In an effort to produce more stable infectious laryngotracheitis virus (ILTV) vaccines recombinant strains with deletion of genes associated with virulence had been evaluated for attenuation and protection efficacy. Among viral genes associated with virulence a cluster of five open reading frames (ORFs A through E) have been identified. An attenuated ILTV recombinant strain with deletion of the ORF C gene induced protection comparable to that elicited by the tissue culture origin (TCO) vaccine when administered via eye-drop. The objective of this study was to evaluate the attenuation and protection efficacy of the ∆ORF C strain when delivered in ovo to maternal antibody negative (MAb-) and maternal antibody positive (MAb+) embryos. In ovo delivery of the ∆ORF C strain did not affected hatchability or weight-gain, and virus transmission to contact chickens was negligible. However, nine mortalities occurred in the MAb- vaccinated groups of chickens as the result of extensive replication of the ∆ORF C strain in the trachea. Moreover, although still not appropriately attenuated for in ovo vaccination, the ∆ORF C strain induced satisfactory protection in MAb- chickens and marginal protection in MAb+ chickens. The reduced protection observed for MAb+ groups of chickens was likely caused by the interference of maternally derived antibodies. This report presents the use of a genetically attenuated ILTV strain delivered in ovo as a potential new approach in the control of ILTV.

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