What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Infectious Coryza in the UK: Epidemiology, Clinical Signs, and Control

Introduction

Infectious coryza is an acute respiratory disease of chickens, turkeys, and occasionally other avian species, caused by the bacterium Avibacterium paragallinarum (formerly Haemophilus paragallinarum) [1, 2]. The disease is characterised by catarrhal inflammation of the upper respiratory tract, with prominent facial swelling and ocular discharge [1, 3]. In the United Kingdom, infectious coryza has been recognised as a sporadic but economically significant condition in commercial layer flocks, breeder flocks, and smallholder poultry [2, 4]. Despite advances in biosecurity and vaccination, the pathogen remains endemic in some regions, posing diagnostic and control challenges for veterinary practitioners [3, 5].

This article provides a detailed, evidence-based review of the epidemiology, clinical presentation, diagnostic approaches, treatment options, and control measures for infectious coryza in UK poultry, with particular emphasis on ocular involvement and flock-level management.

Etiology

Avibacterium paragallinarum is a Gram-negative, non-motile, pleomorphic coccobacillus belonging to the family Pasteurellaceae [1, 6]. The bacterium is fastidious, requiring nicotinamide adenine dinucleotide (NAD, V factor) for in vitro growth, and is typically cultured on chocolate agar or supplemented blood agar under microaerophilic conditions (5-10% CO₂) [1, 3]. Based on haemagglutinin serotyping, three serovars (A, B, and C) are recognised internationally, with serovars A and C being the most prevalent in European poultry flocks [2, 6]. Serovar B strains are less common but have been associated with more severe clinical outbreaks in some regions [1, 4].

The bacterium possesses multiple virulence factors, including a polysaccharide capsule that inhibits phagocytosis, fimbriae for adhesion to respiratory epithelium, and a lipopolysaccharide component that triggers a host inflammatory response [3, 6]. The capsule and haemagglutinin antigens are targets for protective immunity and form the basis of current vaccine formulations [1, 5].

Epidemiology in the United Kingdom

Infectious coryza is distributed worldwide, with higher prevalence in regions of intensive poultry production [2, 4]. In the UK, the disease is most frequently reported in multi-age commercial layer flocks and breeder rearing units, where management systems allow for prolonged pathogen persistence [3, 5]. Sporadic outbreaks also occur in free-range and organic flocks, often following the introduction of replacement birds or contact with infected fomites [2, 6].

The national incidence of infectious coryza in UK poultry is not systematically monitored, as it is not a notifiable disease under current UK legislation [4, 7]. However, diagnostic laboratory data from the Animal and Plant Health Agency (APHA) and private veterinary laboratories indicate that cases are submitted year-round, with a slight increase during autumn and winter months, likely related to higher stocking densities and reduced ventilation in housed flocks [3, 5]. A 2018 survey of commercial layer farms in England reported a seroprevalence of approximately 12% for antibodies to A. paragallinarum serovar C, suggesting subclinical circulation in some populations [2, 6].

Risk factors for infectious coryza in UK flocks include poor biosecurity, inadequate all-in/all-out practices, stress from co-morbid infections (e.g. Mycoplasma gallisepticum, infectious bronchitis virus), and the presence of chronic carrier birds [1, 4]. Carrier birds, which may harbour the bacterium in the infraorbital sinuses or nasal cavities for months after clinical recovery, serve as a reservoir for new outbreaks [3, 7].

Further background on the broader context of this pathogen can be found in the articles Infectious Coryza in Chickens and Quail: Avibacterium paragallinarum Etiology, Clinical Signs, Treatment, and Prevention and Infectious Coryza in Poultry and Ducks: Etiology, Clinical Signs in Chickens, Differential Diagnosis from Avian Influenza, and Prevention Strategies.

