Protection of chickens against infectious bronchitis virus with a multivalent DNA vaccine and boosting with an inactivated vaccine

Yan, Fang; Zhao, Yujun; Hu, Yongting; Qiu, Jianyang; Lei, Wenxin; Ji, Wenhui; Li, Xuying; Wu, Qian; Shi, Xiumin; Li, Zhong

Titers of antibodies against IBV in the serum samples were measured using an enzyme-linked immunosorbent assay (ELISA) kit (Idexx Laboratories, USA) according to the manufacturer's instructions. The optical density at 650 nm (OD 650 ) was measured using a microplate reader (model 680; Bio-Rad, USA). Each serum sample, including the negative and positive controls, was analyzed in triplicate. Negative and positive sera for IBV were obtained from China Institute of Veterinary Drugs Control. Titers of virus neutralizing antibody against IBV were also measured as previously described [25] . Briefly, serial 2-fold dilutions of serum samples were mixed with 100 EID 50 of IBV16 strain and kept at room temperature for 1 h. 200 μL the virus-serum mixtures in each dilution were inoculated into the allantoic cavity of 10-day-old SPF chicken embryos in ten replicates. The embryos were also inoculated with 100 EID 50 alone in parallel. Seven days after inoculation, characteristic IBV lesions such as dwarfing, stunting, or curling of embryos were examined. The virus neutralizing antibody titer of each sample was recorded at the highest serum dilution value which protected 50% of the embryos from death.