Examination of the Australian infectious laryngotracheitis virus vaccines

© 2010 Mauricio Juan Carlos CoppoInfectious laryngotracheitis (ILT) is a viral disease of chickens distributed worldwide and characterised by respiratory signs and decreased egg production and/or decreased weight gain. It is caused by Gallid herpesvirus 1, a member of the Alphaherpesvirinae sub-family, and like other viruses in this sub-family, produces latency in nervous or respiratory tissue of infected individuals. Vaccination using attenuated live viruses propagated in tissue culture or chicken embryos has been extensively used for the prevention of the disease together with strict biosecurity measures; yet ILT remains a threat to the poultry industry. Vaccines attenuated by passage in chicken embryos have been associated with residual virulence, reversion to higher levels of virulence after bird-to-bird passage, establishment of latency with subsequent shedding of virus and outbreaks of disease. In the last few years new vaccines using recombinant technology have emerged as an alternative to conventional vaccines in the prophylaxis of this disease, with specific gene-deletion a promising approach for better control of ILT without the disadvantages of attenuated vaccines. This project aimed to characterise and assess the safety and efficacy of all currently available vaccines against ILT in Australia as well as the safety and efficacy of a novel glycoprotein G deleted infectious laryngotracheitis virus mutant (ΔgG ILTV) developed in this University. This project also aimed to characterise and compare ILTV replication patterns in tracheal mucosa and transmission to naïve chickens following vaccination using different ILTV vaccine strains and different routes of inoculation. For this, three in vivo experiments using SPF chickens were carried out. Results from this study revealed that the levels of pathogenicity of ILTV vaccine strains available in Australia, measured in terms of body weight gain, clinical signs and viral replication in tracheal mucosa assessed by quantitative PCR, varied considerably. Comparatively lower levels of pathogenicity were observed in the group of birds vaccinated with the novel vaccine candidate ΔgG ILTV than those observed in other vaccine groups, this difference was most notable in respect to persistence of vaccine virus in the trachea. Comparison of viral replication and transmission following vaccination via different routes using the same vaccine strain (Serva ILTV) showed that drinking-water vaccination was associated with delayed viral replication compared to eye drop vaccination; however, transmission to in-contact birds was not considerably different. Comparison of viral replication and transmission following vaccination using different vaccine strains via the same route (drinking water) revealed that Serva ILTV was more effectively transmitted to in contact birds soon after primary infection while SA2 ILTV was more effectively transmitted at later time points of the experiment, associated with a period of viral re-excretion. This study also showed that in terms of body weight gain, clinical signs, histopathological lesions and viral replication in tracheal mucosa, all currently available ILT vaccines and the novel vaccine candidate conferred similar levels of protection against challenge with virulent virus. Findings from this project support the safety and efficacy of the novel vaccine candidate ΔgG-ILTV and provide valuable practical information regarding the use of vaccines available in Australia and vaccination procedures, in prospects of enhanced control of ILT