Transmission

Transmission of A. paragallinarum occurs primarily through direct contact between infected and susceptible birds via aerosolised respiratory droplets or contaminated drinking water [1, 3]. The bacterium does not survive for extended periods outside the host; under typical environmental conditions (20-25°C, moderate humidity), viability on fomites is lost within 24-48 hours [2, 6]. However, in moist organic material such as faecal-contaminated litter or residual feed, survival may be prolonged for several days [4, 7].

Indirect transmission via contaminated equipment, footwear, and clothing is a recognised route in multi-site production systems [3, 5]. Airborne transmission over distances greater than 5 metres is considered unlikely, but lateral spread within a house can be rapid once clinical signs appear [1, 2]. The incubation period ranges from 1 to 3 days under experimental conditions, but may extend to 7 days in field situations depending on the infectious dose and host immune status [4, 6].

Clinical Signs

General Presentation

The hallmark clinical signs of infectious coryza are acute onset of serous to mucoid nasal discharge, facial oedema (particularly of the infraorbital sinuses and periorbital region), and conjunctivitis [1, 3]. Affected birds exhibit depression, anorexia, and a drop in egg production (10-40% in layers) [2, 5]. Respiratory signs include sneezing, snicking, and rales, often exacerbated by secondary bacterial infections such as Escherichia coli or Mycoplasma species [4, 6]. In severe cases, the infraorbital sinuses become markedly distended with purulent exudate, leading to occlusion of the nasal passages and open-mouth breathing [1, 7].

Infectious Coryza Eye Involvement

Ocular involvement is a consistent and clinically significant feature of infectious coryza, particularly in younger birds and those infected with serovar C strains [2, 6]. The term "infectious coryza eye" describes the pronounced conjunctivitis, periorbital oedema, and excessive lacrimation that characterise the ocular component [1, 4]. In some birds, the oedema may be so severe that it causes temporary closure of the palpebral fissure, a condition sometimes referred to as "swollen head syndrome" (though this term is more commonly associated with pneumovirus infection) [3, 5].

The ocular discharge is initially clear and serous but progresses to a mucopurulent consistency within 48-72 hours, often matting the feathers around the eye [2, 7]. Bacterial culture of conjunctival swabs from acutely affected birds yields high loads of A. paragallinarum, confirming the tropism of the pathogen for the ocular mucosa [1, 6]. Chronic or incompletely treated cases may develop caseous deposits in the conjunctival sac and, rarely, keratitis or panophthalmitis secondary to opportunistic infections [3, 4].

Morbidity within an affected flock is typically high (40-90%), but mortality is low (1-5%) unless exacerbated by concurrent infections or poor environmental conditions [2, 5]. For a comparative perspective on ocular signs in other poultry respiratory diseases, refer to Infectious Coryza in Poultry: Clinical Presentation, Diagnosis, and Management and Avian Mycoplasmosis in Poultry: Clinical Signs and Control.

Serovar-Specific Variation

Serovar A infections tend to produce the most severe facial swelling and nasal discharge, while serovar C is more frequently associated with conjunctivitis and ocular discharge [1, 6]. Serovar B strains, though less common, have been linked to protracted clinical courses and higher mortality in layers [2, 4]. Experimental challenge studies in specific-pathogen-free birds have demonstrated that the clinical signs are dose-dependent, with higher inocula resulting in more rapid onset and greater severity [3, 5].

Pathogenesis

Following inhalation or direct mucosal contact, A. paragallinarum adheres to the ciliated epithelial cells of the nasal cavity, infraorbital sinuses, and conjunctiva via fimbrial adhesins [1, 6]. The capsule and lipopolysaccharide then trigger an intense neutrophilic inflammatory response, leading to vasodilation, increased vascular permeability, and accumulation of oedema fluid in the subcutaneous tissue of the face and periorbital region [3, 7]. Histologically, the lesion is characterised by a severe acute fibrinosuppurative rhinitis and sinusitis, with epithelial necrosis, exudation of fibrin and neutrophils, and subsequent hyperplasia of the mucosal epithelium [2, 5].

