Efficacy of different vaccination programs of live 4/91 strain against Thai QX-like infectious bronchitis virus in broiler chickens

Thai QX-like infectious bronchitis virus (IBV) has caused problems in commercial chicken farms as well as small farms in many parts of Thailand. This study investigated the level of protection generated by vaccination with different IBV vaccination programs, 4/91 strain against Thai QX-like IBV. One hundred 1 day old female broiler chickens were randomly divided into 5 groups of 20 chickens each. The chickens in Group 1 were vaccinated with Ma5 strain IBV vaccine at 1 day old and with 4/91 strain IBV vaccine at 14 days old. The chickens in Group 2 were vaccinated with Ma5 and simultaneously with 4/91 strain IBV vaccine at 1 day old. The chickens in Group 3 were vaccinated with a combined Newcastle disease virus, C2 strain and IBV, B48 strain vaccine at 1 day old and with 4/91 strain IBV vaccine at 14 days old. The chickens in Groups 4 and 5 did not receive IBV vaccine and served as positive and negative control groups, respectively. At 28 days of age, the chickens in Groups 1-4 were individually challenged with 104.2 EID50 of Thai QX-like IBV (isolate THA80151). Protection was evaluated at 7 days post-inoculation (DPI). Results revealed that the clinical sign and histopathological lesion score of the tracheas of the vaccinated chickens were significantly (p<0.05) lower than those of the non-vaccinated, challenged control chickens. These results indicate that improvement in protection was achieved against the challenge with Thai QX-like IBV by the vaccination programs used in this study. However, complete protection was not observed.