Molecular Characterization of Infectious Laryngotracheitis Virus from Broiler Diseased Chickens in Northern Vietnam During 2023–2025

PCR-based surveillance identified the infectious laryngotracheitis virus (ILTV) genome in 8 out of 14 (57.14%) commercial broiler farms across three Northern Vietnam provinces. Partial sequencing of the infected cell protein 4 (ICP4) and glycoprotein B, G, and J (gB, gG, and gJ) genes from the ILTV isolates obtained during 2023–2025 indicated that eight Vietnamese ILTV-positive samples were field ILTV strains. Additionally, high nucleotide identity was observed among the isolates, with ranges of 99.74–100% for the gB, 99.63–100% for the gJ, 99.66–100% for the gG, and 99.34–100% for the ICP4 genes. Phylogenetic analyses based on four genes (gB, gJ, gG, and ICP4) revealed that the eight ILTV strains belonged to two distinct lineages, except for the gJ gene. Analysis of the ICP4 gene showed that the five ILTV strains (VNUA-ILTV-02, VNUA-ILTV-05 to VNUA-ILTV-08) clustered with the United States Laryno Vac, Poulvac-ILT, and ON-2462242/1987 strains, whereas the remaining strains (VNUA-ILTV-01, VNUA-ILTV-03, and VNUA-ILTV-04) were in a distinct group. Furthermore, several amino acid changes were observed in the gB, gG, and gJ proteins of the characterized ILTV strains.