Protection induced by commercially available live-attenuated and recombinant viral vector vaccines against infectious laryngotracheitis virus in broiler chickens

Viral vector vaccines using fowl poxvirus (FPV) and herpesvirus of turkey (HVT) as vectors and carrying infectious laryngotracheitis virus (ILTV) genes are commercially available to the poultry industry in the USA. Different sectors of the broiler industry have used these vaccines in ovo or subcutaneously, achieving variable results. The objective of the present study was to determine the efficacy of protection induced by viral vector vaccines as compared with live-attenuated ILTV vaccines. The HVT-LT vaccine was more effective than the FPV-LT vaccine in mitigating the disease and reducing levels of challenge virus when applied in ovo or subcutaneously, particularly when the challenge was performed at 57 days rather than 35 days of age. While the FPV-LT vaccine mitigated clinical signs more effectively when administered subcutaneously than in ovo, it did not reduce the concentration of challenge virus in the trachea by either application route. Detection of antibodies against ILTV glycoproteins expressed by the viral vectors was a useful criterion to assess the immunogenicity of the vectors. The presence of glycoprotein I antibodies detected pre-challenge and post challenge in chickens vaccinated with HVT-LT indicated that the vaccine induced a robust antibody response, which was paralleled by significant reduction of clinical signs. The chicken embryo origin vaccine provided optimal protection by significantly mitigating the disease and reducing the challenge virus in chickens vaccinated via eye drop. The viral vector vaccines, applied in ovo and subcutaneously, provided partial protection, reducing to some degree clinical signs, and challenge VIRUS replication in the trachea.