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Etiology and Transmission
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.
IBDV may be isolated in 8- to 11-day-old, antibody-free chicken embryos with inocula from birds in the early stages of disease. The chorioallantoic membrane is more sensitive to inoculation than is the allantoic sac. IBDV also may be isolated in cell cultures derived from the cloacal bursa and established avian and mammalian cell lines, and some strains may be isolated in chicken embryo fibroblasts. Cell-culture-adapted strains of IBDV produce a cytopathic effect and may be used for quantitative serologic tests. Two serotypes of IBDV have been identified; within them, antigenic variation between strains is considerable. Serotype 2 infects chickens and turkeys but does not cause clinical disease or immunosuppression.
“Variant” strains of IBDV, which have major antigenic differences from the “standard” strains, have been identified.
Clinical Findings
Infectious bursal disease is highly contagious; results of infection depend on age and breed of chicken and virulence of the virus. Infections may be subclinical or clinical. Infections before 3 wk of age are usually subclinical. Chickens are most susceptible to clinical disease at 3–6 wk, but severe infections have occurred in Leghorn chickens up to 18 wk old.
Early subclinical infections are the most important form of the disease because of economic losses. They cause severe, long-lasting immunosuppression due to destruction of immature lymphocytes in the bursa of Fabricius, thymus, and spleen. The humoral (B cell) immune response is most severely affected; the cell-mediated (T cell) immune response is affected to a lesser extent. Chickens immunosuppressed by early IBDV infections do not respond well to vaccination and are predisposed to infections with normally nonpathogenic viruses and bacteria. Common diseases are usually exacerbated by IBDV infections. Subclinical infections by the “variant” strains occur in young birds, and severe longterm immunosuppression and bursal atrophy result from early infections.
In clinical infections, onset of the disease is sudden after an incubation of 3–4 days. Chickens exhibit severe prostration, incoordination, watery diarrhea, soiled vent feathers, vent picking, and inflammation of the cloaca. Losses may be >20%. Recovery occurs in <1 wk, and broiler weight gain is delayed by 3–5 days. The presence of maternal antibody will modify the clinical course of the disease. Virulence of field strains of the virus varies considerably. Very virulent (vv) strains of the virus that cause high mortality and morbidity were detected first in Europe. These spread throughout the Old World and in 1999 were detected in South America. They were detected in the USA in 2009.
Lesions
At necropsy, the cloacal bursa is swollen, edematous, yellowish, and occasionally hemorrhagic, especially in birds that have died of the disease. Congestion and hemorrhage of the pectoral, thigh, and leg muscles is common. Chickens that have recovered from IBDV infections have small, atrophied, cloacal bursas due to the destruction and lack of regeneration of the bursal follicles.
Control
There is no treatment. Rigorous disinfection of contaminated farms after depopulation has achieved limited success. Live vaccines of chicken embryo or cell-culture origin and of varying low pathogenicity can be administered by eye drop, drinking water, or SC routes at 1–21 days of age. The immune response can be altered by maternal antibody, and the more virulent vaccine strains can override higher levels of antibody.
High levels of maternal antibody during early brooding of chicks in broiler flocks (and in some commercial layer operations) can minimize early infection, subsequent immunosuppression, or both. Breeder flocks should be vaccinated one or more times during the growing period, first with a live vaccine and again just before egg production with an oil-adjuvanted, inactivated vaccine. Inactivated vaccines of chicken embryo, bursa, or cell-culture origin are available. The latter vaccines induce higher, more uniform, and more persistent levels of antibody than do live vaccines. The immune status of breeder flocks should be monitored periodically with a quantitative serologic test such as virus neutralization or ELISA. If antibody levels fall, hens should be revaccinated to maintain adequate immunity in the progeny.
Last full review/revision March 2012 by Y. M. Saif, DVM, PhD
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