian, environmental, and human health. Human infection typically results from occupational or recreational exposure to infected birds, with sporadic outbreaks linked to pet shops, aviaries, and poultry processing facilities [1, 12]. The zoonotic potential is underscored by case reports of severe pneumonia in elderly patients without direct avian contact, suggesting possible environmental transmission via contaminated dust [5, 6]. The incubation period in humans has been estimated through retrospective observational studies to range from 5 to 14 days, with a median of 7 days [11].

Clinical Signs in Psittacines

The clinical presentation of psittacosis in birds varies from acute fatal disease to chronic subclinical infection. Common clinical signs include:

• Ocular signs: conjunctivitis, periorbital edema, serous to mucopurulent discharge.
• Respiratory signs: dyspnea, nasal discharge, sinusitis, air sacculitis.
• Gastrointestinal signs: anorexia, weight loss, greenish diarrhea, vomiting.
• Systemic signs: lethargy, ruffled feathers, depression, sudden death.

Chronic carriers may exhibit intermittent shedding of EBs without overt clinical signs, complicating detection and control. C. psittaci infection can also present initially with gastrointestinal symptoms, mimicking enteric disease [1]. In severe cases, organizing pneumonia and systemic vasculitis may develop, particularly in immunocompromised birds [6].

Diagnostic Approaches

Accurate diagnosis of C. psittaci relies on a combination of molecular, serological, and culture-based methods. The sensitivity and specificity of each assay vary by sample type, disease stage, and laboratory capability.

Nucleic Acid Amplification Tests (NAATs)

Real-time PCR targeting the ompA gene or the 16S rRNA gene remains the gold standard for detecting C. psittaci in avian clinical specimens. The analytical sensitivity of real-time PCR reaches as low as 10 genome copies per reaction, with 100% specificity when using species-specific probes [8]. Conventional PCR with Sanger sequencing of the ompA gene enables genotyping to distinguish avian strains (e.g., genotype A from psittacines, genotype B from pigeons) [13].

Targeted next-generation sequencing (tNGS) has emerged as a powerful tool for detecting C. psittaci directly from respiratory samples, particularly in cases where conventional PCR is negative [8, 10]. tNGS amplifies pathogen-specific genomic regions using a primer panel, followed by high-throughput sequencing, allowing simultaneous detection of co-infections. Metagenomic next-generation sequencing (mNGS) provides an unbiased approach, capturing both pathogen and host nucleic acid, and has successfully diagnosed psittacosis in patients with atypical presentations and no known avian exposure [5].

Serological Assays

Serological testing by enzyme-linked immunosorbent assay (ELISA) detects antibodies against C. psittaci lipopolysaccharide (LPS) or outer membrane proteins. However, cross-reactivity with other Chlamydia species (e.g., C. trachomatis, C. pneumoniae) reduces specificity. Complement fixation tests and microimmunofluorescence are more specific but require paired acute and convalescent sera, limiting utility in clinical decision-making [15]. In avian medicine, serology is best used for population-level surveillance rather than individual diagnosis.

Culture and Antigen Detection

Cell culture isolation using McCoy or BGM cells remains a reference method but is labor-intensive and requires biosafety level 3 containment due to high infectiousness. Antigen detection via direct immunofluorescence or immunohistochemistry on tissue smears is rapid but suffers from low sensitivity.

Comparison of Diagnostic Modalities

Diagnostic Algorithm

The following Mermaid diagram outlines a decision tree for diagnosing C. psittaci infection in suspect psittacine birds.

flowchart TD
    A[Clinical signs: respiratory, ocular, gastrointestinal], > B{Collect samples}
    B, > C[Choanal/cloacal swab, feces, or tissue]
    C, > D[Real-time PCR for C. psittaci]
    D, > E{Result}
    E, >|Positive| F[Confirmed infection; initiate treatment and biosecurity]
    E, >|Negative or equivocal| G[Consider tNGS or mNGS]
    G, > H{Pathogen detected?}
    H, >|Yes| F
    H, >|No| I[Perform serology (paired sera) or repeat PCR after 2 weeks]
    I, > J{Seroconversion?}
    J, >|Yes| F
    J, >|No| K[Consider other respiratory pathogens: Avian Influenza, Mycoplasma, Aspergillus]

One Health Control Strategies

One Health control of psittacosis integrates surveillance, biosecurity, and public health education. Key measures include:

• Quarantine and screening: Newly acquired psittacines should be quarantined for 30 days with PCR testing of pooled cloacal and choanal swabs. Individuals from high-risk sources (e.g., wild-caught, feral populations) warrant tNGS or mNGS testing [13].
• Environmental decontamination: EBs are inactivated by 70% ethanol, 1% sodium hypochlorite, and quaternary ammonium compounds. Regular cleaning of cages and removal of dried feces reduces aerosolized particles.
• Antimicrobial therapy: Doxycycline administered in feed or water for 45 days is the first-line treatment in birds. Treatment failure rates are low but can be monitored by PCR post therapy.
• Occupational health: Avian workers should use N95 respirators, gloves, and eye protection when handling birds or cleaning enclosures. Serological monitoring of at-risk personnel is recommended [4, 14].
• Vaccine development: Lipid nanoparticle-delivered mRNA vaccines encoding the major outer membrane protein (MOMP) have shown protective immune responses in BALB/c mice, indicating potential for future avian vaccines [7]. Such vaccines could reduce shedding and transmission in flock settings.

