Evaluation of vaccination against infectious laryngotracheitis (ILT) with recombinant herpesvirus of turkey (rHVT-LT) and chicken embryo origin (CEO) vaccines applied alone or in combination.

The chicken embryo origin (CEO) infectious laryngotracheitis (ILT) live attenuated vaccines, although capable of protecting against disease and reducing challenge virus replication, can regain virulence. Recombinant ILT vaccines do not regain virulence but are partially successful at blocking challenge virus replication. The objective of this study was to evaluate the effect of rHVT-LT vaccination on CEO replication and how this vaccination strategy enhances protection and limits challenge virus transmission to naïve contact chickens. The rHVT-LT vaccine was administered at 1 day of age subcutaneously and the CEO vaccine was administered at 6 weeks of age via eye-drop or drinking water. CEO vaccine replication post vaccination, challenge virus replication and transmission post challenge were evaluated. After vaccination, only the group that received the CEO via eye-drop developed transient conjunctivitis. A significant decrease in CEO replication was detected for the rHVT-LT + CEO groups as compared to groups that received CEO alone. After challenge, reduction in clinical signs and challenge virus replication were observed in all vaccinated groups. However, among the vaccinated groups, the rHVT-LT group presented higher clinical signs and challenge virus replication. Transmission of the challenge virus to naïve contact chickens was only observed in the rHVT-LT vaccinated group of chickens. Overall, this study found that priming with rHVT-LT reduced CEO virus replication and the addition of a CEO vaccination provided a more robust protection than rHVT alone. Therefore, rHVT-LT + CEO vaccination strategy constitutes an alternative approach to gain better control of the disease.