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

Isolates of infectious laryngotracheitis virus ( ILTV) obtained from field disease outbreaks in Korea from 1982 to 1998 were compared by virulence tes ting and by examining restriction endonuclease (REN) cleavage patterns of v iral DNA. Based on pathogenicity tests, eight of 11 ILTV strains were class ified as virulent, because these strains caused 40 to 80% mortality in spec ific pathogen free chickens, while three strains were classified as low vir ulence because these did not cause mortality. The REN cleavage patterns of the low virulence strains were identical with those of two reference vaccin e strains, which were of chicken embryo origin. However, the DNA cleavage p atterns of the virulent strains differed from those of both the low virulen ce and the vaccine strains. Furthermore, one virulent Korean strain N87278 had REN cleavage patterns that were clearly different from other virulent s trains. In the present study, ILTV strains examined could be classified int o two groups (virulent and low virulence strains) by pathogenicity testing, and three groups based on their REN cleavage patterns. These results sugge st that most outbreaks of infectious laryngotracheitis were not likely to b e associated with vaccine strains, but some were associated with viruses in distinguishable from commercial vaccine strains. At least three genetically distinct groupings of ILTV have been involved in outbreaks of infectious l aryngotracheitis in Korea.