It is economically important to the poultry industry worldwide due to increased susceptibility to other diseases and negative interference with effective vaccination.

In recent years, very virulent strains of IBDV (vvIBDV), causing severe mortality in chicken, have emerged in Europe, Latin America, South-East Asia, Africa, and the Middle East.

Viruses belonging to one of these antigenic subtypes are commonly known as variants, which were reported to break through high levels of maternal antibodies in commercial flocks, causing up to 60 to 100 percent mortality rates in chickens.

The greatest bursal mass is mostly a result of a large population of maturing IgM-bearing B-lymphocytes (lymphoblasts), the main target of infection.

However, the B-cell destruction is usually most severe in subclinically infected young, as virus will destroy a smaller population and most cells in one place (the bursa).

Chickens infected with the disease generally have the following symptoms: pecking at other chickens, high fever, ruffled feathers, trembling and slow walking, found lying together in clumps with their heads sunken towards the ground, diarrhea, yellow and foamy stool, difficulty in excretion, reduced eating or anorexia.

[6] A preliminary diagnosis can usually be made based on flock history, clinical signs and post-mortem (necropsy) examinations.

RT-PCR (as mentioned above) was designed for the detection of IBDV genome, such as VP1 coding gene, with the possibility of PCR product sequences be determined for genetically comparing isolates and producing phylogenetic trees.

Serological tests such as agar gel precipitation and ELISA, for detecting antibodies, are used for monitoring vaccine responses and might be additional information for diagnosis of infection of unvaccinated flocks.

Necropsy examination will usually show changes in the bursa of Fabricius such as swelling, oedema, haemorrhage, the presence of a jelly serosa transudate and eventually, bursal atrophy.

Breeder flocks may be immunised against IBD so that they would transfer protective antibodies to their progenies, such as broiler and pullet chicks.