The bacterium does not invade beyond the upper respiratory tract; bacteraemia is not a feature of infectious coryza [1, 4]. However, the destruction of the mucociliary barrier facilitates secondary bacterial infections, particularly by E. coli and Ornithobacterium rhinotracheale, which may descend into the lower respiratory tract causing airsacculitis and pneumonia [3, 6]. The drop in egg production observed in layers is attributed to the systemic effects of inflammation and anorexia rather than direct ovarian infection [2, 7].

Differential Diagnosis

The acute onset of facial swelling, nasal discharge, and ocular signs in a poultry flock necessitates differentiation from several other respiratory pathogens that may present similarly in UK flocks [1, 4]. Key differential diagnoses include:

Fowl cholera (caused by Pasteurella multocida) : typically produces septicaemia, high mortality, and cyanosis of comb and wattles, findings not seen in infectious coryza [3, 5]. For details, see Fowl Cholera in Poultry: Pasteurella multocida Pathogenesis, Clinical Signs, Prevention, Control, and WOAH Classification.
Mycoplasmosis (especially M. gallisepticum) : produces chronic respiratory disease with tracheal rales, gasping, and airsacculitis; facial swelling is less pronounced than in infectious coryza [2, 6]. Refer to Mycoplasma gallisepticum in Poultry: Chronic Respiratory Disease and Control Strategies.
Infectious bronchitis virus (IBV) : causes respiratory distress, sneezing, and tracheal mucus production, but facial oedema is not a typical feature [1, 4]. See Infectious Bronchitis Virus.
Avian influenza (H5/H7 highly pathogenic strains) : induces severe respiratory and systemic signs with high mortality; must be ruled out by PCR in notifiable disease investigations [3, 7].
Pneumovirus infections (avian metapneumovirus) : cause swollen head syndrome with pronounced periorbital oedema, but are more common in turkeys [2, 5].
Vitamin A deficiency : causes ocular and nasal discharges and caseous deposits in the conjunctiva, but is non-contagious and responds to dietary correction [1, 6].

A thorough diagnostic workup is essential, as multiple pathogens may co-circulate in a flock, complicating clinical assessment [4, 7]. The article Bacterial Poultry Diseases: An Overview of Common Pathogens and Clinical Signs provides a comparative overview.

Diagnosis

Sample Collection and Culture

The definitive diagnosis of infectious coryza relies on the isolation and identification of A. paragallinarum from clinical specimens [1, 3]. Optimal samples include nasal swabs, sinus exudate, or conjunctival swabs collected from acutely affected birds (within 3-5 days of clinical onset) before antibiotic therapy is initiated [2, 6]. Swabs should be placed in a transport medium (e.g. Amies with charcoal) and transported to the laboratory refrigerated, ideally within 24 hours [4, 7].

Culture is performed on chocolate agar or blood agar supplemented with a nurse strain (e.g. Staphylococcus aureus) to supply NAD factor, incubated at 37°C in 5-10% CO₂ for 18-48 hours [1, 3]. Colonies of A. paragallinarum appear as small, greyish, dewdrop-like colonies after 24 hours, and are positive for oxidase and catalase [2, 5]. Biochemical confirmation of the species identity can be achieved using commercial identification kits (e.g. API NH strip) or by PCR targeting the 16S rRNA gene or the hagA haemagglutinin gene [3, 6].

Serotyping and Molecular Methods

Serotyping of isolates is performed using the haemagglutinin-inhibition (HI) test with serovar-specific antisera, and is important for guiding vaccine selection [1, 4]. Multiplex PCR assays targeting serovar-specific regions of the capsular polysaccharide biosynthesis locus are now available and allow for rapid serovar identification directly from swab samples [2, 7]. Real-time PCR assays for A. paragallinarum have been developed and validated, offering higher sensitivity than culture, particularly in samples with low bacterial load or after antibiotic treatment [3, 5].

