Abstract: This study aimed to understand the molecular characteristics of infectious laryngotracheitis virus (ILTV) in China. TK, ICP4, gB, UL32 gene sequences of ILTV BT strain isolated from domestic farms was determined, and its pathogenicity to 2-week-old SPF chickens was studied. Phylogenetic analysis results showed that the nucleotide sequence of BT strain has a consistency with other epidemic strains in China was more than 97%, which were consisted within one branch and there was no significant variation. However, it is quite different from American virulent strain 1874C5 and domestic standard strain WG, which mainly focuses on ICP4 gene. There is a deletion of 4 amino acids at position 87-90 of (AAQD), which is also found in other domestic isolates. The deletion of (QPQEPQ), with 6 amino acids at position 862-868 was consistent with attenuated vaccine strains such as K317, Serva, Laeyngo and LTBlen. When the strain was inoculated back into the larynx and trachea, no chicken death was observed, only a few chickens showed conjunctival secretion. Autopsy showed that the larynx trachea was normal. It is suggested that domestic ILTV epidemic strains are prevalent in chicken flocks in a mild form and show unique genetic characteristics.

Key words: infectious laryngotracheitis virus, sequence analysis, pathogenicity