Efficacy of a glycoprotein G-deficient infectious laryngotracheitis virus vaccine delivered in the drinking-water.

Infectious laryngotracheitis virus (ILTV) remains a significant viral disease in the poultry industry worldwide and vaccination has proven to be an invaluable tool for disease control. Vaccine type, dose and route of administration are important parameters that determine the success of vaccination programmes and control strategies. The current study aimed to investigate the optimal dose for drinking-water vaccination with ΔgG-ILTV, an attenuated glycoprotein G-deficient ILTV vaccine that is efficacious when administered by eye-drop. Three groups of one-week old specific-pathogen-free chickens were vaccinated with increasing doses of ΔgG-ILTV (10^3.8, 10^4.3 and 10^5.0 plaque forming units per bird) via the drinking-water. Additional groups of birds included an eye-drop vaccination control (n = 20), and two unvaccinated control groups (n = 20 and 10, respectively). Three weeks after vaccination, all groups, except one unvaccinated control group (n = 10), were challenged with virulent ILTV. Vaccine efficacy was assessed after challenge by recording mortality rate and scoring of clinical signs and gross tracheal pathology. Challenge resulted in severe clinical disease and a high mortality rate in unvaccinated birds. Eye-drop vaccination resulted in complete clinical protection against this specific challenge. The efficacy of drinking-water vaccination showed a direct association with the administered vaccine dose. Results from this study highlight the need for improved understanding of virus-host interactions and immunological responses that occur following drinking-water vaccination, in order to improve the efficacy of vaccination strategies that use this route.