Avian Coryza in Poultry: Etiology, Clinical Signs, Diagnosis, and Control

Introduction

Avian coryza, also known as infectious coryza, is an acute upper respiratory tract disease of chickens and occasionally other avian species caused by the bacterium Avibacterium paragallinarum (formerly Haemophilus paragallinarum) [21]. The disease is characterized by serous to purulent nasal discharge, facial edema, conjunctivitis, and a marked drop in egg production in laying flocks [1, 21]. Avian coryza results in significant economic losses due to reduced egg output, culling, increased mortality from secondary infections, and costs associated with treatment and vaccination [2, 31]. It is distributed worldwide, with documented outbreaks across Asia [3, 2, 27], the Americas [1, 19, 25], Europe [28], Africa [22], and the Middle East [4]. A comprehensive understanding of its etiology, epidemiology, clinical presentation, diagnostic approaches, and control measures is critical for veterinary practitioners and poultry health managers.

Etiology

Avibacterium paragallinarum is a Gram-negative, non-motile, pleomorphic, facultatively anaerobic rod that requires nicotinamide adenine dinucleotide (NAD) for growth (V-factor dependence) [21, 24]. The bacterium is catalase-negative, oxidase-positive, and reduces nitrates [21]. Colony morphology on chocolate agar appears as small, dewdrop-like colonies after 24 to 48 hours of incubation in a 5% carbon dioxide-enriched atmosphere [5]. Isolation can be enhanced by selective culture media containing antibiotics such as cloxacillin, bacitracin, and nalidixic acid, which suppress competing flora [5].

Serotyping and Genotyping

Classical serotyping of A. paragallinarum identifies three serovars (A, B, and C) based on hemagglutination inhibition assays using the HMTp210 protein [32, 33]. The hemagglutinin HMTp210 is a major immunogenic antigen, and specific regions of this protein are critical for both hemagglutination activity and serotype specificity [32]. Molecular genotyping based on the HMTp210 gene sequence has been proposed as a reliable alternative to classical serotyping, often revealing greater diversity and the existence of multiple genotypes within a single serovar [33]. For example, Iranian poultry farms harbor novel genotypes that are not captured by traditional serotyping alone [4]. In China, molecular serotyping of circulating strains has demonstrated that serovar C and serovar A are predominant, with some isolates exhibiting novel virulence profiles [3]. Non-pathogenic variants of A. paragallinarum have also been identified in naive, healthy layer flocks in the United States [6, 7]; these isolates are genetically distinct from pathogenic strains and lack key virulence genes, complicating both diagnostics and vaccination strategies [8].

Surface Structures and Virulence Factors

The lipooligosaccharide (LOS) and capsular polysaccharide (CPS) of A. paragallinarum are critical for serotype specificity and host immune evasion. Biosynthetic loci for LOS and CPS have been characterized, and their genetic organization correlates with serovar identity [24]. Biofilm formation is another virulence mechanism; genes involved in biofilm development have been identified through random transposon mutagenesis, and biofilm production enhances bacterial persistence in the respiratory tract [23]. Natural transformation competency has been described in A. paragallinarum, facilitating horizontal gene transfer and acquisition of antibiotic resistance determinants [34]. Outer membrane vesicles (OMVs) also mediate the transfer of antibiotic resistance genes between A. paragallinarum and other Gram-negative bacteria, contributing to the spread of resistance [35].

Epidemiology

Avian coryza is primarily a disease of chickens, though quail and pheasants may also be susceptible [21]. The disease spreads horizontally via direct contact, aerosolized respiratory droplets, and contaminated fomites such as feed, water, and equipment [18, 21]. Recovered birds can act as asymptomatic carriers for prolonged periods, serving as a reservoir for new outbreaks [21].

Prevalence and Risk Factors

A meta-analysis of poultry populations in China covering 1993 to 2024 estimated a pooled prevalence of infectious coryza at approximately 15%, with higher rates in layer flocks than in broilers [2]. In California, a retrospective analysis of outbreaks between 2016 and 2022 identified that multi-age layer farms, poor biosecurity compliance, and proximity to other poultry operations were significant risk factors [19]. A case-control survey in the United States confirmed that farm characteristics such as lack of all-in/all-out management, shared equipment, and visitor access increased the odds of disease occurrence [18]. In Ethiopia, molecular detection rates of A. paragallinarum from suspected cases ranged from 20% to 40% depending on the region, suggesting widespread circulation [22]. Coinfection with other respiratory pathogens such as Ornithobacterium rhinotracheale and Escherichia coli is common and can exacerbate clinical severity [28].

