Infectious Bronchitis in Chickens: A Veterinary Overview

Introduction

Infectious bronchitis (IB) is one of the most economically significant viral diseases affecting commercial and backyard poultry worldwide. The disease was first identified in North Dakota, USA, when Schalk and Hawn reported a new respiratory disease in young chickens, and although it was initially believed to occur primarily in young birds, chickens of all ages are susceptible. Beyond its impact on the respiratory tract, IB can affect the kidneys and reproductive system, leading to renal disease and reduced egg production .

Causative Agent

Infectious bronchitis virus (IBV) is an avian gamma coronavirus belonging to the order Nidovirales, family Coronaviridae, and subfamily Orthocoronavirinae. It is a single-stranded, positive-sense, enveloped RNA virus ranging from roughly 27–32 kb in length. While the chicken (Gallus gallus) is the primary host of clinical concern, pheasants are also recognized as natural hosts, though clinical disease is predominantly reported in chickens. Related IBV-like coronaviruses have also been detected in other avian species including turkeys, geese, ducks, pigeons, quail, and peafowl, and antigenic similarities have been demonstrated between turkey coronavirus and IBV. Notably, while antibodies to IBV have been found in humans with close poultry contact, the virus has not been shown to cause clinical disease in people.

Pathogenesis and Tissue Tropism

A key feature of IBV is its broad and strain-dependent tissue tropism. The variability of the spike (S1) protein’s amino acid sequence largely determines host and tissue tropism, as this protein engages host cell receptors—α2,3-linked sialic acids and/or heparan sulfate—for cell entry. The virus’s ability to infect particular tissues appears to be governed by the binding affinity of S1 for these sialic acid receptors, which is itself influenced by glycosylation patterns on the receptor-binding domain.

Over time, distinct IBV genotypes have emerged with differing organ predilections. Various IBV genotypes are now associated with reproductive, renal, gastrointestinal, muscular, and immunosuppressive disease manifestations, contributing to IBV’s substantial economic impact on global poultry production. Infected chickens commonly present as depressed, with varying degrees of respiratory difficulty and ruffled feathers, and younger birds tend to show the most severe clinical signs.

Pathogenicity is not static. IBV strains are generally categorized as low, moderate, or highly pathogenic, and this pathogenicity can shift depending on secondary or opportunistic infections.

Strain-Specific Pathology: Italy 02 Example

Research on specific serotypes illustrates how age affects disease severity. The Italy 02 serotype was shown to cause severe respiratory and renal damage in one-day-old chicks, but in adult hens it produced only respiratory disease accompanied by a drop in egg production. Studies using in situ hybridization found viral RNA appeared in the nasal turbinates before the trachea, with more persistent replication in other tissues, and viral shedding in feces was more consistent and prolonged than nasal shedding.

Sex-Based Differences in Pathogenesis

A 2023 study examined whether chicken sex influences IBV pathogenesis. Researchers found no significant difference in the percentage of IBV-positive tissue area between infected male and female chickens across most tissues, except in the kidney, where infected males showed increased viral antigen at both 4 and 11 days post-infection compared to females. B lymphocyte percentages did not differ significantly by sex in any examined tissue, though CD8+ T cell recruitment in the trachea was higher in females than males at 11 days post-infection. Overall, the study concluded that sex does not play a major role in IBV pathogenesis or host immune response in young chickens, though marginal differences suggest infection may be somewhat more severe in males.

Clinical Signs

Clinical presentation varies by strain and bird age but generally includes:

  • Respiratory signs: coughing, sneezing, tracheal rales, and nasal discharge
  • Reduced activity and ruffled feathers
  • In layers: drops in egg production and egg quality (thin, soft-shelled, or misshapen eggs)
  • In some strains: nephritis and renal damage, particularly severe in young chicks

In addition to respiratory signs, decreased egg production and egg quality are common clinical signs, and nephritis can be caused by some strains of IBV.

Diagnosis

Diagnosis relies on a combination of clinical observation, virus detection, and serology. Diagnostic tests include ELISA and hemagglutination inhibition testing for serum antibodies, virus detection by reverse transcription PCR (RT-PCR) and reverse transcription quantitative real-time PCR, and virus isolation in embryonated eggs.

Alternatively, IBV can be isolated in tracheal organ cultures, with viral growth indicated by cessation of ciliary motility, though isolation of some field strains may require several blind passages. RT-PCR is the most common diagnostic approach, used to detect viral RNA in tracheal, cecal tonsil, or kidney tissue samples.

Serotyping is particularly important for outbreak investigation, since vaccine mismatch is a common cause of disease in vaccinated flocks. Typing viruses is critical for diagnosing outbreaks caused by serotypes distinct from those in vaccines used on a flock, and serotyping has traditionally relied on virus neutralization tests using sera from SPF chickens inoculated with known serotypes—though this method is expensive, time-consuming, and not widely available.

Vaccination and Control

Vaccination remains the cornerstone of IB control, though it presents unique challenges due to the genetic diversity of IBV. Live attenuated and killed (inactivated) vaccines are both available; however, different antigenic types of IBV do not cross-protect, which complicates control efforts.

