Chlamydia gallinacea: Emerging Chlamydiosis in Poultry

Introduction

Avian chlamydiosis has historically been attributed to Chlamydia psittaci, a Gram-negative obligate intracellular bacterium that causes respiratory and systemic disease in birds and zoonotic psittacosis in humans. Within the past decade, novel chlamydial species have been identified in poultry populations, most notably Chlamydia gallinacea. This emerging agent is now recognized as a widespread colonizer of chickens, turkeys, and other galliform birds, often in the absence of overt clinical signs but with demonstrated zoonotic potential [1, 2]. The present article reviews the current state of knowledge regarding C. gallinacea as an agent of emerging chlamydiosis in poultry, with emphasis on its etiologic characteristics, epidemiologic patterns, transmission biology, diagnostic detection, and control strategies.

Etiology and Taxonomy

Chlamydia gallinacea belongs to the family Chlamydiaceae, order Chlamydiales. It is an obligate intracellular bacterium with a biphasic developmental cycle alternating between infectious elementary bodies (EBs) and metabolically active reticulate bodies (RBs). Phylogenetic analyses based on the outer-membrane protein A (ompA) gene and 16S rRNA sequences place C. gallinacea in a distinct clade separate from C. psittaci and C. avium [3, 2]. The bacterium was first identified in chickens and turkeys in Europe and has since been detected on multiple continents [1].

The organism shares approximately 95% 16S rRNA gene sequence identity with C. psittaci, yet exhibits a markedly different host range and clinical phenotype. C. gallinacea appears to be highly adapted to gallinaceous birds, particularly Gallus gallus domesticus (chickens) and Meleagris gallopavo (turkeys). Unlike C. psittaci, which is frequently associated with respiratory distress and high mortality in psittacines, C. gallinacea typically establishes subclinical or mild infections in poultry, although it may potentiate disease under conditions of coinfection or immunosuppression [4, 5].

Epidemiology and Geographic Distribution

Surveys conducted across Europe, Asia, and the Americas have demonstrated that C. gallinacea is widely distributed in commercial and backyard poultry flocks. In a large-scale molecular survey in Guangxi, southwestern China, the overall positivity rate for avian chlamydia among apparently healthy birds was 28.20% (492/1744). Among poultry species, pigeons showed the highest prevalence (62.30%), followed by chickens (25.05%), geese (18.12%), and ducks (14.14%). Importantly, C. gallinacea was detected in chicken and duck samples, with the latter representing the first documented occurrence of this species in ducks in China [3].

In Brazil, a study of backyard chicken farms found C. gallinacea to be the predominant chlamydial species, with molecular detection rates exceeding 50% in some flocks [6]. Similarly, surveys in free-range indigenous chicken flocks in the Philippines identified C. gallinacea as the main avian chlamydia species present [7]. In Italy, backyard chickens tested positive for C. gallinacea using ompA-based PCR, confirming its circulation in European small-holding systems [8]. In Slovakia, C. psittaci was detected in clinically healthy chickens, but molecular typing also revealed the presence of C. gallinacea in the region [9]. These findings collectively indicate that C. gallinacea is a globally distributed emerging pathogen in poultry, with a particularly high prevalence in backyard and free-range management systems.

Transmission Routes

Elucidation of C. gallinacea transmission dynamics has been critical for understanding its epidemiology. Experimental studies using specific-pathogen-free (SPF) broilers demonstrated that C. gallinacea is efficiently transmitted via the fecal-oral route. Over 90% of sentinel birds co-housed with inoculated broilers for 15 days became positive for C. gallinacea DNA in oropharyngeal and cloacal swabs. In contrast, airborne transmission was not observed when birds were housed in separate isolators connected only by ventilation tubing; infectious bronchitis virus (IBV) was transmitted under identical conditions, confirming the adequacy of the model but demonstrating that C. gallinacea does not spread efficiently via aerosol [10].

