Abstract
This study was designed to examine the impact of competitive non-protective antigens in a bivalent killed vaccine of Newcastle disease virus and infectious bronchitis (IB) virus on seroconversions against protective fusion protein of Newcastle disease (ND) virus (NDV), in free-range layers primed by live ND-clone 30 and IB-H120 vaccines. The experimental design included two free-range layer farms in which sera of randomly chosen layers were collected on two occasions from each of the two farms namely: at the time of administration of the killed booster vaccine (23 weeks of age) and three weeks later. The Western immunoblotting technique was used to react the individual pooled sera collected at different times from each farm with antigens used in priming, namely those of the ND-clone 30 virus and the IB-H120 virus. The optical density bands formed by reactions were compared statistically between seroconverted antibodies at 23 weeks with those of layers aged 26 weeks. The killed booster vaccine offered a significant seroconversion on both farms to the non-protective L-protein (248.0 kDa) of NDV and on one of the two farms to the non-protective NDV-matrix protein (40.0 kDa) (p<0.05). However, seroconversion to the protective fusion protein of NDV (60 kDa) failed on both farms (p<0.05). In addition, on one farm, a statistical significance was revealed by the killed booster vaccine seroconversion to non-protective IBV-nucleocapsid protein (510 kDa) and, on the other farm, to another non-protective IBV-glycoprotein (28.0 kDa) (p<0.05). The impact of competitive seroconversions to non-protective antigens and seroconversion failures to low molecular weights of NDV protective fusion protein is discussed.