The emergence and evolution of viral pathogens cause a major problem in the poultry industry. Mutation and recombination processes are involved in the genetic and phenotypic variations of infectious bronchitis virus (IBV) in chickens, leading to the emergence of new variant strains, and give rise to virus population diversity to be modelled by the host, particularly by the immune system. The consequence is the continuous emergence of new variants with regard to pathotypes, serotypes, and protectotypes. The viral genes encoding the spike, replicase and nucleocapsid proteins can be considered the main genomic regions, which indicate the evolution processes of IBV. Despite the use of vaccination, new IBV variants are evolving and circulating in the field and should be considered as initial candidates for vaccine development. The new generation vaccines developed against locally prevailing IBV strains may be more helpful and avoid the reversion of virulence in live vaccine viruses. Understanding the mechanisms underlying the evolution of IBV has basic relevance and, without doubt, is essential to appropriately control and prevention of the disease.

Infectious bronchitis virus (IBV) is one of the foremost causes of economic loss within the poultry industry. IBV is a commonly occurring, economically significant pathogen of commercial chicken. Economic consequences to the poultry industry comprise mortality, growth retardation and high condemnation rates in meat-type birds. In addition, decreased egg production, reduced internal and external egg quality, and reduced hatchability have been documented in layers and breeders affecting the performance of both meat-type and egg-laying birds. Apart from this some nephro-pathogenic strains cause kidney damage. Secondary pathogens can complicate the disease resulting in increased morbidity and mortality. Being a single stranded RNA virus, IBV has an enormous capacity to change both by spontaneous mutation and by genetic recombination resulting into the emergence of new variants. Since the first isolation of virus in 1937, it has been found almost all over the world. In addition, most countries are now known to have their own indigenous IBV variants. Despite the use of currently available live and inactivated vaccines, one of the most important difficulties to control IB is related to emergence of variant strains. The following paper reviews the current status of research into IBV.