Vertical transmission has also been documented. In a study examining embryonated eggs from a breeding farm, 97.6% of eggshells (287/294) were positive for C. gallinacea DNA. Detection rates in albumen increased from 7.6% before incubation to 44.4% after 7 days of incubation, with parallel increases in yolk. After 19 days of incubation, C. gallinacea DNA was detected in heart, liver, spleen, lung, kidney, and intestine of chicken embryos, confirming trans-shell penetration and subsequent systemic dissemination within the developing embryo [10].

These transmission characteristics have important implications for control. The dominance of fecal-oral spread and vertical transmission means that biosecurity measures must focus on breaking the oral-fecal cycle, improving egg sanitation, and preventing contamination of feed and water with infected feces. Respiratory aerosol precautions, while relevant for C. psittaci, appear less critical for C. gallinacea.

Clinical Signs and Pathology

One of the defining features of C. gallinacea infection in poultry is its largely subclinical nature. Most naturally infected chickens and turkeys show no overt signs of disease, which has led to the view of C. gallinacea as a commensal or relatively low-virulence organism [1, 2]. However, mild respiratory signs, reduced feed conversion, and slight growth depression have been anecdotally observed in some flocks. In co-infections with other respiratory or enteric pathogens, such as Escherichia coli, Avibacterium paragallinarum (causing infectious coryza), or immunosuppressive viruses, C. gallinacea may act as a contributing agent to clinical disease [4, 5].

Postmortem findings in experimentally infected birds are often minimal. Mild conjunctivitis, serous nasal discharge, and slight airsacculitis have been reported in some studies. Histopathologic examination may reveal lymphoplasmacytic infiltration in the conjunctiva and respiratory mucosa, with intracytoplasmic chlamydial inclusions visible on Giemsa or modified Ziehl-Neelsen staining. In egg-laying hens, vertical transmission has been associated with reduced hatchability and increased embryonic mortality, although the magnitude of this impact requires further quantification [10, 2].

As C. gallinacea is not a notifiable pathogen in most jurisdictions, its role in economic losses for the poultry industry remains underappreciated. Nevertheless, the high prevalence observed in backyard flocks worldwide suggests that the pathogen may contribute to suboptimal flock performance and heightened susceptibility to secondary bacterial infections.

Diagnostic Approaches

Accurate diagnosis of C. gallinacea infection requires molecular methods, as the organism cannot be reliably distinguished from C. psittaci by serology or clinical presentation alone. The diagnostic workflow typically proceeds from sample collection to nucleic acid extraction, PCR amplification targeting conserved chlamydial genes, and sequencing or species-specific real-time PCR for final identification.

The following diagram summarizes the recommended diagnostic algorithm.

flowchart TD
    A[Sample collection: oro-pharyngeal and cloacal swabs, eggshells, tissue], > B[Nucleic acid extraction]
    B, > C[Pan-Chlamydia real-time PCR: 23S rRNA or ompA target]
    C, Negative, > D[Report negative; consider re-sampling if high suspicion]
    C, Positive, > E[Species differentiation by ompA sequencing or species-specific qPCR]
    E, > F[Identify C. gallinacea, C. psittaci, or C. avium]
    F, > G[Confirm with ompA phylogenetic analysis if required for research/outbreak]

Sample Collection

Oropharyngeal and cloacal swabs are the specimens of choice for live birds. For investigation of vertical transmission, eggshell membranes, albumen, yolk, and embryonic tissues (heart, liver, spleen, lung, kidney) can be tested. Swabs should be placed in a transport medium (e.g., phosphate-buffered saline with antibiotics to suppress bacterial overgrowth) and stored at 4°C for short-term transport or at -20°C for longer storage.

Molecular Detection

Real-time PCR targeting the 23S rRNA gene or ompA gene is widely used as a screening tool for all Chlamydiaceae. A positive pan-Chlamydia PCR requires further species identification. This can be achieved by conventional PCR amplifying the ompA gene (approximately 1.1 kb) followed by Sanger sequencing. Alternatively, species-specific real-time PCR assays that discriminate C. gallinacea from C. psittaci and C. avium have been developed and validated [3, 1].

