Spread of the newly emerging infectious laryngotracheitis viruses in Australia

• The recent status of ILT in Australia was used here as a model to evaluate the evolution of the ILT viruses (ILTVs).
• Many of the recent outbreaks were caused by two recombinant ILTVs with higher virulence and replication capacity.
• Yet another recombinant ILTV emerged and appeared to become a predominant genotype in one state of the country.
• In another state however, another recombinant virus became the predominant virus, replacing the local historical strain.
• ILTV is fast evolving towards a greater transmissibility and therefore greater capacity to spread into ILTV-free areas.

Infectious laryngotracheitis (ILT) is a significant viral disease of chickens in many countries around the globe. In this report the status of ILT in Australia has been used as a model to evaluate the evolution of the ILT viruses (ILTVs). Due to its geographical isolation, Australia harbored a distinct lineage of ILT viruses (ILTV) up to 2007. However examination of the ILT viruses (ILTV) involved in outbreaks between 2007 and 2009 has revealed that many of the outbreaks were caused by two new viral genotypes, class 8 and class 9. These two recombinant viruses were found to emerge as a result of recombination between previously existing live vaccine strains (SA2 and A20), and another live vaccine strain (Serva) introduced into the country in 2007. The new recombinant ILTVs were also shown to possess significantly higher virulence and replication capacity compared with a previously predominant ILTV, class 2. In the current study, examination of a large number of ILTVs isolated from outbreaks between 2009 and 2015 revealed the emergence of yet another recombinant virus (class 10) that appears to have become a predominant genotype in New South Wales. In Victoria however, the recombinant class 9 gradually became the predominant virus, replacing class 2. Therefore, there was an unusual pattern in geographical spread of the newly emerged viruses in different states of the country. These results suggest that ILTV is fast evolving towards a greater transmissibility and therefore greater capacity to spread into ILTV-free areas.