Abstract
Attenuated live infectious laryngotracheitis (ILT) virus (ILTV) vaccines have been used to prevent and control the outbreak of ILT worldwide. Recent studies using high-throughput sequencing technology have increased the number of complete genome sequences of ILTVs, enabling comparative genome analysis. Although 37 complete genome sequences of ILTV, including vaccine strains, have been reported, the complete genome sequence of any field strain of ILTV in South Korea is yet to be published. In this study, we determined and analyzed the complete genome sequences of three virulent Korean field strains of ILTV (40798/10/Ko, 0206/14/Ko, and 30678/14/Ko). Two of the Korean field strains (40798/10/Ko and 0206/14/Ko) displayed fewer non-synonymous single nucleotide polymorphisms than those of the Serva vaccine strain, indicating that these Korean field strains of ILTV most likely originated from the vaccine strain. The third ILTV strain, 307678/14/Ko, had two regions in the genome showing recombination between the Serva vaccine-like strain and the Australian A20 vaccine-like strain. Comparative genome analysis of ILTV using the Korean field strains with variable virulence can shed light on the recent trend of the emergence of virulent ILTV strains in the field. A few amino acid changes in the genome of ILTV vaccines could enhance the virulence in the vaccine strain, and natural recombination should be considered one of the major risks for the generation of revertant strains of ILTV under field conditions.
Highlights
The double-stranded DNA virus family Herpesviridae is classified into three subfamilies— Alphaherpesvirinae, Betaherpesvirinae, and Gammaherpesvirinae [1]
The vaccine itself can convert to virulent strains through bird-to-bird passages and outbreaks [34]
Understanding the molecular epidemiology of Infectious laryngotracheitis virus (ILTV) is essential to distinguish the revertant field strains from attenuated chicken embryo origin (CEO) vaccine strains [35]
Summary
The double-stranded DNA virus family Herpesviridae is classified into three subfamilies— Alphaherpesvirinae, Betaherpesvirinae, and Gammaherpesvirinae [1]. Infectious laryngotracheitis virus (ILTV) is an alphaherpesvirus that is responsible for respiratory diseases in chickens worldwide. The clinical signs of ILTV infection include conjunctivitis, coughing, nasal discharge, expectoration of bloody mucus, and a reduction in egg production in layers [2]. The nucleotide sequence of a large genomic region of ILTV was first reported in 1990 [3]. Nucleotide sequences of the remaining genomic regions were determined in multiple studies using different strains of ILTV [4,5,6,7,8,9,10,11,12,13]. The first complete genome sequence of ILTV was assembled by overlapping genome fragments of different virus strains [14].
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