ReviewResearch on infectious bursal disease—the past, the present and the future
Section snippets
Forty years of “Gumboro disease”
When infectious bursal disease (IBD) appeared in chickens in 1962, the disease was designated as “Gumboro disease” after the geographic location of the first recorded outbreaks. Infections caused by IBD virus (IBDV) may exacerbate infections with other etiologic agents, and reduce the chicken’s ability to respond to vaccination. Since the first report, IBD has gained the attention of the poultry industries all over the world. The economic impact of IBD is influenced by strain of virus,
Structure and molecular biology of IBDV
IBDV particles have a non-enveloped, icoasahedral capsid with a diameter of about 60 nm. The structure of the virus is based on a T=13 lattice and the capsid subunits are predominantly trimer clustered (Böttcher et al., 1997). Characterization of the viral genome as a bi-segmented double-stranded (ds) RNA (Müller et al., 1979) allowed placing IBDV into a new family of viruses, the Birnaviridae (Dobos et al., 1979); there it represents the prototype member of the genus Avibirnavirus (Leong et
IBDV antigenicity
Two serotypes of IBDV can be differentiated by virus neutralization test (McFerran et al., 1980). Serotype 1 contains the pathogenic strains, whereas serotype 2 strains, mainly isolated from turkeys, cause neither disease nor protection against serotype 1 strains in chickens. Antigenic variant strains have been reported in the US (Snyder et al., 1988), Central America (Jackwood and Sommer, 1999) and, only recently, in Australia (Sapats and Ignjatovic, 2000). The antigenic site responsible for
IBDV pathogenicity
Experiments in which bursectomized chickens survived IBDV infections lethal for normal chickens demonstrate that the BF is the target organ for the pathogenic serotpye 1 strains. High concentrations of viral antigens and high infectivity titres have been demonstrated in the BF, whereas only traces of antigen and low virus titres were detected in the thymus and spleen. Analogous results were obtained with lymphoid cells isolated from these organs. In vitro infection studies showed that IBDV
Virulence and attenuation
The outcome of an IBDV infection largely depends on the strain and the amount of the infecting virus, the age and the breed of the bird, the route of inoculation, and the presence or absence of neutralizing antibodies. Reassortant serotype 1/serotype 2 IBDV show that genome segment A determines bursa tropism whereas segment B is involved in the efficiency of virus replication (Zierenberg et al., 2003). The pathogenic serotype 1 field isolates can be grouped into classical virulent (cv) or vv
Diagnosis
In chicken flocks, the clinical picture and the course of the disease usually are indicative of an IBDV infection. Pathological changes observed at the BF are characteristic, and histopathological investigations combined with the demonstration of viral antigens by immunohistochemistry confirm an IBDV infection. IBDV can be isolated by the inoculation of antibody-free embryonated chicken eggs. Viral antigens can be demonstrated by the agar-gel precipitin assay or by the antigen-capture
Vaccination
IBDV is highly infectious and very resistant to inactivation. Therefore, despite strict hygienic measures, vaccination is inevitable under high infection pressure and mandatory to protect chickens against infection during the first weeks after hatch. To induce high titres of maternally derived antibodies that persist over the whole laying period, layers are vaccinated with inactivated oil-emulsified vaccines. After hatching, chickens are immunized with live vaccines. The time-point of
Concluding remarks
During the last decades, chicken meat has increased its proportion of the total meat market because of price advantage and a positive health image among consumers. Consumer demand in the area of food safety appears to be for a product without “chemicals” and without pathogens. In the future, microbial contamination may become a major issue for the chicken industry. Consumers also expect chicken meat to be produced from flocks in which the needs of animal health and welfare have been fulfilled.
Acknowledgements
M.R. Islam was supported by the Alexander von Humboldt Foundation (Georg Forster Research Fellowship Programme).
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