Introduction
Infectious bursal disease (IBD) has been observed in chickens since 1957. Birds which survive the disease are permanently immunosuppressed. Therefore they are more susceptible to other disease causing agents and don't respond adequately to vaccinations which are an essential part of poultry management systems.
The bursa of Fabricius (BF) is the primary target organ of IBDV. The virus replicates in immature bursa-derived lymphocytes (B-lymphocytes) of chickens. Chickens infected with IBDV at one day of age are completely devoid of serum immunoglobulin G (IgG) and produce only monomeric IgM. A permanent decrease in the number of peripheral blood B cells was observed following an IBDV infection.
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Molecular Characteristics
Infectious bursal disease viruses are nonenveloped icosahedrons, approximately 58-60 nm in diameter. The genome consists of two segments of double-stranded RNA. The larger mRNA codes for a polyprotein which is cleaved following translation into the major structural protein of the virus.
Birnaviruses have at least four structural proteins. The designation and molecular weights are: VP1 (90,000), VP2 (41,000), VP3 (35,000) and VP4 (28,000). Initially only one serotype was identified, then a second was identified in turkeys. Both serotypes are present in commercially reared chickens, but only serotype 1 viruses are known to cause disease.
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