Avian Coryza (Infectious Coryza) in Poultry: Clinical Signs and Control

Introduction

Avian coryza, also referred to as infectious coryza, is an acute upper respiratory tract disease of chickens and other avian species caused by the bacterium Avibacterium paragallinarum [1]. The disease is characterized by serous to mucoid nasal discharge, facial edema, conjunctivitis, and a marked drop in egg production in laying flocks [2, 3]. Although mortality is generally low, morbidity can reach 100% in susceptible populations, leading to significant economic losses due to reduced growth rates, decreased egg yield, and increased culling [4, 5]. The term "avian coryn" is occasionally encountered in lay literature as a misspelling of avian coryza, but the correct nomenclature remains infectious coryza caused by A. paragallinarum [1]. This article provides a detailed, evidence-based review of the etiology, epidemiology, clinical signs, pathology, diagnostics, treatment, and control of infectious coryza, with emphasis on recent molecular and epidemiological findings.

Etiology

The causative agent, Avibacterium paragallinarum, is a Gram-negative, non-motile, pleomorphic coccobacillus belonging to the family Pasteurellaceae [1]. The bacterium requires nicotinamide adenine dinucleotide (NAD) for growth, a characteristic that distinguishes it from other avian pasteurellas [6]. Three classical serovars (A, B, and C) have been defined based on hemagglutination inhibition (HI) assays using the HMTp210 protein [7]. However, molecular serotyping based on the HMTp210 gene has revealed greater genetic diversity, with novel genotypes identified in various geographic regions [8, 9, 10]. For example, isolates from Iran and China have been classified into distinct sequence types that do not always correlate with classical serovars [9, 11]. Nonpathogenic variants of A. paragallinarum have also been isolated from healthy flocks, complicating diagnostic interpretation [12, 13, 14]. These nonpathogenic strains lack key virulence determinants, such as the capsular polysaccharide and lipooligosaccharide biosynthetic loci [15]. The complete genome sequences of several pathogenic and nonpathogenic isolates are now available, facilitating comparative genomics and the development of refined typing schemes [13, 16, 17].

Epidemiology

Infectious coryza occurs worldwide, with prevalence varying by region, management system, and biosecurity practices [4, 18]. A meta-analysis of Chinese poultry populations from 1993 to 2024 reported an overall prevalence of 12.3%, with higher rates in layer flocks compared to broilers [4]. In the United States, outbreaks have been documented in California (2016–2022) and Pennsylvania, often associated with multi-age layer operations and backyard flocks [18, 3]. A case-control survey in the northeastern U.S. identified farm characteristics such as lack of all-in/all-out management, presence of other poultry species, and inadequate visitor biosecurity as significant risk factors [19]. In Canada, an outbreak in a table egg layer flock in Alberta was linked to the introduction of replacement pullets from an infected source [2]. In Ethiopia, molecular detection confirmed A. paragallinarum in 18.5% of suspected cases, with serovar C being predominant [20]. Coinfections with Ornithobacterium rhinotracheale and other respiratory pathogens are common and can exacerbate clinical severity [21]. The bacterium is transmitted horizontally via direct contact, aerosolized droplets, and contaminated fomites; vertical transmission has not been demonstrated [1]. Carrier birds, including those infected with nonpathogenic strains, may serve as reservoirs [14].

Clinical Signs

The incubation period ranges from 24 to 72 hours following experimental infection [22, 23]. The hallmark clinical signs include serous to mucoid nasal discharge, sneezing, facial edema (particularly periorbital and infraorbital sinuses), and conjunctivitis [2, 1]. In laying hens, a rapid drop in egg production of 10% to 40% is common, often accompanied by an increase in shell-less and misshapen eggs [2, 3]. Affected birds may exhibit depression, anorexia, and reluctance to move [5]. In broilers, the disease is less severe but can lead to reduced weight gain and increased condemnation at slaughter due to airsacculitis [21]. The severity of clinical signs is influenced by the virulence of the infecting strain, host immune status, and concurrent infections [23]. Highly virulent isolates, such as those characterized by Mei et al., produce more pronounced facial swelling and higher morbidity [23]. Coinfection with Mycoplasma gallisepticum or infectious bronchitis virus can result in a more severe respiratory syndrome [1]. In small chicken flocks, the consequences of infectious coryza are often grave, with prolonged morbidity and high culling rates [5].