Phylogenetic analysis of ompA sequences has revealed that C. gallinacea isolates from chickens and ducks cluster separately from C. psittaci genotypes A and B. In the Guangxi study, C. gallinacea was identified in chickens and ducks, with duck-derived sequences grouping within the C. gallinacea clade [3]. Such genetic characterization is essential for tracking cross-species transmission events and emergence of novel variants.

Serology

Serological methods, including enzyme-linked immunosorbent assays (ELISAs) and complement fixation tests, are available for detection of anti-Chlamydia antibodies. However, these assays do not differentiate between species and are more useful for flock-level exposure assessment than for individual diagnosis. Cross-reactivity between C. gallinacea and C. psittaci antibodies limits their specificity. The use of species-specific antigens for serodiagnosis is an area of ongoing development.

Differential Diagnosis

Clinical signs compatible with chlamydiosis, such as conjunctivitis, respiratory distress, and diarrhea, can be caused by other avian pathogens. Differential diagnoses include Avibacterium paragallinarum (infectious coryza), Mycoplasma gallisepticum, Pasteurella multocida (fowl cholera), and avian influenza virus. Laboratory confirmation by PCR is essential to distinguish C. gallinacea infection from these overlapping conditions.

Treatment and Antimicrobial Considerations

Tetracyclines, particularly doxycycline and chlortetracycline, remain the antimicrobials of choice for treating chlamydiosis in poultry. However, C. gallinacea has demonstrated susceptibility to tetracyclines in vitro, and field reports indicate that medication of drinking water with chlortetracycline (at doses of 400-500 mg/L) for 7-14 days can reduce shedding. The intracellular location of the pathogen necessitates prolonged treatment to ensure adequate intracellular concentrations.

Antimicrobial resistance among Chlamydiaceae is still considered rare, but there are concerns that the widespread agricultural use of tetracyclines for growth promotion and disease prevention may select for reduced susceptibility in C. gallinacea [5]. Macrolides (e.g., tylosin, tilmicosin) and fluoroquinolones (e.g., enrofloxacin) have been used empirically, but standardized minimum inhibitory concentration (MIC) testing for obligate intracellular bacteria is technically challenging and not routinely performed in veterinary diagnostic laboratories.

An important limitation is that no effective vaccine is currently available for C. gallinacea or any avian chlamydial species. Experimental inactivated vaccines have shown partial protection against C. psittaci but have not been developed for C. gallinacea [4, 2]. Thus, control relies on biosecurity and antimicrobial therapy.

Control and Biosecurity

Given the fecal-oral transmission pathway and the potential for vertical transmission, control measures should be multi-pronged.

Flock Management

• Hygiene: Regular removal of litter and disinfection of housing with quaternary ammonium compounds or bleach solutions effective against Chlamydiaceae. Feeders and drinkers should be positioned to minimize fecal contamination.
• Biosecurity: Separate footwear and clothing for each poultry house, rodent and wild bird control to prevent introduction of chlamydial strains, and quarantine of newly introduced birds.
• Testing and Culling: In high-value breeding flocks, periodic PCR testing of pooled swabs can identify positive birds. Culling of seropositive or PCR-positive birds may be considered in elimination programs.

Egg Sanitation

Because C. gallinacea can penetrate eggshells, eggs intended for hatching should be collected frequently, cleaned of visible soiling, and fumigated with formalin or treated with a disinfectant dip (e.g., hydrogen peroxide or peracetic acid) to reduce bacterial load on the shell surface.

Zoonotic Risk and Occupational Health

Although this article does not focus on human medicine, it is relevant to note that C. gallinacea has been detected in poultry workers, indicating zoonotic transmission potential [1]. Poultry farmers, slaughterhouse workers, and veterinarians should use respiratory protection (N95 masks) and gloves when handling birds from known positive flocks.

