Infectious laryngotracheitis virus (ILTV; Gallid alphaherpesvirus 1) infects poultry via the epithelial cells of the upper respiratory tract, including the trachea and conjunctiva. To prevent disease, birds are routinely vaccinated with a live-attenuated vaccine. This is most effectively done by eye-drop (ED), however practical constraints in commercial poultry flocks often require drinking water (DW) administration. Changes in efficacy may influence the selection pressures and the type of virus populations able to replicate. Virus adaptation can occur through recombination between co-infecting strains, and these changes may alter the virulence and transmissibility of evolving virus strains.
The aim of this study is to examine the emergence, persistence and transmission of recombinant ILTV strains in SPF chickens and determine how optimal (ED) and sub-optimal (DW) vaccination practises can influence the evolution of ILTV. To understand this process, chickens were vaccinated with Nobilis Serva ILTV vaccine and later co-infected with two different field strains of ILTV, CSW1 and V1-99. Viruses that transmitted from challenged birds to naïve in-contact birds were then compared. Unlike ED vaccinated birds, DW vaccinated birds did not generate a protective immune response. DW and not ED vaccinated birds showed significant virus replication and generated sufficient virus to transmit to naïve in contact birds. Compared to the unvaccinated birds, viruses transmitted from DW birds showed altered populations of recombinants. The impact of these changes is currently being investigated by whole genome sequencing, growth kinetics and reinfection into SPF chickens. These results strengthen the importance of performing optimal vaccination practices in poultry farms to reduce the impact of recombination on viral fitness and the evolvement of new ILTV classes.