Serological Testing

Serological detection of antibodies by HI test or commercial ELISA kits can be used for flock-level surveillance, especially in breeder flocks [1, 6]. However, the HI test has limitations, including cross-reactivity between serovars and variability in sensitivity [2, 4]. ELISA-based detection of antibodies to A. paragallinarum is available but has not been standardised across all serovars [3, 7]. Serology is therefore most valuable for confirming exposure after an outbreak rather than for early diagnosis [1, 5].

Diagnostic Workflow

The following Mermaid diagram outlines a rational diagnostic workflow for a suspected outbreak of infectious coryza in a UK flock.

flowchart TD
    A[Flock showing facial swelling,\nocular discharge, sneezing], > B{Notifiable disease?}
    B, >|Yes| C[Rule out AI and NDV\nvia statutory PCR testing]
    B, >|No| D[Collect nasal/conjunctival\nswabs from 5-10 acute birds]
    D, > E[Culture on chocolate agar\nwith nurse strain, 37°C, 5% CO2]
    E, > F[Colonies resembling\nA. paragallinarum?]
    F, >|No| G[Perform Mycoplasma and\nother bacterial cultures]
    F, >|Yes| H[Biochemical confirmation\n(oxidase, catalase, API NH)]
    H, > I[Serotyping via HI or\nmultiplex PCR]
    I, > J[Vaccine selection and\ntreatment planning]
    G, > K[Consider alternative diagnoses:\nIBV, pneumovirus, mycoplasmosis]

Treatment

Antimicrobial therapy is the mainstay of treatment for acute infectious coryza in UK flocks, though its effectiveness is limited by the development of drug resistance and the presence of carrier birds [1, 3].

Antibiotic Selection

Traditional first-line antibiotics include sulphonamides (e.g. sulphadimethoxine), tetracyclines (e.g. oxytetracycline or doxycycline), and tylosin, administered in drinking water for 3-5 days [2, 4]. Erythromycin and lincomycin-spectinomycin combinations have also been used with variable efficacy [3, 5]. In recent years, reduced susceptibility of UK isolates to tetracyclines and tylosin has been noted, and susceptibility testing is recommended before selecting therapy [1, 6]. Fluoroquinolones (e.g. enrofloxacin, danofloxacin) are highly effective in vitro, but their use in poultry is restricted under UK antibiotic stewardship guidelines to prevent emergence of resistance [2, 7]. Ceftiofur and other third-generation cephalosporins are not authorised for oral administration in chickens in the EU and UK and should not be used [3, 4].

Antibiotics should be administered early in the clinical course to reduce morbidity and mortality; however, they do not eliminate the carrier state and clinical relapses are common after withdrawal of medication [1, 5]. Details of treatment protocols are discussed further in Infectious Coryza in Chickens: Drugs, Treatment Protocols, and Differential Diagnosis.

Supportive Care

Supportive measures include improving ventilation to reduce ammonia levels, increasing ambient temperature, and ensuring easy access to feed and water [2, 6]. Anti-inflammatory drugs (e.g. meloxicam, acetylsalicylic acid) may alleviate severe facial swelling but their use in laying birds must adhere to withdrawal periods for eggs [3, 7].

Antimicrobial Resistance Considerations

The UK Veterinary Medicines Directorate and poultry industry guidelines recommend performing culture and sensitivity testing whenever possible [4, 5]. Resistance to sulphonamides and tetracyclines is widespread in some regions, and multi-drug resistant isolates of A. paragallinarum have been reported in continental Europe and may be emerging in the UK [1, 6]. The prudent use of critically important antibiotics (e.g. fluoroquinolones, macrolides) is essential to preserve their efficacy [2, 7].

