EFFICACY OF VACCINES AGAINST VARIANT INFECTIOUS BURSAL DISEASE VIRUSES TO CONTROL IMMUNOSUPPRESSION IN THE BROILER CHICKEN INDUSTRY IN CANADA

Infectious bursal disease (IBD) is a highly immunosuppressive viral disease in chickens. Currently, the antigenically variant strains of infectious bursal disease virus (vIBDV) are the most prevalent strains of IBDV circulating in Canada. The main objective of the of my research is to investigate the protective efficacy of vaccines against the immunosuppressive effects of IBDV currently circulating in Canada. In the second chapter, we conducted a series of controlled challenge experiments in chickens using vIBDV-SK09, which represents one of the most prevalent field strains of vIBDV in Canada. In this study, we challenged specific pathogen free (SPF) leghorns, maternal antibody (MAb) free broilers, and MAb carrying broilers. Our results revelaed that vIBDV-SK09 is pathogenic and capable of breaking through MAbs. In the third chapter, we found that vIBDV–SK09 can cause immunosuppression, resulting in significantly higher mortality and disease severity in pre-exposed chickens upon challenge with a virulent strain of Escherichia coli. In the fourth chapter, we evaluated two commercial broiler vaccines, recombinant herpes virus of turkey (rHVT)-IBDV and modified live vaccine (MLV), and found that both vaccines failed to confer complete protection against vIBDV-SK09 infection in broilers. However, the MLV but not the rHVT-IBDV vaccine was able to delay vIBDV-SK09 pathogenesis. We also revealed the potential of immunosuppression by rHVT-IBD that allowed early replication of challenged IBDV, thus increasing the viral load in the bursa. In the fifth chapter, we tested five circulating strains of vIBDVs (SK09, SK10, SK11, SK12, and SK13) as potential broiler-breeder vaccine candidates. Progeny challenge using SK09 as challenge virus demonstrated homologous and heterologous protection by SK09 as a vaccine candidate. In conclusion, the overall findings in this thesis demonstrate that vIBDV-SK09 is pathogenic, not amenable to current commercial IBD vaccines, and can cause immunosuppression which in turn may increase the susceptibility of birds to secondary infections such as E. coli. Our data suggest that antigenically relevant vaccine candidate(s) such as vIBDV-SK09 may be useful in controlling IBDV infections in the Canadian broiler chicken industry. Although, we have demonstrated the efficacy of vIBDV-SK09 under laboratory conditions further studies are needed under field situations