Comparison of virulence and restriction endonuclease cleavage patterns of infectious laryngotracheitis viruses isolated in Korea

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Abstract
Isolates of infectious laryngotracheitis virus (ILTV) obtained from field disease outbreaks in Korea from 1982 to 1998 were compared by virulence testing and by examining restriction endonuclease (REN) cleavage patterns of viral DNA. Based on pathogenicity tests, eight of 11 ILTV strains were classified as virulent, because these strains caused 40 to 80% mortality in specific pathogen free chickens, while three strains were classified as low virulence because these did not cause mortality. The REN cleavage patterns of the low virulence strains were identical with those of two reference vaccine strains, which were of chicken embryo origin. However, the DNA cleavage patterns of the virulent strains differed from those of both the low virulence and the vaccine strains. Furthermore, one virulent Korean strain N87278 had REN cleavage patterns that were clearly different from other virulent strains. In the present study, ILTV strains examined could be classified into two groups (virulent and low virulence strains) by pathogenicity testing, and three groups based on their REN cleavage patterns. These results suggest that most outbreaks of infectious laryngotracheitis were not likely to be associated with vaccine strains, but some were associated with viruses indistinguishable from commercial vaccine strains. At least three genetically distinct groupings of ILTV have been involved in outbreaks of infectious laryngotracheitis in Korea.