Unveiling the genetic landscape of infectious laryngotracheitis virus in Switzerland: Evidence for vaccine-like and wild-type strains.

Infectious laryngotracheitis (ILT) is a respiratory disease affecting chickens worldwide. Unlike many countries, Switzerland does not vaccinate against ILT. This study analysed ILT samples from 21 natural outbreaks in Switzerland using restriction fragment length polymorphism (RFLP) and multiple gene sequencing. Chicken embryo origin (CEO) and tissue culture origin (TCO) vaccine strains were included as references. Both vaccine strains were distinguishable, and 14 out of 21 samples resembled the CEO vaccine. Additionally, four distinct non-vaccine-like groups were identified. Sequencing of three genes from selected Swiss samples and those from neighbouring countries revealed four phylogenetic clades. Notably, four Swiss field strains formed two unique clades, not closely related to vaccine strains or ILTV from neighbouring countries. Overall, RFLP results were supported by sequencing data. This study demonstrates the presence of both vaccine-like and wild-type ILT viruses in Switzerland, where vaccination is de facto prohibited.