Mutant Strains of Infectious Bronchitis Virus in Poultry: Epidemiology, Pathogenesis, and Control Strategies
Dr. Nahlan Juwair Hassan / Poultry Diseases
24-3-2026
Abstract
Infectious bronchitis virus (IBV) is one of the most important viruses affecting poultry due to the significant economic losses it causes in the poultry production sector. The continuous emergence of mutant strains resulting from genetic mutations and recombination has led to the failure of many traditional vaccination programs and the continued occurrence of infections in farms. Some mutant strains, such as QX, 793B, and IS/1494, are characterized by their ability to induce different pathogenic changes and weak cross-protection with classical vaccines. This study aims to review the most important aspects related to the evolution, spread, pathogenesis, and diagnostic methods of these strains, with a focus on modern control strategies, particularly the concept of multiple protection (protectotype) and the importance of continuous molecular surveillance.
Introduction
Infectious bronchitis (IBV) is a highly contagious acute viral disease in chickens caused by a virus belonging to the avian coronavirus family. The disease primarily affects the respiratory system and can extend to the kidneys and reproductive system, leading to reduced productivity and significant economic losses.
The virus’s high capacity to generate new strains stems from its nature as an RNA virus, making it susceptible to frequent mutations and genetic recombination, particularly in the gene responsible for the S1 spike protein, a key determinant of the immune response.
Evolution and Emergence of Mutant Strains
The diversity of IBV results mainly from:
- Point mutations in the genetic material
- Recombination between field and vaccine strains
Recent studies have shown that these changes lead to the emergence of new strains with differing antigenic characteristics, reducing the effectiveness of vaccines and contributing to immune escape. Epidemiology
Various IBV strains exhibit varying geographic distribution, although some strains have become widespread globally. Among the most important are:
- QX strain (associated with severe kidney infection)
- 793B strain
- Variant 2 (GI-23) strain, common in the Middle East
Field studies indicate that some of these strains may represent the majority of viral isolates in their areas of prevalence, necessitating the adoption of continuous surveillance programs to identify dominant strains.
Pathogenicity and Clinical Effects
Mutant strains are characterized by:
- The ability to infect multiple organ systems (respiratory, renal, and reproductive)
- Increased severity of infection in young birds
- Variation in symptom severity depending on the strain.
Some studies have reported high mortality rates in young chicks, in addition to permanent damage to the reproductive system, such as oviduct atrophy, leading to the development of unproductive hens (false layers).
Diagnosis
Accurate diagnosis relies on:
- RT-PCR techniques
- Genetic sequencing to determine the genotype
- Differentiation between field and vaccine strains. Clinical signs alone are insufficient due to the similarity between different strains.
Control and Prevention
First: Immunization
Traditional vaccines (such as Mass) show limited protection against mutated strains. Therefore, the following is recommended:
The Protectotype approach, which relies on:
- Combining different live vaccines
- Selecting vaccines that are antigenically close to field strains.
Studies have shown that this approach provides a broader range of protection compared to using a single vaccine.
Second: Hygiene Measures
- Implementing a full all-in/all-out system
- Strengthening biosecurity measures
- Reducing cross-poll transmission
Third: Epidemiological Surveillance
Continuous molecular surveillance is essential for:
- Detecting new strains
- Updating immunization programs
Challenges
- Continuous viral mutation
- Potential for recombination with vaccine strains
- Weak cross-protection
- Strain variations across regions
Conclusion
Mutated strains of infectious bronchitis virus pose a growing challenge to the poultry industry due to their ability to constantly mutate and evade acquired immunity. The best control strategies remain based on a combination of thoughtful immunization based on the concept of multiple protection, continuous molecular monitoring, and strict application of biosecurity measures.
References
- Gallardo RA. Infectious bronchitis virus variants in chickens: evolution, surveillance, control and prevention. Austral J Vet Sci. 2021;53(1):55–62.
- Zhang X, et al. A pathogenic and recombinant infectious bronchitis virus variant with multiorgan tropism. Vet Res. 2023;54:xx.
- Wang Y, et al. Pathogenicity of recombinant IBV variants in chickens. 2023.
- Recent surveillance of IBV Variant 2 (GI-23) in broilers. 2025.
- Veterinary Research Review. IBV evolution and respiratory disease complex. 2025.
- Molecular characterization of IBV variants. Genes. 2024;15(11):1480.
