Attenuation and protection efficacy of ORF C gene-deleted recombinant of infectious laryngotracheitis virus.

Infectious laryngotracheitis (ILT) is a highly contagious respiratory disease of chickens caused by infectious laryngotracheitis virus (ILTV). The disease is controlled by the use of live-attenuated vaccines. Previously we reported the complete nucleotide sequence of the ILTV vaccine strain (TCO) and identified a nonsense mutation in the gene encoding the ORF C protein. This suggested that the ORF C protein might be associated with viral virulence. To investigate this, an ILTV recombinant with a deletion in the gene encoding ORF C was constructed using the genome of the virulent United States Department of Agriculture (USDA) challenge strain (USDAch). Compared to the parental virus, the ΔORF C recombinant replicated in chicken kidney (CK) cells with similar kinetics and generated similar titres. This demonstrated that the ORF C deletion had no deleterious effects on replication efficacy in vitro. In chickens, the recombinant induced only minor microscopic tracheal lesions when inoculated via the intra-tracheal/ocular route, while the parental strain induced moderate to severe microscopic tracheal lesions, even though virus load in the tracheas were comparable. Groups of chickens vaccinated via eye-drop with the ∆ORFC-ILTV were protected to levels comparable to those elicited by TCO vaccination. To our knowledge, this is the first report that demonstrates the suitability of ∆ORFC as a live-attenuated vaccine to prevent the losses caused by ILTV.

Comparison of the pathogenicity of the USDA challenge virus strain to a field strain of infectious laryngotracheitis virus.

Infectious laryngotracheitis virus (ILTV) causes respiratory disease in chickens. This alphaherpesvirus infects laryngeal tracheal epithelial cells and causes outbreaks culminating in decreases in egg production, respiratory distress in chickens and mortality. There are several different vaccines to combat symptoms of the virus, including chicken embryo origin, tissue culture origin and recombinant vaccines. All vaccines licensed for use in the U.S. are tested for efficacy and potency according to U.S. federal regulation using a vaccine challenge assay involving the use of an ILT challenge virus. This challenge virus is provided to biologics companies by the Center for Veterinary Biologics (CVB), United States Department of Agriculture (USDA). The current USDA challenge virus originated from a vaccine strain and has been subjected to multiple passages in eggs, and may not represent what is currently circulating in the field. The objective of this study was to evaluate and compare the pathogenicity of USDA's challenge virus strain to the pathogenicity of a recent ILT field isolate. Using the challenge virus and various dilutions of the field isolate, clinical signs, mortality and pathology were evaluated in chickens. Results indicate that the field isolate at a 1:20 dilution is comparable in pathogenicity to the USDA challenge virus at a 1:4 dilution, and that the ILTV field isolate is a viable candidate that could be used as a challenge virus when evaluating vaccine efficacy.

Glycoprotein G deficient infectious laryngotracheitis virus is a candidate attenuated vaccine.

Infectious laryngotracheitis virus (ILTV), an alphaherpesvirus, causes respiratory disease in chickens and is currently controlled by vaccination with conventionally attenuated virus strains. These vaccines have limitations because of residual pathogenicity and reversion to virulence, suggesting that a novel vaccine strain that lacks virulence gene(s) may enhance disease control. Glycoprotein G (gG) has recently been identified as a virulence factor in ILTV. In this study the immunogenicity and relative pathogenicity of gG deficient ILTV was investigated in SPF chickens. Birds vaccinated with gG deficient ILTV were protected against clinical signs of disease following challenge with virulent ILTV and gG deficient ILTV was also shown to be less pathogenic than currently available commercial vaccine strains. Thus gG deficient ILTV appears to have potential as a vaccine candidate.