Enhancing control of virulent recombinant strains of laryngotracheitis virus using vaccination

Abstract

Infectious laryngotracheitis virus (ILTV) is an alphaherpesvirus that causes acute upper respiratory tract disease in chickens. Attenuated live ILTV vaccines are often used to help control the disease, but these vaccines have well-documented limitations including natural recombination between different vaccine strains. Recently, two novel ILTV field strains (class 8 and 9 ILTV viruses) emerged in Australia due to natural recombination involving two distinct commercial ILTV vaccines. These recombinant field strains became dominant in important poultry producing areas and caused severe disease in commercial poultry flocks, showing that more options are needed to enable control of ILTV. The work described in this thesis investigated tools to better control disease due to ILTV.

Firstly, different commercial ILTV vaccines and a developmental candidate vaccine, glycoprotein G-deficient ILTV (ΔgG ILTV, registered as Vaxsafe ILT, Bioproperties Pty Ltd) were investigated for their ability to protect commercial broiler chickens against challenge with the virulent recombinant class 9 ILTV after drinking water vaccine delivery. All vaccines induced partial protection by direct (drinking-water) and indirect (contact) exposure when birds were subsequently challenged with the virulent class 9 challenge strain.

A vaccination and challenge study was then performed to determine the minimum effective dose of ∆gG ILTV that, when delivered by eye-drop to layer birds, would protect the birds from a robust challenge with class 9 ILTV. A dose of 103.8 plaque forming units per bird was the lowest dose capable of providing a high level of protection against challenge, as measured by clinical signs of disease, tracheal pathology and viral replication after challenge.

Finally, attempts were made to develop suitable tools to measure the level of immunity induced by ILTV vaccination. To this end, an ELISA that measures the amount of chicken interferon gamma (IFN-γ) was developed and used to quantitate IFN-γ production from splenocytes stimulated with control mitogens, or with ILTV antigen. The assay could detect IFN-γ released from chicken splenocytes after stimulation by concanavalin A. However, when splenocytes were incubated with semi-purified ILTV antigens in vitro, there was no increase in the level of ILTV specific IFN-γ production by splenocytes from ILTV infected birds, compared to uninfected birds. A number of potential avenues for further development of this assay were identified.

The work described in this thesis demonstrates that currently available vaccines and the new Vaxsafe ILT vaccine can be used to help control class 9 ILTV when delivered by drinking water. When delivered by eye-drop the Vaxsafe ILT vaccine candidate can induce a high level of protection against class 9 ILTV at a commercially feasible dose. Taken together, the results from this work lay the foundations on which a commercial vaccine may be developed, thereby offering the potential to provide producers with another important tool to help control ILTV. Future development of a tool to measure protective immunity after vaccination is needed and would be a valuable addition to disease control programmes