Electron micrograph of Infectious Bronchitis virus particle
From a practical point of view it may be more relevant to think in terms of protectotypes rather than serotypes.
Infectious Bronchitis serotypes and protectotypes
Many different serotypes or genotypes of Infectious Bronchitis Virus are recognized. In general, different serotypes of the virus do not cross-protect. However, some strains of the virus are quite effective at inducing cross protection against other serotypes and are referred to as protectotypes.
IBV serotypes
Isolates of IBV have been divided into different serotypes based on the virus neutralization test in embryonated eggs or tracheal organ cultures. IB isolates can also be identified molecularly using RT-PCR and the RFLP test or nucleotide sequencing. This groups strains according to their genotype. Although serotyping and genotyping do not always give exactly the same groupings of strains, in general these is good correlation between the two typing methods. Many different serotypes or genotypes of IBV are recognized and they are important because in general, different serotypes of the virus do not cross-protect. However, some strains of the virus are quite effective at inducing cross protection against other serotypes or genotypes (see protectotypes).
Variant serotypes
IBV has the ability to rapidly change. This can result in new variant viruses or serotypes. The prevalence of variant Infectious Bronchitis (IB) serotypes must first be determined before contemplating the use of a vaccine containing these viruses. Variant viruses may be present when “IB-like” problems are seen in flocks properly vaccinated with Massachusetts type vaccines.
On the other hand, existing vaccines may represent the protectotype needed in order to get protection against the variant strain. When currently available products prove to give insufficient protection against an emerging virus, the development of a homologous vaccine against the new variant is justified (for example IBV 4/91).
The occurrence of variant serotypes emphasises the need for inactivated vaccines because it is problematic to apply different live IB vaccines of various serotypes within a short period of time without provoking interference between them.
Protectotypes
New serotypes can emerge as a result of only a few changes in the amino acid sequence of the S1 part of the spike gene of the virus. Although a new serotype emerges, much of the virus genome remains unchanged. This may be the reason that IB vaccines from a certain serotype may provide protection against IB strains not belonging to that serotype. From a practical point of view it may therefore be more relevant to think in terms of protectotypes rather than serotypes.
Read more in Cross protection - live vaccines and Cross protection - inactivated vaccines.
Assessment of protection
There are different ways to measure protection against IBV infection.
Following challenge with pathogenic IBV, protection can be assessed by:
- clinical signs in the birds
- virus re-isolation
- virus detection (for example, Immunofluorescence test -IFT)
- histological changes in the trachea or nasal tissue (for example by using monoclonal antibodies in immunochemical assays)
- the ciliostasis test
- detection of the viral genome by RT-PCR
For the purpose of our work we will refer basically to the ciliostasis test.
