Infectious bursal disease (IBD) is seen in young domestic chickens worldwide and is caused by infectious bursal disease virus (IBDV). Symptoms of the clinical disease can include depression, watery diarrhea, ruffled feathers, and dehydration. Depending on the IBDV strain and presence of maternal immunity, the disease can also present as a clinical or subclinical disease in young chicks. For both clinical and subclinical forms of the disease, all pathogenic IBDVs cause lesions in the bursa of Fabricious. The cloacal bursa can become enlarged, with a yellowish colored transudate on the surface. Hemorrhages on the serosal and mucosal services are sometimes observed. Atrophy of the bursa, which includes the loss of B-lymphocytes, occurs at approximately about 7-10 days after infection. Immunosuppression is directly related to this loss of B-lymphocytes, but immunosuppression and, related consequently, secondary infections are typically seen in birds that recover from the disease. Severity of the immunosuppression depends on the virulence of the infecting virus and age of the host.
Infectious bursal disease is caused by a birnavirus (infectious bursal disease virus; IBDV) that is most readily isolated from the bursa of Fabricius but may be isolated from other organs. It is shed in the feces and transferred from house to house by fomites. It is very stable and difficult to eradicate from premises.
Two serotypes of IBDV have been identified. The serotype 1 viruses cause disease in chickens and, within them, antigenic variation can exist between strains. Antigenic drift is largely responsible for this antigenic variation, but antigenic differences can also occur through genome homologous recombination. Serotype 2 strains of the virus infect chickens and turkeys but have not caused clinical disease or immunosuppression in these hosts. IBDVs have been identified in other avian species, including penguins, and antibodies to IBDV have been seen in several wild avian species. The contribution of IBDV to disease in these wild birds is unknown.