Date Published: February 7, 2019
Publisher: Public Library of Science
Author(s): Tae-Min La, Eun-Jung Choi, Joong-Bok Lee, Seung-Yong Park, Chang-Seon Song, In-Soo Choi, Sang-Won Lee, Ulrich Melcher.
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.
The double-stranded DNA virus family Herpesviridae is classified into three subfamilies—Alphaherpesvirinae, Betaherpesvirinae, and Gammaherpesvirinae . 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 .
Live attenuated CEO ILTV vaccines have been extensively used worldwide to prevent and control laryngotracheitis infections. However, the vaccine itself can convert to virulent strains through bird-to-bird passages and outbreaks . Therefore, understanding the molecular epidemiology of ILTV is essential to distinguish the revertant field strains from attenuated CEO vaccine strains . In the past, PCR and restriction fragment length polymorphism (RFLP) targeting multiple genomic regions have been used to classify ILTV strains. The increased availability of high-throughput sequencing technology at lower costs has enabled the determination of the complete genome sequence of ILTV and the elucidation of the genetic evolution and relationships among ILTV strains. Comparative genome analyses of closely related strains of ILTV or strains isolated from specific geographical regions have been previously reported [36–38].