Vaccination Strategy Research

Several recent studies have evaluated combined vaccination protocols in commercial layer operations. One study evaluated two vaccination regimes against a Massachusetts (Mass) serotype IBV challenge linked to egg production problems in Western Canada. One program used only live attenuated vaccines, while the other combined live attenuated and inactivated vaccines—priming pullets with a monovalent live Mass vaccine at day-old, followed by bivalent Mass/Connecticut live vaccines at 2, 5, 9, and 14 weeks, with the inactivated Mass vaccine added at 14 weeks for one group.

A related immunological study examined the basis for these protective effects. Although administering inactivated vaccines after priming with live attenuated vaccines in pullets has been shown to protect laying hens against IB, the immunological basis for this protection had not been well studied, prompting researchers to compare two Canadian-industry vaccination strategies for their ability to induce adequate immune responses in IBV-relevant tissues.

Variant strains continue to challenge existing vaccine programs globally. Research on the GI-23 (Var2) strain isolated in Poland tested seven vaccination programs using Mass, 793B, QX, and Var2 live vaccines, alone or in combination, against challenge with a Polish GI-23 field isolate. Two strategies achieved full protection (a ciliostasis score of 0 with 100% protection): Var2 or Mass given to day-old chicks, followed by a boost combining QX and 793B vaccines. This is notable given that IBV is one of the longest-known coronaviruses, first described nearly 90 years ago, and continues to cause significant economic losses through its effects on the respiratory, reproductive, and renal systems.

Biosecurity

Vaccination alone is not sufficient for control. An effective biosecurity program—including rodent control—is considered the best method of preventing infectious bronchitis, in addition to vaccination.

Ongoing Challenges

Despite decades of vaccine development and biosecurity improvements, IB remains a persistent problem. IBV continues to emerge in commercial flocks despite immunization and biosecurity measures, due to the ongoing introduction of novel variants and inadequate cross-protection from currently available vaccines—consistently threatening the growth and profitability of the poultry industry. Continued molecular surveillance and genotype characterization are seen as key to improving regional disease prevention and management strategies.

Conclusion

Infectious bronchitis remains a major endemic threat to poultry health and production economics globally. Its significance stems from the virus’s genetic diversity, broad tissue tropism (respiratory, renal, reproductive, gastrointestinal, and immune systems), and the limited cross-protection offered by existing vaccines against emerging variants. Effective control requires an integrated approach combining strain-matched vaccination programs, robust biosecurity, and ongoing molecular surveillance to detect and respond to new genotypes as they arise.

References

  1. Pathogenesis and Diagnostic Approaches of Avian Infectious Bronchitis. NCBI PMC. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4756178/
  2. Najimudeen, S.M., et al. (2023). Comparison of Infectious Bronchitis Virus (IBV) Pathogenesis and Host Responses in Young Male and Female Chickens. Viruses, 15(12), 2285. DOI: https://doi.org/10.3390/v15122285
  3. Infectious Bronchitis Coronavirus Infection in Chickens: Multiple System Disease with Immune Suppression. Animals (2020), 9(10), 779. https://www.mdpi.com/2076-0817/9/10/779
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  5. Same as reference 2.
  6. Avian infectious bronchitis virus (AIBV) review by continent. Frontiers in Cellular and Infection Microbiology (2024). https://www.frontiersin.org/journals/cellular-and-infection-microbiology/articles/10.3389/fcimb.2024.1325346/full
  7. Same as reference 2 (PMC mirror). https://pmc.ncbi.nlm.nih.gov/articles/PMC10747771/
  8. New insights on infectious bronchitis virus pathogenesis: characterization of Italy 02 serotype in chicks and adult hens. PubMed. https://pubmed.ncbi.nlm.nih.gov/22172297/
  9. Same as reference 3 (PMC mirror). https://pmc.ncbi.nlm.nih.gov/articles/PMC7598688/
  10. Infectious Bronchitis in Chickens. Merck Veterinary Manual, reviewed by Rüdiger Hauck, DVM, PhD, DECPVS (Auburn University), Aug 2024. https://www.merckvetmanual.com/poultry/infectious-bronchitis/infectious-bronchitis-in-chickens
  11. Ali, A., et al. (2023). Efficacy of Two Vaccination Strategies against Infectious Bronchitis in Laying Hens. Vaccines, 11(2), 338. DOI: https://doi.org/10.3390/vaccines11020338
  12. Buharideen, S.M., et al. (2021). Immune Responses in Laying Hens after an Infectious Bronchitis Vaccination of Pullets: A Comparison of Two Vaccination Strategies. Vaccines, 9(5), 531. DOI: https://doi.org/10.3390/vaccines9050531
  13. Infectious Bronchitis in Poultry. Penn State Extension – Small and Backyard Poultry. https://poultry.extension.org/articles/poultry-health/common-poultry-diseases/infectious-bronchitis-in-poultry/
  14. Lisowska, A., et al. (2021). Virulence Properties of GI-23 Infectious Bronchitis Virus Isolated in Poland and Efficacy of Different Vaccination Strategies. Pathogens, 10(5), 522. DOI: https://doi.org/10.3390/pathogens10050522

Note: This article is intended for general informational and educational purposes. For flock-specific diagnostic and treatment decisions, consult a licensed poultry veterinarian.