International Journal of Poultry Science 11 (11): 710-717, 2012 ISSN 1682-8356 © Asian Network for Scientific Information, 2012 Corresponding Author: Awad A. Shehata, Poultry and Rabbit Diseases Department, Faculty of Veterinary Medicine, Minoufiya University, Minoufiya, Egypt 710 Efficacy of HVT-IBDV Vector Vaccine Against Recent Egyptian vvIBDV in Commercial Broiler Chickens Hesham Sultan , Hussein A. Hussein , Alaa G. Abd El-Razik , Sallah El-Balall , 1 2 1 3 Shaima M. Talaat and Awad A. Shehata 1 1,4 Department of Poultry and Rabbit Diseases, Faculty of Veterinary Medicine, 1 Minoufiya University, Minoufiya , Egypt Department of Virology, Faculty of Veterinary Medicine, Cairo University, Giza, Egypt 2 Department of Pathology, Faculty of Veterinary Medicine, Minoufiya University, Minoufiya, Egypt 3 Institute of Microbiology, Faculty of Veterinary Medicine, Leipzig University, Leipzig, Germany 1,4 Abstract: The efficacy of a turkey herpesvirus (HVT-IBD) vector vaccine encoding the VP2 gene of Infectious Bursal Disease (IBD) was evaluated in comparison with classic vaccines on two commercial broiler chicken farms (A and B), with high levels of Maternal Derived Antibodies (MDA). The HVT-IBD vaccine was administrated into chickens of farm A at 1-day-old while intermediate and intermediate-plus vaccines were given at 1-and 14-day-old into chickens of farm B, respectively. An early significant seroconversion was detected at 21-day-old in chickens of farm A, while no significant serological response was detected in farm B at this time. The antibody level was significantly higher in chickens in farm A at 28-and 35-day-old. Moreover, HVT-IBD vaccine provided complete protection against recent Egyptian vvIBDV isolate. In conclusion, HVT-IBD vaccine has the ability to induce an immune response in birds with high levels of MDA and could protect against recent Egyptian vvIBDV isolates. Key words: Recombinant viral vectored vaccines, infectious bursal disease, herpesvirus of turkeys, very virulent IBDV INTRODUCTION Infectious Bursal Disease (IBD) or Gumboro disease, caused by IBDV, is an acute and highly contagious disease in chickens at 3 weeks of age and older. It causes high mortality and immunosuppression in recovered chickens leading to a variety of secondary infections and a decreased response to vaccinations, which results in an important economic impact to the poultry industry worldwide (van den Berg et al., 2000). IBDV targets the bursa of Fabricius, causing damage by destroying developing B-lymphocytes. Several clinical forms of IBD are observed in the field ranging from mild immuno-depression to high mortality (90-100%). The latter is caused by very virulent IBDV (vvIBDV) strains (Chettle et al., 1989). The double stranded RNA (dsRNA) viral genome consists of two segments, A and B (Murphy et al., 1995). Segment B, the smaller, encodes the 97 kDa VP1 with polymerase and capping enzyme activities, while segment A contains a large Open Reading Frame (ORF) encoding a 110 kDa precursor protein that is processed into the mature structural proteins VP2 and VP3 by viral protease VP4 (Boot et al., 2001). The VP2 is responsible for serotype specificity; conversely, VP3 is a group-specific antigen that is recognized by non- neutralizing antibodies, which may cross-react with both serotypes (Oppling et al., 1991). The VP2 has been identified as the major host-protective antigen and conformational dependent immuno-dominant epitopes located in a hyper variable region of VP2 and capable of eliciting viral neutralizing antibodies against IBDV (Becht et al., 1989; Fahey et al., 1989; Islam et al., 2001; Ashraf et al., 2007). Different Modified Live Vaccines (MLVs) containing classical or variant viruses are commercially available and are classified according to their degree of attenuation as "mild", "intermediate", "intermediate plus" and "hot" IBD vaccines (Saif, 1998; van den Berg, 2000; Muller et al., 2003). Mild and intermediate vaccines are safer, in that they cause less bursal damage, than "hot" vaccines, but have a poor efficacy in the presence of MDA and against vvIBDVs. In contrast, less attenuated strains ("intermediate plus" and "hot" vaccines) can overcome higher levels of MDA, but they may cause more severe lesions in the bursa follicles, resulting in immunosuppression. These strains are not recommended for chickens younger than 10 days of age (Prandini et al., 2008). Many new IBDV vaccines were developed, including subunit, DNA and vector vaccines. Most of them are still experimental but some have been used commercially. The main advantage of these vaccines is their ability to overcome difficulties in managing MDA on vaccine intake (Bublot et al., 2007; Hsieh et al., 2007; Rong et al., 2007; Villegas et al., 2008; Le Gros et al., 2009; Rojs et al., 2011). Among these vaccines, a recombinant turkey herpes virus