Research Frontiers and Computational Modeling

Computational tools such as biological foundation models (e.g., ESM-2) are increasingly applied to predict host tropism of zoonotic Chlamydia strains based on amino acid sequences of surface antigens [2]. Spatial epidemiological modeling using geographic information systems can identify high-risk urban clusters (e.g., feral pigeon colonies) and inform targeted culling or vaccination programs. These approaches align with the broader One Health informatics movement, where machine learning algorithms integrate veterinary diagnostic data with environmental and human health records to forecast spillover events [3].

Conclusions

Chlamydia psittaci remains a significant zoonotic pathogen with a complex epidemiology linking psittacine birds, feral pigeon populations, and humans. Advances in molecular diagnostics, particularly tNGS and mNGS, have improved detection accuracy and turnaround times, enabling early case identification in both avian and human patients. A One Health framework emphasizing interspecies surveillance, biosecurity, and vaccine development is essential to mitigate the risk of psittacosis transmission. Continued research into host-pathogen interactions at the molecular level will inform novel therapeutic and preventive strategies.

References

1. Shu Q, Chen Z, Deng Z. Chlamydia psittaci Infection Presenting Initially with Gastrointestinal Symptoms. Clin Lab. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42159113/

2. Zhang J, Wu H, Li Y, et al. From models to medicine: systematic development and evaluation of animal models for Chlamydia psittaci respiratory and genital tract infections. Front Immunol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41948330/

3. Dziedzinska R, Kralik P, Zeman T, et al. Feral pigeons as reservoirs of zoonotic pathogens in the urban environment: Spatial clustering and molecular evidence. Vet Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41930682/

4. Tomić DH, Krkljuš M, Gottstein Ž, et al. Zoonotic potential of Chlamydia psittaci-a case report. Front Vet Sci. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41640945/

5. Sun X, Jiao Q, Fu X, et al. Case Report: Severe psittacosis in an elderly patient without avian exposure: diagnosis via metagenomic next-generation sequencing and rapid response to doxycycline. Front Med (Lausanne). 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41488077/

6. Li Q, Sun X, Lei W, et al. Psittacosis chlamydia pneumonia complicated with organizing pneumonia: a case report and literature review. Front Med (Lausanne). 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41333773/

7. Wang B, Xiao J, Wang J, et al. Lipid nanoparticle-delivered mRNA vaccine encoding the MOMP of Chlamydia psittaci elicits protective immune responses in BALB/c mice. Microbiol Spectr. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41190885/

8. He Z, Chen X, Chu R, et al. Clinical characteristics and outcomes of Chlamydia psittaci pneumonia: Insights from targeted next-generation sequencing diagnosis. Sci Prog. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41148128/

9. Gensch JM, Scholz J, Ingmundson A, et al. Inc/GFP chimera protein-based interactomics reveals host cellular interactions of Cps0558, a novel Chlamydia psittaci inclusion protein. Pathog Dis. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41124021/

10. Yu M, Wang W, Zhou H, et al. Diagnosis of Chlamydia psittaci pneumonia using targeted next-generation sequencing: case report and clinical characteristics statistics. Front Med (Lausanne). 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41114040/

11. Wang Z, Jiang X, Duan Q, et al. Estimating the Incubation Period Distribution of Psittacosis: A Retrospective Observational Study. Health Sci Rep. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41104195/

12. Guillemot Q, Clemens T, Inthasot V, et al. Community-Acquired Chlamydia psittaci Severe Pneumonia: A Case Report. Case Rep Infect Dis. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40949051/

13. Sassa-O'Brien Y, Wu CF, Matsunaga S, et al. Isolation of "pigeon-type" Chlamydia psittaci and detection of Chlamydia-related bacteria in Indian ring-necked parakeets (Psittacula krameri manillensis) in introduced flocks in urban area of Japan. Vet Microbiol. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40850222/

14. Matre TK, Hareide SJØ, Bergane LA, et al. [Ornithosis]. Tidsskr Nor Laegeforen. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40844344/

15. Dutta S, Ray U. Psittacosis, an Uninvited Guest. J Glob Infect Dis. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40727494/