A Cell Line Adapted Infectious Laryngotracheitis Virus Strain (BDORFC) for in ovo and Hatchery Spray Vaccination Alone or in Combination with a Recombinant HVT-LT Vaccine

To produce more-stable, live attenuated vaccines for infectious laryngotracheitis virus (ILTV), deletion of genes related to virulence has been extensively pursued. Although its function remains unknown, the open reading frame C (ORF C) is among the genes potentially associated with viral virulence that is nonessential for replication in vitro. Earlier results indicated that the ILT virus with deletion of the ORF C gene (BDORFC) was suitable and safe for eye drop administration but was not sufficiently attenuated for in ovo administration. The objective of this study was to evaluate the safety and protection efficacy of a cell line-adapted, gene-deleted strain (BDORFC) of ILTV when administered in ovo and/or spray (SP) by itself, or in combination with the recombinant HVT-LT (rHVT-LT) vaccine. Results indicated that vaccination with the BDORFC strain, either by itself or in combination with an rHVT-LT vaccine, did not affect hatchability, and only marginal signs of respiratory distress were recorded for groups of chickens that received the BDORFC strain via SP. The replication and seroconversion induced by the BDORFC strain after in ovo and SP administration was very limited, whereas the replication of the rHVT-LT vaccine was delayed when combined with the BDORFC strain in ovo. Compared to rHVT-LT or BDORFC when administered alone, dual vaccination with rHVT-LT + BDORFC was more effective in mitigating clinical signs of the disease and reducing challenge virus load in the trachea. To our knowledge, this study provides the first proof of concept that ILTV strains can be sufficiently attenuated for early vaccination in ovo or at hatch; also, this study documented the benefits of using a dual (recombinant and live attenuated) hatchery vaccination strategy for ILTV.

Automated summary

This study shows that the deletion strain BDORFC of ILTV, administered alone or in combination with rHVT-LT vaccine, did not affect hatchability and only caused minimal respiratory distress when administered via spray. The replication and seroconversion induced by BDORFC strain after in ovo and spray administration was limited, while replication of the rHVT-LT vaccine was delayed when combined with BDORFC strain in ovo. Dual vaccination with rHVT-LT + BDORFC was more effective in reducing clinical signs and challenge virus load in the trachea compared to single vaccination with rHVT-LT or BDORFC.