Infectious Coryza in Poultry: Clinical Signs and Post-Mortem Lesions

Etiology

Infectious coryza is an acute respiratory disease of poultry caused by the bacterium Avibacterium paragallinarum (formerly Haemophilus paragallinarum), a Gram-negative, non-motile, pleomorphic coccobacillus in the family Pasteurellaceae [1, 2]. The organism requires nicotinamide adenine dinucleotide (NAD, factor V) for in vitro growth but does not require hemin (factor X), a characteristic that differentiates it from other avian Haemophilus-like organisms [1]. Three serovars (A, B, and C) are recognized based on hemagglutination inhibition tests, with serovar A and C strains generally being more pathogenic than serovar B isolates [2]. The bacterium produces a heat-labile capsule and several virulence factors, including a polysaccharide capsule that antiphagocytic activity and a neuraminidase that facilitates epithelial colonization [1].

Epidemiology

Infectious coryza occurs worldwide in chickens, pheasants, and guinea fowl, with occasional reports in turkeys and ducks [1, 2]. The disease is most prevalent in layer flocks and smallholder backyard operations, where multi-age groups and high stocking densities facilitate transmission [1]. The bacterium is transmitted horizontally via direct contact, aerosolized respiratory secretions, and contaminated feed, water, or fomites [2]. Carrier birds can harbor A. paragallinarum in the nasal sinuses and upper respiratory tract for months after clinical recovery, serving as a persistent reservoir within a flock [1]. Vertical transmission has not been documented [2]. Morbidity rates are high (often exceeding 80%) in naive populations, while mortality is usually low (1-10%) unless complicated by concurrent infections such as Mycoplasma gallisepticum or Escherichia coli [1, 2]. The incubation period ranges from 1 to 3 days [2].

Clinical Signs

The hallmark of infectious coryza is the rapid onset of severe upper respiratory signs [1]. Early indicators include serous nasal discharge, ocular discharge (serous to mucoid), and facial edema involving the infraorbital sinuses and periorbital tissues [2]. As the disease progresses, the nasal exudate becomes mucopurulent and may crust over the nares, leading to dyspnea with open-mouth breathing and audible snicking or rales [1]. Affected birds often exhibit conjunctivitis, photophobia, and swelling of the wattles and comb (avian coryn) [2]. Bilateral involvement is common and results in a characteristic "puffy face" appearance [1].

Systemic signs are milder than those seen in virulent respiratory viral infections [2]. Birds remain alert but may show reduced feed and water intake due to impaired vision and discomfort, leading to a transient drop in egg production (typically 10-40%) in layer flocks [1]. In young birds, the disease can cause stunting and unthriftiness [2]. Mortality is generally low unless secondary infections (e.g., colibacillosis, mycoplasmosis) supervene, in which case case-fatality rates can rise to 20% or more [1]. The clinical course lasts 2-3 weeks in uncomplicated cases, but chronic carriers may shed the organism intermittently [2].

Differential Diagnosis

Several other respiratory pathogens produce clinical signs that overlap with infectious coryza [1]. A table summarizing key differentiating features is provided below.

For a more extensive differential diagnosis, refer to the article on Infectious Coryza in Poultry and Ducks: Etiology, Clinical Signs in Chickens, Differential Diagnosis from Avian Influenza, and Prevention Strategies.

Infectious Coryza Post Mortem Lesions

Necropsy findings in infectious coryza are confined primarily to the upper respiratory tract and associated lymphoid tissues [1]. The characteristic infectious coryza post mortem lesions include the following:

• Nasal cavity and infraorbital sinuses: The mucosa is hyperemic, edematous, and covered by a thick, mucopurulent to caseous exudate [2]. In chronic or severe cases, the sinuses may be distended with inspissated, yellow-green pus that can occupy the entire lumen [1].
• Facial and periorbital tissues: Subcutaneous edema of the comb, wattles, and infraorbital area is common. The edema fluid is serous to serofibrinous and may contain fibrin clots [2].
• Conjunctiva: The conjunctival sacs often contain purulent exudate; the nictitating membrane may be swollen and hyperemic [1].
• Trachea: The lumen may contain a small amount of mucoid exudate, but the mucosa is usually only mildly congested [2]. Severe tracheitis is not typical unless a secondary pathogen is present [1].
• Lungs and air sacs: In uncomplicated infectious coryza, the lungs are grossly normal, with no consolidation or airsacculitis [2]. Extension of inflammation into the lower respiratory tract strongly suggests a mixed infection (e.g., M. gallisepticum, E. coli) [1].
• Other organs: The liver, spleen, kidneys, and gastrointestinal tract lack significant gross lesions [2]. Petechial hemorrhages, necrotic foci, or fibrinous serositis are absent, helping to differentiate infectious coryza from fowl cholera or colibacillosis [1].

Histopathologically, the nasal mucosa exhibits acute suppurative inflammation with extensive infiltration of heterophils and macrophages [1]. The sinus epithelium undergoes hyperplasia and squamous metaplasia in chronic cases [2]. Edema of the submucosa and desquamation of epithelial cells are prominent [1]. In the presence of secondary infections, fibrinopurulent airsacculitis, pneumonia, or pericarditis may be observed [2].

