Infectious Bronchitis in the USA

An update for 2007, provided by Dr. Mark Jackwood.

Avian infectious bronchitis costs the U.S. poultry industry millions of dollars annually. So much so, that the American Association of Avian Pathologists continues to list avian infectious bronchitis as the number one research priority for commercial poultry. 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.

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 allows IBV to quickly adapt and change in response to selection pressures such as specific host immune responses (antibodies and T-cells).

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 embryonating 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 different types of the virus do not cross-protect so vaccines must be produced to many different types of the virus.

The most common serotypes of IBV in the USA are Arkansas, Connecticut, Delaware, and Massachusetts. However, those IBV types can undergo changes called genetic drift. 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. Variant viruses are defined as previously unrecognized IBV types that are clearly different from the common IBV serotypes. Variants can rapidly emerge from recombination events that occur by a template switching mechanism along conserved regions of the viral genome when two different viruses infect the same cell. 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.

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 recently 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. Highlights from that study indicated that Ark-like isolates continue to emerge each year.

Our most recent data from 2005 and 2006 also shows that the Arkansas viruses continue to evolve, and are commonly isolated in both broilers and layer type birds. But, it appears that sound vaccination with Arkansas type vaccines (See 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 is not clear if those Ark-DPI isolates are field viruses or vaccines. Given that vaccine type viruses can be easily isolated from recently vaccinated birds, it is not clear why only Ark-DPI type viruses are being identified. Further research is being conducted to answer that question, and to provide a possible mechanism for the high percentage of Ark-DPI virus isolations in the USA.

In 1989, 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 and recently released by Intervet, Inc. As an added advantage, MILDVAC-GA-98 was also found to provide good protection against Delaware type strains. Now for the first time we have the tools necessary to control this devastating strain of IBV.

Geographically restricted variant viruses have been reported in California since 1975. Recent findings in our laboratory 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 isolated in 2003 and 2004, appear to fall into different and unique molecular and serotypic groups. At this time, the viruses have only been identified in California. But, unfortunately none of the currently available vaccines are effective against those variant viruses.

Variant IBV types are constantly emerging and circulating in the field, and disease outbreaks associated with variant types continue to occur periodically. 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.

Top of page