Non-Pathogenic Isolates

The discovery of non-pathogenic A. paragallinarum strains in naive layer flocks across multiple US states challenges our understanding of transmission and immunity [7]. These isolates lack the ability to cause clinical disease despite colonizing the upper respiratory tract [8]. Their presence can interfere with PCR-based diagnostics that do not discriminate between pathogenic and non-pathogenic strains [20].

Clinical Signs

The incubation period for avian coryza is 1 to 3 days following natural exposure [21]. Clinical signs primarily involve the upper respiratory tract and include:

• Serous to mucopurulent nasal discharge
• Sneezing and snicking
• Facial edema, particularly around the infraorbital sinuses
• Conjunctivitis with frothy ocular exudate
• Swelling of the wattles and comb
• Dyspnea and open-mouth breathing in severe cases
• Marked drop in egg production (10% to 40%)
• Reduced feed and water intake
• In laying hens, decreased eggshell quality occasionally noted

Morbidity is high (often approaching 100% in unvaccinated flocks), while mortality is typically low except when secondary infections with E. coli or Avibacterium endocarditis occur [1, 21, 28]. A case study from Alberta, Canada described an outbreak in a table egg layer flock where facial edema and a 20% drop in egg production were the presenting signs [1]. In broiler chickens, coinfection with Ornithobacterium rhinotracheale resulted in more severe respiratory distress and increased mortality [28]. The clinical signs of avian coryza can be indistinguishable from those of fowl cholera (caused by Pasteurella multocida) and mild forms of Avian Influenza and Avian Mycoplasmosis; therefore, laboratory confirmation is essential.

Pathology

Gross lesions are confined to the upper respiratory tract. The nasal passages and infraorbital sinuses contain fibrinous to mucopurulent exudate [21]. The mucosa of the nasal turbinates is congested and edematous. In chronic cases, caseous plugs may be present in the sinuses. Conjunctivitis and subcutaneous edema of the face and wattles are common. Histologically, there is a marked infiltration of heterophils and mononuclear cells in the submucosa, along with hyperemia and desquamation of the respiratory epithelium [21]. Fibrin exudation is prominent in acute stages. In cases of septicemic spread, fibrinous pericarditis and perihepatitis may be observed, often attributable to concurrent E. coli infection [28].

Diagnosis

A definitive diagnosis of avian coryza requires isolation and identification of A. paragallinarum or detection of its nucleic acid in clinical samples (e.g., nasal swabs, sinus exudate).

Bacterial Isolation

Samples are inoculated onto chocolate agar or selective media such as blood agar with a nurse colony (e.g., Staphylococcus aureus) to supply V-factor, or onto commercial selective media [5]. After 24 to 48 hours of incubation at 37 deg C in 5% CO2, small, dewdrop-like colonies appear. Identification is confirmed by Gram stain, colony morphology, NAD requirement, and biochemical tests (catalase-negative, oxidase-positive) [21]. The recently developed selective culture medium significantly improves recovery of A. paragallinarum from field samples, especially when competing microbiota are abundant [5].

Molecular Diagnostics

Polymerase chain reaction (PCR) assays targeting the HMTp210 gene or the 16S rRNA gene are widely used for detection [20, 22]. A key advance is the development of PCR assays that can differentiate pathogenic from non-pathogenic A. paragallinarum isolates, based on the presence or absence of specific genetic markers [20]. This differentiation is critical for interpreting positive PCR results from apparently healthy flocks. A genome-guided multilocus sequence typing (MLST) scheme has been standardized, providing enhanced epidemiological typing resolution compared to conventional approaches [9]. For serotype determination, molecular methods based on HMTp210 sequences are increasingly used [33]. Genotyping reveals high genetic diversity among field isolates [4, 10]. Genomic characterization of isolates from Hubei Province, China, demonstrated distinct sequence types and antimicrobial resistance gene profiles [11].