For further comparison, see the article on Fowl Cholera in Chickens: Clinical Signs, Postmortem Lesions, Treatment, Vaccination, and Prevention.

Diagnostic Approaches

A tentative diagnosis is based on characteristic clinical signs and gross lesions, but definitive diagnosis requires laboratory confirmation [1, 2]. The following diagnostic methods are used:

1. Bacterial culture: Swabs of the infraorbital sinus or nasal exudate are streaked on chocolate agar or blood agar cross-streaked with a Staphylococcus nurse colony (for NAD supply) and incubated in 5-10% CO2 at 37°C for 24-48 hours [2]. A. paragallinarum colonies are small, dewdrop-like, and require NAD for growth; a satellite phenomenon is observed on blood agar [1].
2. Identification: Presumptive identification is based on Gram stain (Gram-negative pleomorphic rods), oxidase positivity, and catalase negativity [2]. Confirmation is achieved by species-specific PCR targeting the 16S rRNA gene or the haemagglutinin gene [1].
3. Serotyping: Hemagglutination inhibition (HI) tests using serovar-specific antisera are performed to determine serovar A, B, or C, which is important for vaccine selection [2].
4. Serology: Serum plate agglutination and ELISA tests are available for flock screening, though cross-reactions with other Pasteurellaceae can occur [1].
5. Pathology: Histological examination of sinus mucosa confirms suppurative inflammation. Immunohistochemistry can detect bacterial antigen in formalin-fixed tissues [2].

A diagnostic decision tree is presented in the Mermaid diagram below.

flowchart TD
    A[Clinical signs: facial edema, nasal discharge, ocular discharge], > B{Post-mortem exam}
    B, > C[Sinus exudate + facial subcutaneous edema]
    C, > D[Sample collection: sinus swab / exudate]
    D, > E{Culture on chocolate agar + NAD dependency}
    E, > F[Growth with satellite phenomenon?]
    F, >|Yes| G[Gram-negative pleomorphic rods]
    G, > H[Oxidase positive, catalase negative]
    H, > I[PCR for A. paragallinarum]
    I, > J[Serotyping by HI]
    F, >|No| K[Consider other pathogens: Mycoplasma, Pasteurella, viruses]
    J, > L[Report: Avibacterium paragallinarum serovar X]

Treatment

Antimicrobial therapy reduces clinical severity and duration but does not eliminate carriage [1]. Treatment decisions should be guided by antibiotic susceptibility testing because resistance to sulfonamides, tetracyclines, and erythromycin has been reported [2]. Commonly used antimicrobials include tylosin (10-20 mg/kg IM or via drinking water at 0.5 g/L for 3-5 days), oxytetracycline (water-soluble formulation at 10-20 mg/kg for 5 days), and sulfadimethoxine combined with trimethoprim (30 mg/kg per day for 5-7 days) [1]. Florfenicol and enrofloxacin have shown efficacy in vitro but regulation varies by jurisdiction [2]. Treatment of individual birds in small flocks can be achieved by injectable tylosin or oral dosing; flock-wide water or feed medication is standard in commercial operations [1]. Relapses are common if treatment is stopped prematurely [2].

Control and Prevention

Control of infectious coryza relies on biosecurity and vaccination [1]. Essential measures include:

• Biosecurity: All-in/all-out management, isolation of new birds for 4 weeks, prevention of fomite transmission, and control of wild bird access [2].
• Vaccination: Commercial bacterins (inactivated whole-cell vaccines) containing serovars A, B, and C are available. Vaccination of pullets at 10-14 weeks of age with a second booster at 18 weeks provides protection for the laying period [1]. Autogenous vaccines may be used for endemic serovars [2].
• Eradication: In breeding flocks, test-and-remove programs using sinus culture and serology can eliminate the pathogen [1]. Depopulation of infected flocks followed by thorough cleaning and disinfection (quaternary ammonium compounds, 2% sodium hypochlorite) is effective for small premises [2].

The article Infectious Coryza in the UK: Epidemiology, Clinical Signs, and Control provides additional regional details.

Conclusion

Infectious coryza remains a significant economic concern for poultry producers worldwide, particularly in layer flocks and multi-age operations. Accurate recognition of infectious coryza post mortem lesions coupled with swift laboratory confirmation enables appropriate antimicrobial therapy and implementation of vaccination protocols. Differentiation from other respiratory diseases, including fowl cholera, mycoplasmosis, and viral infections, is essential for effective management and biosecurity. The condition typifies the need for integrated diagnostic and preventive strategies in modern poultry medicine.

References

[1] Saif, Y.M., Fadly, A.M., Glisson, J.R., McDougald, L.R., Nolan, L.K., and Swayne, D.E. (eds). Diseases of Poultry, 13th Edition. Wiley-Blackwell, Ames, IA.

[2] Merck & Co., Inc. The Merck Veterinary Manual, 11th Edition. Kenilworth, NJ.

[3] World Organisation for Animal Health (WOAH). Manual of Diagnostic Tests and Vaccines for Terrestrial Animals. Chapter 3.3.4: Infectious Coryza. *** 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.