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. |
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. |