Conclusion

Chlamydia gallinacea is an emerging pathogen of poultry that is globally distributed, highly prevalent in backyard and free-range flocks, and transmitted predominantly via fecal-oral and vertical routes. Unlike C. psittaci, it causes minimal clinical disease in infected birds, likely contributing to its underdiagnosis. Molecular diagnostics, particularly pan-Chlamydia PCR with species-specific follow-up, are essential for accurate identification. Control relies on strict biosecurity, antimicrobial therapy with tetracyclines, and egg sanitation. Continued surveillance and genomic characterization of C. gallinacea strains are needed to monitor its evolution, host range expansion, and zoonotic risk. The integration of this emerging agent into routine poultry diagnostic panels will be essential for understanding its true economic and public health impact.

References

[1] Li L, Luther M, Macklin K, et al. Chlamydia gallinacea: a widespread emerging Chlamydia agent with zoonotic potential in backyard poultry. Epidemiology and Infection. 2017. URL: https://www.semanticscholar.org/paper/15ea5f38a4c4c8e5203a4960285f4812c0d3c963

[2] Sachse K, Laroucau K, Vanrompay D. Avian Chlamydiosis. Current Clinical Microbiology Reports. 2015. URL: https://www.semanticscholar.org/paper/9baf8e19552a3981c99d8375d4cab5aa6c0527db

[3] Long J, Zhong H, Deng Y, et al. Prevalence and Genetic Characteristics of Avian Chlamydia in Birds in Guangxi, Southwestern China. Microorganisms. 2025. URL: https://www.semanticscholar.org/paper/fa8d9e0e04b40991129c72f13a8b34907d178596

[4] El-Ghany WA. Avian Chlamydiosis: A World-wide Emerging and Public Health Threat. Journal. 2020. URL: https://www.semanticscholar.org/paper/d28e166fd45faae10af113615f324c1d72eb8bc0

[5] Ravichandran K, Anbazhagan S, Karthik K, et al. A comprehensive review on avian chlamydiosis: a neglected zoonotic disease. Tropical Animal Health and Production. 2021. URL: https://www.semanticscholar.org/paper/b07a3df257c6e9a8db3bc2626897c403ac2c7705

[6] Ospina-Pinto MC, Alves BF, Soares H, et al. Chlamydia gallinacea in Brazilian backyard chicken farms. Brazilian Journal of Microbiology. 2024. URL: https://www.semanticscholar.org/paper/c183341cc0ffba6fef6bb64ba170208ba404b474

[7] Quilicot A, Tomic D, Gottstein Z, et al. Chlamydia gallinacea - main avian chlamydia species detected in free-range indigenous chicken flocks in Bohol, Philippines. Journal. 2017. URL: https://www.semanticscholar.org/paper/983970b0a42c94c7cfb2a62b871a32b9af5c5df7

[8] Donati M, Laroucau K, Guerrini A, et al. Chlamydiosis in Backyard Chickens (Gallus gallus) in Italy. Vector Borne and Zoonotic Diseases. 2018. URL: https://www.semanticscholar.org/paper/33bce9d625b7417d2ce6b4200b2277263cbd2f3b

[9] Čechová L, Halánová M, Babinská I, et al. Chlamydiosis in farmed chickens in Slovakia and zoonotic risk for humans. Annals of Agricultural and Environmental Medicine. 2018. URL: https://www.semanticscholar.org/paper/2d49e9024f72bb34971ec95f988aa7a9dda59339

[10] You J, Wu Y, Zhang X, et al. Efficient fecal-oral and possible vertical, but not respiratory, transmission of emerging Chlamydia gallinacea in broilers. Veterinary Microbiology. 2019. URL: https://www.semanticscholar.org/paper/128463a30d255b2fb1ed122e75240234c7635ca0