Antimicrobial Susceptibility Testing

A standardized broth microdilution method for A. paragallinarum has been recommended to enable consistent resistance monitoring across laboratories [29]. This method defines minimum inhibitory concentration (MIC) breakpoints specific to this species and is essential for guiding rational antibiotic therapy.

Serological Tests

Serological assays (e.g., hemagglutination inhibition, ELISA) are available but are generally used for flock-level surveillance rather than individual diagnosis, due to variable sensitivity and the presence of non-pathogenic strains that can induce cross-reactive antibodies [21].

Differential Diagnosis

The clinical signs of avian coryza overlap with several other respiratory diseases of poultry. A table summarizing key differentials is provided below.

Treatment

Antimicrobial therapy remains the primary intervention during acute outbreaks. Commonly used antibiotics include tetracyclines, tylosin, erythromycin, and sulfonamides [21, 27]. However, antimicrobial resistance in A. paragallinarum is increasing. A genomic-based resistance analysis of isolates from Guangdong Province, China, revealed widespread resistance to tetracyclines and sulfonamides, with resistance genes often carried on plasmids or mobile genetic elements [27]. Broth microdilution testing is recommended to guide therapy [29]. In addition to conventional antibiotics, alternative approaches have been explored. Probiotics combined with berry phenolic extracts showed inhibitory effects against A. paragallinarum in vitro [12]. Chinese herbal medicine extracts also exhibited bacteriostatic activity in laboratory studies [13]. These alternatives may reduce reliance on antibiotics and help mitigate resistance development.

Control

Control of avian coryza relies on a combination of biosecurity, management, and vaccination.

Biosecurity and Management

Strict biosecurity protocols are essential: all-in/all-out stocking, cleaning and disinfection of premises, control of visitor access, and provision of dedicated equipment [18, 21]. Quarantine of new birds and isolation of sick flocks reduce transmission. A retrospective analysis in California emphasized that early detection and removal of affected birds, along with enhanced sanitation, reduced the duration of outbreaks [19]. Because recovered birds can remain carriers, complete depopulation and thorough disinfection between flocks are recommended for eradication.

Vaccination

Both inactivated (bacterin) and live attenuated vaccines are available. Bacterins containing serovars A, B, and C are commonly used and provide protection against homologous serovars [21]. Live attenuated vaccines offer the advantage of local mucosal immunity. A candidate live attenuated vaccine developed by Guo et al. (2025) showed protective efficacy against experimental challenge in chickens [14]. Vaccination strategies must account for serovar variation; a study from Argentina evaluated different vaccination plans against a serovar B variant and found that a two-dose regimen with a homologous bacterin provided the best protection [15].

Adjuvants can enhance vaccine immunogenicity. Polymeric nanocarrier-based adjuvants improved the mucosal antibody response when delivered intranasally in a locally produced vaccine [26]. Furthermore, the probiotic Enterococcus faecium has been shown to improve immune responses to infectious coryza vaccine in chickens [16]. The presence of non-pathogenic A. paragallinarum strains in naive flocks may interfere with vaccine efficacy by inducing tolerance or cross-reactive antibodies without protective immunity [8, 7].

Genetic manipulation tools, such as a counterselection system for marker-free mutagenesis, have been developed, enabling the construction of defined mutants for future vaccine development [17]. A comprehensive review by El-Gazzar et al. (2025) summarizes current vaccine approaches and their limitations [21].

Diagnostic and Control Decision Workflow

A schematic representation of the recommended diagnostic and control workflow for avian coryza is presented below.

flowchart TD
    A[Clinical signs: swelling, nasal discharge, egg drop], > B{Immediate?}
    B, >|Yes| C[Collect nasal/sinus swabs]
    B, >|No| D[Monitor flock]
    C, > E[PCR for A. paragallinarum]
    E, > F{Test result}
    F, >|Positive| G[Diferentiate pathogenic vs. nonpathogenic]
    G, >|Pathogenic| H[Isolate on selective media]
    G, >|Nonpathogenic| I[No treatment needed; review biosecurity]
    H, > J[Antimicrobial susceptibility testing]
    J, > K[Select appropriate antibiotic therapy]
    K, > L[Implement treatment: antibiotics +/- probiotics]
    F, >|Negative| M[Consider differentials: fowl cholera, avian influenza, mycoplasmosis]
    M, > N[Perform additional tests: PCR, culture, serology]
    L, > O[Post-treatment monitoring]
    O, > P[Vaccination planning / revaccination]
    P, > Q[Review biosecurity and prevent recurrence]

