Infectious Coryza in Chickens: Drugs, Treatment Protocols, and Differential Diagnosis

Etiology and Pathogenesis

Infectious coryza is an acute respiratory disease of chickens caused by the Gram-negative, non-motile, pleomorphic rod Avibacterium paragallinarum (formerly Haemophilus paragallinarum). The bacterium is a member of the family Pasteurellaceae and requires nicotinamide adenine dinucleotide (NAD, V factor) for in vitro growth, a characteristic that aids in its laboratory identification. The organism colonizes the upper respiratory tract, specifically the nasal passages, sinuses, and conjunctival mucosa. Pathogenesis involves adherence to ciliated epithelial cells via fimbriae and subsequent production of a polysaccharide capsule that confers resistance to phagocytosis. The host inflammatory response, driven by neutrophil infiltration and cytokine release, results in the characteristic serous to mucopurulent nasal discharge and facial edema.

Three serovars (A, B, and C) are recognized based on hemagglutination inhibition assays, with serovar A being the most prevalent in commercial poultry operations. Cross-protection between serovars is incomplete, which complicates vaccination strategies and influences the selection of therapeutic agents.

Clinical Signs and Pathological Findings

The incubation period ranges from 24 to 48 hours following natural exposure. Clinical signs include serous nasal discharge that progresses to mucopurulent exudate, sneezing, conjunctivitis, periorbital and facial edema, and infraorbital sinus distension. In laying hens, a marked drop in egg production (10% to 40%) is observed. Feed and water intake are reduced, leading to weight loss in growing birds. Morbidity is high (up to 100%), but mortality is typically low (less than 5%) unless complicated by secondary infections such as Escherichia coli or Mycoplasma gallisepticum.

Gross pathological lesions are confined to the upper respiratory tract. The nasal passages and infraorbital sinuses contain catarrhal to purulent exudate. The conjunctival sacs may be filled with caseous material. In chronic or complicated cases, airsacculitis and pneumonia may be present due to secondary bacterial invaders.

Differential Diagnosis

Differentiating infectious coryza from other respiratory diseases of poultry is critical for implementing appropriate therapy and control measures. The table below summarizes key differentiating features.

The most critical differentiation is from fowl cholera and highly pathogenic avian influenza (HPAI). Fowl cholera typically presents with acute mortality and septicemic lesions absent in uncomplicated coryza. HPAI is characterized by rapid spread, high mortality, and systemic hemorrhagic lesions. A detailed discussion of these distinctions is provided in the article Infectious Coryza in Poultry and Ducks: Etiology, Clinical Signs in Chickens, Differential Diagnosis from Avian Influenza, and Prevention Strategies. Additional guidance on differentiating from Pasteurella multocida infections is available in Fowl Cholera in Poultry: Pasteurella multocida Pathogenesis, Clinical Signs, Prevention, Control, and WOAH Classification.

Laboratory Diagnosis

Definitive diagnosis relies on isolation and identification of A. paragallinarum. Swabs from the infraorbital sinus or nasal passages of acutely affected birds are inoculated onto chocolate agar or blood agar with a nurse colony of Staphylococcus aureus (satellitism phenomenon). The organism requires NAD for growth and produces small, dewdrop-like colonies after 24 to 48 hours of incubation at 37 degrees Celsius in a 5% to 10% carbon dioxide atmosphere. Biochemical identification includes positive catalase and oxidase reactions, nitrate reduction, and fermentation of glucose without gas production.

Molecular detection using polymerase chain reaction (PCR) targeting the HMTp210 gene is highly sensitive and specific. PCR can differentiate serovars and is particularly useful for detecting the organism in carrier birds or when culture is negative due to prior antibiotic therapy. Serological tests, including the hemagglutination inhibition test and commercial ELISA kits, are available for flock-level surveillance but are less reliable for individual diagnosis.

Antimicrobial Therapy

Treatment of infectious coryza is primarily aimed at reducing clinical signs, limiting the drop in egg production, and preventing secondary bacterial infections. The selection of antimicrobial agents should be guided by in vitro susceptibility testing due to the emergence of resistance. The table below lists commonly used drugs, their mechanisms of action, and typical dosages for water or feed administration.

Treatment Protocols

The following protocol is recommended for acute outbreaks in commercial flocks.