Control and Prevention

Biosecurity

Because the bacterium is primarily transmitted through direct contact and contaminated fomites, strict biosecurity is the cornerstone of prevention [1, 3]. Key measures include:

All-in/all-out management of houses with thorough cleaning and disinfection between placements [2, 5].
Dedicated footwear and clothing for each house, with footbaths containing disinfectant active against Gram-negative bacteria (e.g. quaternary ammonium compounds, peracetic acid) [3, 6].
Controlled introduction of replacement stock from sources known to be free of A. paragallinarum; isolation of new birds for at least 3 weeks [1, 4].
Rodent and wild bird control to prevent mechanical transmission [2, 7].
Water line sanitation with regular flushing and use of approved disinfectants [3, 5].

Vaccination

Inactivated bacterin vaccines are available and widely used in UK layer and breeder flocks, particularly in multi-age sites where eradication is impractical [1, 4]. Vaccines typically contain at least serovars A and C, and some products include serovar B [2, 6]. Vaccination is recommended at 8-12 weeks of age, followed by a booster 4-6 weeks later, and then every 3-6 months in long-lived layers [3, 5]. Protection is serovar-specific, and vaccine failure can occur if the challenge serovar is not included in the formulation or if the flock is immunocompromised due to concurrent infections [1, 7].

Autogenous vaccines may be prepared from locally isolated strains when commercial products are ineffective [2, 4]. Vaccination does not prevent infection but reduces clinical severity, shedding, and the risk of egg production drops [3, 6].

Eradication

Eradication of infectious coryza from a farm is difficult due to the persistence of carrier birds and the bacterium's ability to survive in the environment for short periods [1, 5]. In breeder flocks, depopulation, thorough cleaning and disinfection, and restocking with pathogen-free birds is the most reliable approach [2, 4]. In laying flocks, a test-and-cull strategy using serology or PCR to detect carriers has been attempted but is logistically challenging and expensive [3, 7].

Integrated Control Program

A comprehensive control programme should combine biosecurity, vaccination, monitoring, and targeted antimicrobial therapy for acute outbreaks [1, 6]. Regular serological monitoring in breeder flocks can detect early exposure and allow for prompt intervention [2, 5]. The article Infectious Coryza in Poultry: Diagnosis, Symptoms, and Causal Pathogen provides additional perspectives on integrated management.

Conclusions

Infectious coryza remains a significant respiratory disease in UK poultry, particularly in commercial layer and breeder flocks. The typical clinical presentation of facial swelling, nasal discharge, and ocular involvement (infectious coryza eye) is distinctive but requires laboratory confirmation to differentiate from other respiratory pathogens [1, 3]. Diagnosis relies on bacterial culture of acute samples, with serotyping and molecular methods guiding vaccine choice [2, 4]. Treatment with appropriate antimicrobials can reduce morbidity, but resistance is a growing concern [3, 5]. Effective control hinges on robust biosecurity, strategic vaccination, and the management of carrier birds [1, 6]. Given the fragmented structure of UK poultry production and the persistence of subclinical infection, ongoing surveillance and veterinary vigilance are essential to minimise the economic impact of this disease [4, 7].

References

[1] Swayne, D.E. (ed.). Diseases of Poultry. 14th ed. Wiley-Blackwell, 2020. (Chapter on Infectious Coryza)

[2] Merck & Co. The Merck Veterinary Manual. 11th ed. Kenilworth, NJ, 2016. (Section on Avian Respiratory Diseases)

[3] Quinn, P.J., et al. Veterinary Microbiology and Microbial Disease. 2nd ed. Wiley-Blackwell, 2011.

[4] Jordan, F.T.W., et al. Poultry Diseases. 6th ed. Saunders Ltd., 2008.

[5] World Organisation for Animal Health (OIE). Manual of Diagnostic Tests and Vaccines for Terrestrial Animals. 12th ed. Paris, 2023. (Chapter on Infectious Coryza)

[6] Saif, Y.M. (ed.). Diseases of Poultry. 13th ed. Wiley-Blackwell, 2013.

[7] Pattison, M., et al. (eds.). Poultry Diseases. 6th ed. Elsevier, 2008. *** 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.