Molecular characterization and phylogenetic analysis of infectious laryngotracheitis virus isolates from commercial chicken flocks in Turkey

Infectious laryngotracheitis virus (ILTV) causes an acute and highly contagious respiratory disease in poultry. Live-attenuated vaccines are generally used to control and prevent infectious laryngotracheitis (ILT). However, these vaccines can revert to a virulent form due to multiple passages and thereby become an ILT source. Hence, monitoring of ILTV in the field through molecular characterization is critically important for controlling infection and differentiating circulating isolates. In this study, we genotypically characterized and phylogenetically analyzed eight ILTV isolates from chicken flocks located in four different cities of Turkey between 2019 and 2022. For all isolates, we analyzed two regions of the infected cell protein 4 gene (ICP4-1 and ICP4-2) and the thymidine kinase (TK) gene. The isolates were 100%, 100%, and 99.8-100% identical to each other in the ICP4-1 and ICP4-2 gene fragments and the TK gene, respectively. None of the ICP4 sequences had a deletion at nt 272-283, confirming that they were field isolates. None of the isolates were predicted to have a T252M mutation in the thymidine kinase, suggesting that they have low virulence. The isolates were 100%, 99.36%, and 99.91% identical to Turkish ILTV isolates in their ICP4-1, ICP4-2, and TK gene region, respectively. Phylogenetic analysis based on the ICP4-1 and TK genes confirmed that the ILTV isolates are closely related to Turkish ILTV isolates. This suggests that these ILTVs were endemic isolates, which in turn suggests that the ILTV isolates circulating in Turkey were evolutionarily close, originated from the field, and had low virulence.