1. Immediate Isolation. Remove clinically affected birds to a separate facility if feasible. This reduces the infectious load and allows for targeted therapy.
2. Water Medication. Administer a potentiated sulfonamide (trimethoprim-sulfadiazine at 30 mg/kg combined dose) via drinking water for 5 to 7 consecutive days. Ensure adequate water intake by withholding water for 1 to 2 hours before medication.
3. Supportive Care. Provide supplemental electrolytes and vitamins A and C to support mucosal integrity and immune function. Increase ambient temperature by 2 to 3 degrees Celsius to reduce metabolic stress.
4. Secondary Infection Control. If airsacculitis or pneumonia is suspected, add a macrolide (tylosin at 25 mg/kg) to the water medication regimen for the first 3 days.
5. Reassessment. If clinical signs do not improve within 48 hours, perform bacterial culture and susceptibility testing on samples from untreated birds. Adjust therapy based on the antibiogram.

For flocks with recurrent outbreaks, a strategic approach involving vaccination (bacterins containing serovars A, B, and C) combined with periodic medication during high-risk periods (e.g., cold weather, introduction of new stock) is recommended. Detailed prevention strategies are covered in Infectious Coryza in Chickens and Quail: Avibacterium paragallinarum Etiology, Clinical Signs, Treatment, and Prevention.

Antimicrobial Resistance Considerations

Resistance to sulfonamides and tetracyclines has been documented in A. paragallinarum isolates from multiple geographic regions. Mechanisms include plasmid-mediated acquisition of resistance genes (e.g., sul1, sul2, tetA, tetB) and chromosomal mutations. Fluoroquinolone resistance, mediated by mutations in the gyrA and parC genes, is emerging in areas where enrofloxacin is used extensively. Therefore, routine susceptibility testing is essential for guiding therapy and preserving the efficacy of available drugs.

Decision Tree for Diagnosis and Treatment

The following Mermaid diagram outlines a clinical decision tree for managing suspected infectious coryza outbreaks.

flowchart TD
    A[Respiratory signs in flock], > B{Nasal discharge, facial edema?}
    B, >|Yes| C[Collect sinus swabs for culture/PCR]
    B, >|No| D[Consider other respiratory diseases]
    C, > E{Acute mortality >5%?}
    E, >|Yes| F[Rule out fowl cholera, HPAI]
    E, >|No| G[Presumptive diagnosis: Infectious Coryza]
    F, > H[Perform necropsy, bacterial culture, virus isolation]
    G, > I[Initiate water medication: potentiated sulfonamide]
    I, > J{Response within 48 hours?}
    J, >|Yes| K[Complete 5-7 day course]
    J, >|No| L[Perform susceptibility testing]
    L, > M[Adjust antibiotic based on antibiogram]
    M, > N[Consider adding macrolide for Mycoplasma]
    K, > O[Monitor for recurrence; implement biosecurity]
    N, > O

Prognosis and Flock Recovery

With appropriate antimicrobial therapy, clinical signs typically resolve within 3 to 5 days. Egg production returns to pre-infection levels within 2 to 3 weeks, although full recovery may be delayed in older hens. Carrier birds can shed the organism intermittently for months after clinical recovery, serving as a reservoir for future outbreaks. Therefore, depopulation of recovered flocks followed by thorough cleaning and disinfection is recommended for eradication. The organism is susceptible to common disinfectants, including quaternary ammonium compounds, chlorhexidine, and sodium hypochlorite.

Conclusion

Infectious coryza remains an economically significant respiratory disease of chickens worldwide. Effective management requires rapid and accurate differential diagnosis, particularly from fowl cholera and avian influenza, followed by targeted antimicrobial therapy guided by susceptibility testing. The emergence of antimicrobial resistance underscores the need for judicious drug use and the integration of vaccination and biosecurity measures into comprehensive control programs.

References

1. Blackall, P. J., & Matsumoto, M. (2003). Infectious coryza. In Y. M. Saif, H. J. Barnes, J. R. Glisson, A. M. Fadly, L. R. McDougald, & D. E. Swayne (Eds.), Diseases of Poultry (11th ed., pp. 691-703). Iowa State Press.
2. Bragg, R. R. (2002). Virulence of South African isolates of Haemophilus paragallinarum. Part 1: NAD-dependent growth characteristics. Onderstepoort Journal of Veterinary Research, 69(2), 163-169.
3. Soriano, V. E., & Terzolo, H. R. (2004). Infectious coryza: A review of the disease and its causative agent, Avibacterium paragallinarum. Veterinary Research Communications, 28(6), 491-512.
4. World Organisation for Animal Health (WOAH). (2021). Infectious coryza. In Manual of Diagnostic Tests and Vaccines for Terrestrial Animals (Chapter 3.3.2). WOAH.