Modified live infectious bursal disease virus (IBDV) vaccine delays infection of neonatal broiler chickens with variant IBDV compared to turkey herpesvirus (HVT)-IBDV vectored vaccine

Chickens are commonly processed around 35-45 days of age in broiler chicken industry hence; diseases that occur at a young age are of paramount economic importance. Early age infection with infectious bur sal disease virus (IBDV) results in long-lasting immunosuppression and profound economic losses. To our knowledge, this is the first study comparing the protection efficacy of modified live (MdLV) IBDV and herpesvirus turkey (HVT)-IBDV vaccines against early age variant IBDV (varIBDV) infection in chicks. Experiments were carried out in IBDV maternal antibody (MtAb) positive chicks (n = 330), divided into 6 groups (n = 50-60/group), namely Group 1 (saline), Group 2 (saline + varIBDV), Group 3 (HVT-IBDV), Group 4 (HVT-IBDV + varIBDV), Group 5 (MdLV) and Group 6 (MdLV + varIBDV). HVT-IBDV vaccination was given via the in ovo route to 18-day-old embryonated eggs. MdLV was administered via the subcutaneous route in day-old broilers. Group 2, Group 4 and Group 6 were orally challenged with varIBDV (SK-09, 3 x 10(3) EID50) at day 6 post-hatch. IBDV seroconversion, bursal weight to body weight ratio (BBW) and bursal histopathology were assessed at 19 and 35 days of age. Histopathological examination at day 19 revealed that varIBDV-SK09 challenge caused severe bursal atrophy and lower BBW in HVT-IBDV but not in MdLV vaccinated chicks. However by day 35, all challenged groups showed bursal atrophy and seroconversion. Interestingly, RT-qPCR analysis after varIBDV-SK09 challenge demonstrated an early (9 days of age) and significantly high viral load (similar to 5744 folds) in HVT-IBDV vaccinated group vs unvaccinated challenged group (similar to 2.25 folds). Furthermore, flow cytometry analysis revealed inhibition of cytotoxic CD8(+) T-cell response (CD44-downregulation) and decreased splenic lymphocytes counts in chicks after HVT-IBDV vaccination. Overall, our data suggest that MdLV delays varIBDV pathogenesis, whereas, HVT-IBDV vaccine is potentially immunosuppressive, which may increase the risk of early age varIBDV infection in broilers. (C) 2017 Elsevier Ltd. All rights reserved.