Evaluation of five circulating strains of variant infectious bursal disease virus (varIBDV) for their immunogenicity as broiler breeder vaccines and protective efficacy in neonatal broiler chicks.

The majority of infectious bursal disease virus (IBDV) strains circulating in the broiler chicken industry in Canada are variant strains (varIBDV). Despite high levels of maternally derived antibodies (MtAb), the circulating varIBDVs can establish infection and cause severe immunosuppression in broiler chicks. The objective of this study was to evaluate circulating varIBDVs as broiler breeder vaccine candidates and investigate their protective efficacy against varIBDV challenge in their progeny chicks. Six groups of breeders (20 females/group) were vaccinated with varIBDV strains, SK09, SK10, SK11, SK12, and SK13 or saline at the age of 13 weeks and antibody response was determined by ELISA at 3-7-, and 20- weeks post-vaccination. We also included commercial chicks for the comparison. Results showed that SK-09 is the most antigenic strain, followed by SK-10, SK-12, and SK-13. In contrast, SK-11 showed the lowest antibody response, and over time, antibody titers steadily decreased. Eggs from breeders were collected at 21-week post-vaccination and incubated to produce their respective progenies. The serum antibody titer in day-old chicks showed a successful MtAb transfer. Progeny chicks (n = 40/group) were orally challenged with varIBDV-SK-09 strain at 6 days of age and serum antibody titer (19 d and 35 d of age), bursa to body weight ratio (19 d and 35 d of age), bursal viral load (9 d and 19 d of age) was examined to assess the protection against IBDV. Following the challenge, we found a significant increase in the antibody titers in MtAb-free and commercial vaccine groups than in the varIBDV groups, both at 19 d and 35 d of age. The BBW ratio and viral load data indicated a significant homologous and heterologous protection against varIBDV-SK-09 challenge by SK-09 and SK-10 MtAbs, respectively. Overall, this study demonstrated the feasibility of developing breeder vaccines using circulating varIBDV as candidate vaccine antigens.