Conclusion

Avian coryza remains a significant respiratory disease of poultry worldwide, with Avibacterium paragallinarum as its etiologic agent. The emergence of non-pathogenic strains, increasing antimicrobial resistance, and genetic diversity among field isolates complicate diagnosis and control. Advances in molecular diagnostics, including PCR assays that distinguish pathogenic from non-pathogenic forms [20] and standardized MLST schemes [9], provide improved epidemiological tools. Control strategies should integrate strict biosecurity, targeted antimicrobial use guided by susceptibility testing, and vaccination with appropriate serovar coverage. Future research into novel vaccines, alternative therapeutics, and the role of non-pathogenic isolates in flock immunity will further refine management protocols.

References

[1] Gupta A, Girard T, Bowling H, et al. Infectious coryza outbreak in a table egg layer flock in Alberta. Can Vet J. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41716501/

[2] Wang Y, Si M, Qiu J, et al. Prevalence, risk factors, and regional insights of infectious coryza among poultry populations in China during 1993-2024: a meta-analysis. J Vet Med Sci. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41320253/

[3] Wen J, Liao Z, Yin L, et al. Molecular serotyping and virulence assessment of Avibacterium paragallinarum strains circulating in Chinese poultry flocks. Front Vet Sci. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41815498/

[4] Bashashati M, Moradi Haghgou L, Nouri A, et al. Genetic diversity of Avibacterium paragallinarum: uncovering novel genotypes in Iranian poultry farms. Avian Pathol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41178666/

[5] Srednik ME, Shelkamy MMS, Hashish A, et al. Development of selective culture media for efficient isolation of Avibacterium paragallinarum from chickens. J Clin Microbiol. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40679850/

[6] Shelkamy MMS, Hashish A, Srednik ME, et al. Eight complete and four draft genome sequences of nonpathogenic Avibacterium paragallinarum isolates from naive, healthy layer chickens in the USA. Microbiol Resour Announc. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40815000/

[7] Shelkamy MMS, Hashish A, Srednik ME, et al.

[8] Shelkamy MMS, Fay C, Hashish A, et al. Investigating the pathogenicity of novel non-pathogenic Avibacterium paragallinarum isolates and their protective potential against infectious coryza. Avian Pathol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42210853/

[9] Ghanem M, Harris A, Timilsina M, et al. A standardized, genome-guided MLST scheme for Avibacterium paragallinarum: enhanced epidemiological typing and validation against existing methods. J Clin Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41665375/

[10] Chen L, Hu J, Dai N, et al. Genotypic and Biochemical Divergence of Avibacterium paragallinarum Isolates in China. Avian Dis. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40643936/

[11] Guo Y, Yin M, Zhang T, et al. Research Note: Isolation and genomic characterization of Avibacterium paragallinarum from the Hubei Province, China. Poult Sci. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40644909/

[12] Thapa K, Phan A, Lin S, et al. Implication of probiotics with berry phenolic extracts against Avibacterium paragallinarum. Microb Pathog. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42097187/

[13] Yue Z, Li Y, Chen L, et al. Study on bacteriostasis of Chinese herbal medicine extracts to Avibacterium paragallinarum. Poult Sci. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41579597/

[14] Guo M, Wang H, Liu D, et al. Development and evaluation of an attenuated Avibacterium paragallinarum strain as a live vaccine candidate for infectious coryza. Vet Res. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40490807/

[15] Huberman YD, Méndez LL, Méndez AM, et al. Efficacy of Different Vaccination Plans Against Experimental Infection with a Serovar B Variant of Avibacterium paragallinarum from Argentina in Laying Hens. Avian Dis. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41003436/

[16] Cui W, Wang C, Wu Y, et al. Epidemiological Investigation of Infectious Coryza in Central China and the Effect of Enterococcus faecium on Improving Vaccine Immunity. Poult Sci. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40769017/

[17] Sun J, Chen L, He G, et al. Development of a counterselection system for efficient marker-free genetic manipulation in Avibacterium paragallinarum. Appl Environ Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41910236/