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Infectious Bronchitis in the USA

An update for 2009, provided by Dr. Mark Jackwood.

Avian infectious bronchitis costs the U.S. poultry industry millions of dollars annually and it remains one of the top research priorities for commercial poultry worldwide. Avian infectious bronchitis is a highly contagious upper-respiratory disease in chickens that is extremely difficult to control because different types of the virus that cause the disease do not cross-protect. Different types of infectious bronchitis virus (IBV) include multiple serotypes and variants of the virus, which emerge due to genetic mutations and recombination events between different strains of the virus during replication and adaptation to the host.

The best strategy for control of this disease is the use of attenuated (non-pathogenic) IB viruses. Typically attenuated viruses are developed by passage of the virus in embryonated eggs. As the virus adapts to grow in eggs, it looses its ability to cause disease in chickens and thus can be developed into safe and effective vaccines. But, because different types of the virus do not cross-protect vaccines must be produced to many different types of the virus.

Picking the correct vaccine type is extremely important for sound protection against IBV. Vaccine type viruses commonly used in the United States are Ark-DPI, Conn, DE072, GA98, and Mass41. Making informed decisions regarding vaccine type to be used in a particular flock of chickens depends on information obtained from constantly monitoring the different types of IBV circulating in the field. We previously published data from 11 years of IBV isolation and identification, collected in the USA from July 1994 to December 2004 (Avian Dis. 49: 614-618, 2005). In that study, we examined a total of 1,511 IBV isolates and not surprisingly, we found that the virus continues to evolve and cause disease outbreaks.

Genetic diversity in Infectious Bronchitis Virus

Infectious bronchitis virus like most RNA viruses has an extremely high mutation rate because the RNA-dependent RNA-polymerase, the enzyme that copies the viral genome during replication, does not have proofreading capability. So when a mistake in copying the genome is made, the enzyme can’t 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). We see this adaptation as variant viruses or emerging new serotypes of the virus.

The most common serotypes of IBV in the USA are Arkansas, Connecticut, Delaware, and Massachusetts. However, when those IBV types undergo changes, also called 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 amount of genetic change, which can accumulate over time, reaches a critical point, the Arkansas vaccine strain (Ark-DPI) may no longer provide sound protection against the Arkansas-like viruses.

In addition to genetic drift, genetic shift can lead to a dramatic change that results in a unique variant virus. These 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. The resulting virus is a hybrid of the two parent viruses that, under the right conditions, can break through immunity induced by the common vaccine types.

Infectious bronchitis virus Arkansas variants

Our data from 2005 and 2006 in the USA, shows that the Arkansas viruses continue to evolve to produce Ark-like viruses, which are commonly isolated in both broilers and layer type birds. But, it appears that sound vaccination with Arkansas type vaccines ( Mildvac Ark) continues to protect against them. We also found that the Ark-DPI strain was and still is the most frequently identified type of IBV in the field. Since that strain is used in the Arkansas vaccine, it was not clear at the time, if those Ark-DPI isolates were field viruses or vaccines.

Recent data (Jackwood et al. Avian Dis. 53:175-183, 2009) indicates that the Ark-DPI isolates are indeed vaccine viruses persisting in the vaccinated flock. It is not clear why only the Arkansas vaccine viruses were persisting in the flocks because other IBV vaccine types were also given to the birds. Further research is being conducted to answer that question, and to provide a possible mechanism for the persistence of Ark-DPI vaccines in broilers in the USA. This work is extremely important because the longer IBV persists in the field the more opportunity it has to undergo genetic drift and shift resulting in new variant viruses.

Infectious bronchitis virus variant GA98

In 1998, a variant of IBV was identified as the cause of a severe and widespread outbreak of the disease and was designated GA98. Molecular and serotypic characterization of the virus showed that it was related to the Delaware strain of IBV. However, currently available vaccines were not providing adequate protection against this new variant virus so a new vaccine, MILDVAC-GA-98 was developed by Intervet, Inc. As an added advantage, MILDVAC-GA-98 was also found to provide good protection against Delaware type strains. Use of this vaccine has lead to the control of this damaging strain of IBV.

Infectious bronchitis virus California variants

Geographically restricted variant viruses have been reported in California since 1975. Recent findings in our laboratory (Jackwood et al. Avian Dis. 51:527-533, 2007) indicate that unique variant strains continue to emerge and cause disease in California chickens. In the 1990’s the California variant (CAV) virus was isolated and identified. That virus was significantly different from the available vaccine strains and caused widespread disease in broilers and breeder type birds. The CAL99 variant viruses were isolated in 1999, and emerged as a distinct genetic and serotypic group from the CAV isolates. In addition, 3 new variant viruses (CA557/03, CA706/03, and CA1737/04) isolated in 2003 and 2004, appear to fall into different and unique molecular and serotypic groups. Unfortunately none of the currently available vaccines are effective against any of the California variant viruses. Until recently, the 2003-2004 California variant viruses appeared to be geographically restricted like the other California variant viruses; however, a recently identified variant virus in Georgia has genetic ties to the California viruses.

Infectious bronchitis virus variant GA08

In 2007-2008, an outbreak of bronchitis in North-Georgia, USA broilers lead to the identification of a new IBV designated GA08. The GA08 virus was genetically similar to the CA557/03 virus. The virus quickly spread throughout Georgia and to surrounding states in the Southeast. None of the commercially available vaccines in the USA either alone or in combination were found to be effective against this new virus. One commercial company is currently using an attenuated live autogenous vaccine and steps are being taken by biologic companies to produce a vaccine against this new IBV type.

Summary

Variant IBV types are constantly emerging and circulating in the field, and disease outbreaks associated with variant types continue to occur. Constantly monitoring IBV types is extremely important because it allows us to follow changes in their incidence and distribution and to identify and control new problematic variant viruses as they arise.

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