Chicken embryo origin-like strains are responsible for Infectious laryngotracheitis virus outbreaks in Egyptian cross-bred broiler chickens.

Infectious laryngotracheitis virus (ILTV) continues to cause respiratory disease in Egypt in spite of vaccination. The currently available modified live ILTV vaccines provide good protection but may also induce latent infections and even clinical disease if they spread extensively from bird-to-bird in the field. Four field ILTV isolates, designated ILT-Behera2007, ILT-Giza2007, ILT-Behera2009, and ILT-Behera2010 were isolated from cross-bred broiler chickens. The pathogenicity based on intratracheal pathogenicity index, tracheal lesion score, and mortality index for chicken embryos revealed that ILT-Behera2007, ILT-Behera2009 and ILT-Behera2010 isolates were highly pathogenic whereas ILT-Giza2007 was non-pathogenic. To study the molecular epidemiology of these field isolates, the infected cell protein 4 gene was amplified and sequenced. Phylogenetic analysis revealed that ILT-Behera2007, ILT-Behera2009, and ILT-Behera2010 are chicken embryo origin (CEO) vaccine-related isolates while ILT-Giza2007 is a tissue culture origin vaccine-related isolate. These results suggest that CEO laryngotracheitis vaccine viruses could increase in virulence after bird-to-bird passages causing severe outbreaks in susceptible birds.