Infectious Bronchitis (IB) is a highly contagious respiratory disease of poultry, that causes widespread economic losses within the industry. Vaccination is key to the effective control of IB, but IBV has an inherently high mutation rate, and the continual emergence of new serotypes makes this challenging. For successful IB control, alternatives to homologous vaccines are necessary and protectotyping is one such concept that has been suggested.
Infectious bronchitis is caused by the Infectious Bronchitis Virus (IBV), a Gammacoronoavirus belonging to the Coronaviridae family, and it was first reported in 1931.1 IBV affects the respiratory, urinary and reproductive systems and clinical signs depend on the tissue tropism of the infecting virus. Coughing, sneezing, tracheal rales and ocular discharge are commonly seen, and the disease tends to be more severe in young birds. In addition, mortality rates are higher in those strains with a renal tropism (nephropathogenic strains), or where secondary infection occurs.
Structure of the Infectious Bronchitis Virus
IBV is an enveloped, single-stranded RNA virus. Three virusspecific proteins (the spike, membrane and nucleocapsid glycoproteins) are encoded by the IB viral genome, and it is the spike protein that is particularly important when it comes to both infectivity of the virus and vaccine efficacy.
The spike proteins are comprised of two glycopolypeptides, termed S1 and S2, and they project from the surface of the virus, enabling attachment to the cell surface and entry into host cells. The S1 glycoprotein determines the serotype and enables the binding of the virus to the host cell receptor. It contains a number of epitopes (or antigenic determinants) to which antibodies attach, and indeed it is known that most haemagglutination inhibition (HI) and serum neutralisation antibodies are directed against this glycoprotein.2
Serotype and the S1 Glycoprotein: An Evolving Threat
While biosecurity measures are important, control of IB is focused on vaccination. However, despite the widespread use of vaccines dating back to the 1950s, the disease continues to circulate widely, with a devastating impact on the poultry industry.
The lack of progress in disease control is largely due to the inherently high mutation rate of IBV, which results in continuous genetic and antigenic changes of the circulating virus, and the emergence of new strains or serotypes.3 The majority of these genetic mutations occur in the S1 glycoprotein, and new serotypes can result from very few changes in the amino acid sequence. In the face of this constant change, successful control of IBV by vaccination is challenging.
Homologous Vaccines and Infectious Bronchitis Control
Having a homologous, serotype-specific vaccine to protect against different serotypes as they arise is the ideal scenario. With an S1 glycoprotein amino acid sequence homologous to the circulating serotype, such vaccines may give full protection. However, given the remarkable ability of IBV to mutate and the continual emergence of new serotypes, it would be impossible to develop such a vaccine against all new strains. Even if it was possible, many of these variant serotypes disappear from circulation and may no longer be of significance by the time a vaccine was ready for use.