Pathology

Gross pathological findings are primarily confined to the upper respiratory tract. The nasal passages and infraorbital sinuses contain copious amounts of catarrhal to purulent exudate [1]. The conjunctivae are hyperemic and edematous. In chronic cases, caseous plugs may form within the sinuses [3]. Histologically, there is acute rhinitis and sinusitis with infiltration of heterophils and mononuclear cells, hyperplasia of the mucosal epithelium, and desquamation of ciliated cells [1]. In severe cases, the inflammation may extend to the trachea and lungs, but pneumonia is rare unless secondary pathogens are involved [21]. Endocarditis caused by A. paragallinarum has been reported in broiler breeding hens, indicating the potential for systemic dissemination under certain conditions [21]. Nonpathogenic strains do not induce significant lesions in experimental models [12].

Diagnostics

Accurate diagnosis of infectious coryza requires isolation and identification of A. paragallinarum from clinical specimens, typically nasal swabs or sinus exudate [6]. Selective culture media, such as blood agar supplemented with NAD and antibiotics, have been developed to improve isolation rates from contaminated samples [6]. The bacterium is fastidious and grows as small, dewdrop-like colonies after 24–48 hours of incubation in a 5% CO2 atmosphere [1]. Biochemical identification can be performed using catalase, oxidase, and sugar fermentation tests, but these may be ambiguous [11].

Molecular diagnostics have largely supplanted traditional methods for confirmation and typing. PCR assays targeting the HMTp210 gene can differentiate pathogenic from nonpathogenic strains, a critical capability given the prevalence of nonpathogenic isolates in healthy flocks [24]. A standardized, genome-guided multilocus sequence typing (MLST) scheme has been developed for enhanced epidemiological typing [25]. This scheme provides higher resolution than classical serotyping and can identify novel genotypes [25, 9]. Real-time PCR and loop-mediated isothermal amplification (LAMP) assays are also available but are not yet standardized across laboratories [24]. Serological tests, such as the hemagglutination inhibition (HI) test, are useful for flock-level surveillance but have limited sensitivity for detecting early infections [7].

The following Mermaid diagram outlines a recommended diagnostic workflow for suspected infectious coryza cases.

flowchart TD
    A[Clinical suspicion: nasal discharge, facial edema, egg drop], > B[Collect nasal swabs or sinus exudate]
    B, > C[Selective culture on NAD-supplemented blood agar]
    C, > D{Growth of dewdrop colonies?}
    D, Yes, > E[Gram stain: Gram-negative coccobacilli]
    E, > F[Biochemical tests: catalase+, oxidase+]
    F, > G[PCR targeting HMTp210 gene]
    G, > H{Pathogenic or nonpathogenic?}
    H, Pathogenic, > I[Confirm as Avibacterium paragallinarum]
    H, Nonpathogenic, > J[Consider carrier state; assess flock history]
    D, No, > K[PCR directly from swab]
    K, > L{Positive?}
    L, Yes, > I
    L, No, > M[Consider other respiratory pathogens]
    I, > N[MLST for epidemiological typing]
    N, > O[Report and implement control measures]

Treatment

Antimicrobial therapy is commonly employed to reduce clinical signs and limit economic losses, but it does not eliminate the carrier state [1]. The bacterium is susceptible to several antibiotics, including tetracyclines, sulfonamides, tylosin, and fluoroquinolones, but resistance has been reported [26, 27]. A standardized broth microdilution method for antimicrobial susceptibility testing of A. paragallinarum has been recommended to enable consistent resistance monitoring [27]. In China, genomic analysis of isolates from Guangdong Province revealed resistance genes against aminoglycosides, beta-lactams, and tetracyclines, often carried on mobile genetic elements [26]. Horizontal transfer of antibiotic resistance genes via outer membrane vesicles has been demonstrated in vitro, raising concerns about the spread of multidrug resistance [17]. Alternative therapeutic approaches, such as probiotics combined with berry phenolic extracts, have shown in vitro inhibitory activity against A. paragallinarum [28]. Chinese herbal medicine extracts also exhibit bacteriostatic effects in vitro, though in vivo efficacy remains to be validated [29]. Treatment should be guided by susceptibility testing whenever possible, and withdrawal periods must be observed for poultry products destined for human consumption [27].

