Page 141 - Avian Virology: Current Research and Future Trends

134 | Liu et al.

identified in Europe, perhaps the ones of major importance are ribosomal frame-shifting, (c) the structural and accessory genes
4/91, 793B, CR88 (Gough et al., 1992; Parsons et al., 1992). downstream of the replicase gene are expressed via a 3′-nested
Some IBV variants may be geographically restricted, and multi- subgenomic messenger RNAs (sgRNAs) and (d) a collection
ple studies have employed molecular methods to identify IBV of viral enzymatic activities are encoded within the replicase-
variants unique to the region (Escorcia et al., 2000; Collison et al., transcriptase protein products.
2001; Gelb et al., 2001; Alvarado et al., 2005). The classification systems of IBV strains are divided into two
major groups, namely the functional and non-functional tests
(Table 5.1). Functional tests look into the biological functions
Nomenclature of the said IBV strains, and categorize IBV into various patho-
IBV is an enveloped, positive sense, single-stranded RNA virus types, protectotypes and antigenic types. On the other hand, the
classified under the family Coronaviridae of the order Nidovirales, non-functional tests probe into the viral genome, and this usu-
along with families Arteriviridae, Mesoniviridae, and Roniviridae ally results in grouping IBVs based on genotype (de Wit, 2000;
(International Committee on Taxonomy of viruses, http://www. Valastro et al., 2016).
ictvonline.org/virustaxonomy.asp). Virus taxonomy of the family Sequence comparisons of the complete genome, spike (S)
Coronaviridae is shown in Fig. 5.1. Nidoviruses are set apart protein subunits 1 and 2 (S1 and S2), envelope (E) protein, mem-
from other RNA viruses by four distinctive characteristics: (a) brane (M) protein and nucleocapsid (N) protein of IBV strains
an invariant genome organization comprising a large replicase from distinct geographical regions have contributed great strides
gene occupying the 5′ two-thirds of the viral genome, (b) transla- in the construction of the IBV phylogenetic tree (Fig. 5.2) (Lin
tion of the replicase-transcriptase polyprotein is performed via et al., 2016).

Nidovirales

Coronaviridae

Coronavirinae Torovirinae

Alphacoronavirus Betacoronavirus Gammacoronavirus Deltacoronavirus Torovirus Bafinivirus

Infectious bronchitis virus (IBV)

Figure 5.1 Taxonomy of the family Coronaviridae. Infectious bronchitis virus (IBV) is a Gammacoronavirus, under the family Coronaviridae
within the order Nidovirales.

Figure 1. Taxonomy of the family Coronaviridae. Infectious bronchitis virus (IBV)
Table 5.1 Classification of infectious bronchitis virus (IBV) into pathotypes, protectotypes, antigenic types and genotypes
is a Gammacoronavirus, under the family Coronaviridae within the order Nidovirales.
Parameter measured Features
Functional tests
Pathotypes Clinical signs, gross lesions, and virus pathogenicity; Pros: practical in the field for vaccine strategy
same pathotype when both tested strains induce similar
pathological signs
Protectotypes Complete immune response against an IBV strain; same Pros: provides valuable information about vaccine efficacy
protectotype when strains induce protection against each Cons: laborious and expensive; requires high-level facilities
other for vaccination-challenge studies
Antigenic types Reaction between IBV strain and chicken-induced IBV Cons: less practical when more IB variants are found in the
(serotypes and serotype-specific antibodies; same serotype when area as every serotype requires its own neutralization test.
epitope type) heterologous neutralization titres differ less than 20-fold For new IB strains, an antiserum has to be raised in SPF
from homologous titres birds.
Non-functional tests
Genotypes Genetic characterization of viral genome; same genotype Pros: objective; provides useful information for
when sequence of tested strain matches epidemiological studies
IB, infectious bronchitis; IBV, infectious bronchitis virus; SPF, specific pathogen free.