Poultry industry is a domineering section of agriculture sector in the world as it provides meat, income and employment. Of the poultry industry, broiler chicken is dominating, as US export was more than 41 billion pounds of chicken (about 16.5 percent of production) in 2017 . In Pakistan, the poultry industry contributes around 1.4 percent to the GDP and 31 percent to total meat production . The global demand for this meat is rising in developing world including Pakistan. To meet the needs, rearing of poultry at both domestic and commercial levels is imperative . However, the industry faces a lot of constraints, preventing it from reaching its maximal potential. Poor welfare, insufficient quality nutrition and devastating diseases are some of these problems. Amongst the myriad of diseases, Infectious Bursal Disease (also known as Gumboro disease, infectious avian nephrosis and infectious bursitis) is an acute, contagious viral disease of chicken [4-6]. It not only affects domesticated but also wild poultry throughout the world by targeting the immune system. [7-9].
Gumboro disease is caused by Infectious Bursal Disease Virus (IBDV), which is a member of genus Avibirnavirus of the Birnaviridae family. IBDV is a non-enveloped virus having a capsid that contains a double stranded RNA genome, split into two segments A and B . Segment A of the genome encodes four proteins (VP2, VP3, VP4 and VP5) while segment B encodes VP1. Based on Pathogenicity the IBDV strains can be grouped into three pathotypes; classical virulent, antigenic variant and very virulent . Though the first outbreak of very virulent IBD virus was reported in Europe in early 1990s , recent pandemics across Asia, Africa and South America  have heavily damaged commercial and wild poultry. Contamination of a rearing site with the IBDV leads to a horizontal transmission of disease across the flocks through contaminated feed and water. However, vertical transmission is not reported yet . Classical virulent strains flaunt a mortality rate as high as 20-30%, mostly due to the widespread bursal damage in infected poultry . Classical virulent strains are the source of commercially available vaccines against IBD viral infections. In early 1980s, new antigenic variants emerged, causing more than 50 percent mortality in birds by rapid bursal damage . Current vaccines prepared from classical strains have failed to control the disease caused by these antigenically variant strains. In the late 1980s, very virulent IBD virus strains were reported that induced IBD with a more pronounced bursal lesion and higher mortality (up to 90 percent) when compared to other IBDV strains . In addition, vaccination against the very virulent strains encounters many challenges as very virulent IBD virus can breakthrough protective antibodies, and therefore requires more efficient vaccination approaches.