Control

Control of infectious coryza relies on a combination of biosecurity, vaccination, and management practices. Biosecurity measures include all-in/all-out flock management, cleaning and disinfection of facilities, control of visitor access, and isolation of newly introduced birds [19, 1]. A retrospective analysis of the California outbreak (2016–2022) emphasized the importance of early detection and depopulation of affected flocks to prevent spread [18]. Vaccination is widely used in endemic areas. Both inactivated (bacterin) and live attenuated vaccines are available [22, 30]. Inactivated vaccines are typically administered to pullets before the onset of lay and provide serovar-specific protection [22]. However, emerging variant strains, such as a serovar B variant from Argentina, may not be fully covered by commercial bacterins [22]. Live attenuated vaccines, developed through marker-free genetic manipulation or natural transformation, offer the advantage of mucosal immunity and easier administration via drinking water [31, 30, 32]. Polymeric nanocarrier-based adjuvants have been shown to enhance the mucosal immune response to locally produced coryza vaccines [33]. Probiotics such as Enterococcus faecium have been reported to improve vaccine immunity in field trials [34]. The presence of nonpathogenic A. paragallinarum strains in naive flocks may interfere with serological monitoring but does not appear to compromise vaccine efficacy [12, 14]. A comprehensive control program should also address concurrent respiratory infections, as coinfections with Ornithobacterium rhinotracheale or mycoplasmas can exacerbate disease [21]. For small chicken flocks, educational outreach on biosecurity and early recognition of clinical signs is critical to reduce the grave consequences of outbreaks [5].

The following table summarizes the key characteristics of the three classical serovars and their geographic distribution based on recent studies.

Conclusion

Infectious coryza remains a significant respiratory disease of poultry, with substantial economic impact on layer and breeder flocks worldwide. Advances in molecular diagnostics, including PCR assays that differentiate pathogenic from nonpathogenic strains and genome-guided MLST, have improved our ability to detect and track outbreaks [25, 24]. The identification of nonpathogenic A. paragallinarum in healthy flocks underscores the need for careful interpretation of diagnostic results [12, 14]. Antimicrobial resistance is an emerging concern, and standardized susceptibility testing is essential for rational therapy [27]. Vaccination, combined with rigorous biosecurity, remains the cornerstone of control, but vaccine formulations must be updated to cover circulating variant serovars [22]. Future research should focus on the mechanisms of biofilm formation, natural transformation, and horizontal gene transfer to better understand the evolution and persistence of this pathogen [35, 32, 17].

References

[1] El-Gazzar M, Gallardo R, Bragg R, et al. Avibacterium paragallinarum, the Causative Agent of Infectious Coryza: A Comprehensive Review. Avian Dis. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40249575/

[2] 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/

[3] Davison S, Tracy L, Kelly DJ, et al. Infectious Coryza in Pennsylvania. Avian Dis. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/39400211/

[4] 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/

[5] Etterlin PE, Comin A, Eriksson H, et al. Questionnaire study suggests grave consequences of infectious laryngotracheitis, infectious coryza and mycoplasmosis in small chicken flocks. Acta Vet Scand. 2023. URL: https://pubmed.ncbi.nlm.nih.gov/37710285/

[6] 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/

[7] Li KP, Tan DH, Ou SJ, et al. Regions Important for Hemagglutination Activity and Serotypes of Avibacterium paragallinarum HMTp210 Protein. Avian Dis. 2023. URL: https://pubmed.ncbi.nlm.nih.gov/37556294/

