Vaccination and viral serotypes

1.2.6.1 Vaccination

The RNA-dependent RNA-polymerase present in most RNA viruses and also in IBV, required for copying of the viral genome, is mainly responsible for the extremely high mutation rate. This enzyme does not have proofreading capability. So, when a mistake in copying the genome is made, the enzyme cannot go back and fix it. This high mutation rate creates a diverse population of virus particles that allows IBV to quickly adapt to selection pressures such as host immune responses (antibodies and T-cells; CAVANAGH 2001). This adaptation is evident clinically as variant viruses or emerging new serotypes of the virus. The control of IBV by vaccination is hampered because of the appearance of new emerging serotypes, which are only weakly affected by vaccination against the original serotypes. Although homologous live vaccines for IB are better than heterologous vaccines in controlling the disease, it is still recommended to develope the vaccines from different local strains to control IB (LIN et al. 2005). The most common serotypes of IBV in the USA are Arkansas, Connecticut, Delaware, and Massachusetts. However, when those IBV types undergo changes, also designated genetic drift, it can result in considerable variation away from the original serotype. The Arkansas type viruses appear to be quite prone to genetic drift resulting in viruses designated Arkansas-like viruses. If the extent of genetic changes, which can accumulate over time, reaches a critical point, the Arkansas vaccine strain (Ark-DPI) may no longer provide sufficient protection against the Arkansas-like viruses. In addition to genetic drift, genetic shift can lead to a dramatic change resulting in a unique variant virus. Such variant viruses are defined as previously unrecognized IBV types that are clearly different from the known IBV serotypes. Genetic shift usually results from recombination between two different parent viruses (that infected the same cell) to produce a distinct progeny virus.

Recombination events occur by a template switching mechanism along conserved regions of the viral genome between two different viruses (CAVANAGH et al. 1992a;

KOTTIER et al. 1995; ESTEVEZ et al. 2003; GELB et al. 2005). The resulting virus is a hybrid of the two parental viruses that, under the right conditions, can break through immunity induced by the common vaccine types.

IBV continues to be one of the major pathogens of chickens throughout Europe. In fact, it is probably endemic in all regions where poultry are reared intensively.

Despite the availability of high quality vaccines, IB continues to be a problem in most regions of Europe. The prevalence of IBV variants in Europe has been recognised as a problem at least since the 1960s.

1.2.6.2 Important serotypes in Europe.

Infectious bronchitis (IB) is still a significant problem in the poultry industry in Europe.

Recently, several authors have reported many different serotype isolates. A survey conducted in UK, France, Holland, Germany, and Spain revealed that the most commonly found genotypes were 4/91 and Massachusetts (Mass), accounting for 34% and 24% of the studied isolates, respectively. The next most frequently detected genotypes were Italy-02 (It-02), QX, D274 and Arkansas (WORTHINGTON et al.

2004; WORTHINGTON and JONES 2006).

Infectious bronchitis virus variant 4/91

The IBV variant 4/91 was firstly reported in Britain in the early 1990s (ADZHAR 1997;

CAVANAGH et al. 1998a). It was associated with outbreaks of respiratory diseases and rapidly spread displacing the D274 serotype that has been dominant in the 1980s (ADZHAR 1997; CAVANAGH et al. 1998). Currently, 4/91 also named 793/B and CR88 is one of the most common IBV serotypes in Europe. Its appearance and spread resulted in great economic losses. This variant has continued to be a major pathogen throughout Europe and also in other parts of the world although it is controlled by a live vaccine, usually followed in older birds by the use of an inactivated vaccine.

Infectious bronchitis virus variant Italy 02

The sequence analysis of this IBV strain isolated in the late 1990's in Italy was studied closely and designated at 2002 as “Italy 02”. Later on this isolate became widespread in Europe. In the early 2000s, in a study carried out by Worthington and co-workers using the RT-PCR technique, the authors found that Italy-02 was one of the most predominant genotypes not only in Italy but also in many other countries (WORTHINGGTON et al. 2004). The studies so far performed have simply detected the virus genome, few virus isolates have been obtained and so it has not been proven that this IBV strain is pathogenic in chickens in the field.

In a recent survey (WORTHINGGTON et al. 2008) “Italy 02” in Spain continues to be the dominant type detected (DOLZ et al. 2006).

Infectious bronchitis virus variant (QX)

The QX serotype was first isolated in China in 1996 from birds with proventriculitis (YU DONG et al. 1998), spread rapidly to several provinces in China (YU et al. 2001;

LIU et al. 2006) then to the European region of Russia (BOCHKOV et al. 2006). In 2005, the QX serotype was isolated in the European continent (BEATO et al. 2005;

LANDMAN et al. 2005; ZANELLA et al. 2006) and spread rapidly to become the most wide spread serotype of non-vaccine origin (WORTHINGTON et al. 2008). In addition to the respiratory lesions, the QX-like serotype has been associated with proventriculitis (YU DONG et al. 1998), severe kidney damage (LIU and KONG 2004;

BEATO et al. 2005; ZANELLA et al. 2006; WORTHINGTON et al. 2008) and false-layer syndrome (LANDMAN et al. 2005) as well as severe egg production problems.

Respiratory signs have been reported in broilers older than four weeks of age. In birds in production, the problems are characterized by a low production rate with peak levels reaching 30% to 55% in apparently healthy flocks. Some of the birds show a peculiar stance (penguin-like) and a pendulous abdomen. These cases were associated with earlier outbreaks of nephropathogenic infectious bronchitis that had occurred in 2003 in broilers and pullets (layer and broiler rearing pullets) from which a variant IB virus, designated D388 by the Animal Health Service in Deventer, The Netherlands, was isolated. Using a PCR test, it was demonstrated that this isolate can be classified with 99% genotypic agreement as a Chinese isolate designated

“QX” and this is the name by which this IB variant is now known (BEATO et al. 2005 and WORTHINGGTON at al. 2008). This original isolate was similar to a Chinese isolate known as QX. When this isolate was inoculated into either SPF or commercial birds it was found to be nephropathogenic and could also be detected in the oviducts of the infected birds.

This IB virus has continued to be a cause of major disease problem in poultry flocks in some parts of Europe, being associated with nephritis and so-called “false layers”

as a result of early infection with this virus. In a recent survey using the RT-PCR technique (WORTHINGTON et al. 2008), the QX IBV variant has been detected frequently in The Netherlands, Belgium, Germany and France.

1.2.6.3 IBV in Egypt

Isolates related to Massachusetts, D3128, D274, D-08880, 4/91 and the novel genotype Egypt/Beni-Suef/01 have been isolated from different poultry farms in Egypt (SHEBL et al. 1986; EL KADY et al. 1989; ABDEL-MONEIM et al. 2002;

SULTAN et al. 2004). The commonly used IBV vaccines are derived from the Massachusetts serotypes.