[8] 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/

[9] 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/

[10] Buter R, Feberwee A, de Wit S, et al. Molecular characterization of the HMTp210 gene of Avibacterium paragallinarum and the proposition of a new genotyping method as alternative for classical serotyping. Avian Pathol. 2023. URL: https://pubmed.ncbi.nlm.nih.gov/37470411/

[11] 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/

[12] 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/

[13] 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/

[14] Shelkamy MMS, Hashish A, Srednik ME, et al. Prevalence of Nonpathogenic Avibacterium paragallinarum in Naïve-Healthy Layer Flocks Across Multiple States in the United States. Transbound Emerg Dis. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40420864/

[15] Chen L, Sun J, Hu J, et al. Identification and characterization of biosynthetic loci of lipooligosaccharide and capsular polysaccharide in Avibacterium paragallinarum. Vet Microbiol. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/39612782/

[16] 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/

[17] Xu J, Mei C, Zhi Y, et al. Comparative Genomics Analysis and Outer Membrane Vesicle-Mediated Horizontal Antibiotic-Resistance Gene Transfer in Avibacterium paragallinarum. Microbiol Spectr. 2022. URL: https://pubmed.ncbi.nlm.nih.gov/36000914/ *** Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.

[18] Nguyen V, Stoute S, Ramsubeik S, et al. A Retrospective Analysis to Identify Epidemiologic Patterns of the Infectious Coryza Outbreak in California 2016-22. Avian Dis. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40249577/

[19] Pierdon M, Prown R, Davison S, et al. A Case Control Survey of Farm Characteristics, Biosecurity Measures, and Risk Events in Flocks With and Without Infectious Coryza. Avian Dis. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40249578/

[20] Deresse G, Assefa E, Tesfaw L, et al. Isolation, molecular detection, and sequence analysis of Avibacterium paragallinarum from suspected cases of infectious coryza infected chickens from different areas of Ethiopia, 2022-2024. BMC Microbiol. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40087555/

[21] Stępień-Pyśniak D, Dec M, Hauschild T, et al. Case reports involving coinfection with Avibacterium paragallinarum and Ornithobacterium rhinotracheale in broiler chickens and Avibacterium endocarditis in broiler breeding hens in Poland. Avian Pathol. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/38385975/

[22] 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/

[23] Mei C, Zhi Y, Xu J, et al. Characterization of a highly virulent Avibacterium paragallinarum isolate. J Anim Sci. 2023. URL: https://pubmed.ncbi.nlm.nih.gov/37882211/

[24] Shelkamy MMS, Hashish A, Chaves M, et al. Development and Validation of PCR Assays for Improved Diagnosis of Infectious Coryza by Differentiating Pathogenic and Nonpathogenic Avibacterium paragallinarum. Avian Dis. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40249576/

[25] 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/

[26] Cao X, Huang X, Lin Y, et al. Prevalence and genomic-based antimicrobial resistance analysis of Avibacterium paragallinarum isolates in Guangdong Province, China. Poult Sci. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/38652951/

[27] Gütgemann F, Heuvelink A, Müller A, et al. Recommendation of a standardized broth microdilution method for antimicrobial susceptibility testing of Avibacterium paragallinarum and resistance monitoring. J Clin Microbiol. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/38363142/

[28] 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/

[29] 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/

[30] 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/

[31] 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/

[32] Liu D, Zhang H, Tan H, et al. Basic Characterization of Natural Transformation in Avibacterium paragallinarum. Microbiol Spectr. 2023. URL: https://pubmed.ncbi.nlm.nih.gov/37212663/

[33] Ibrahim HM, Mohammed GM, Sayed RH, et al. Polymeric nanocarrier-based adjuvants to enhance a locally produced mucosal coryza vaccine in chicken. Sci Rep. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/38961116/

[34] 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/

[35] Guo M, Wang H, Zhang H, et al. Identification of the genes involved in biofilm formation of Avibacterium paragallinarum using random transposon mutagenesis. Vet Microbiol. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/39892023/