Biological and molecular characterization of infectious laryngotracheitis virus (ILTV) from the United States
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Infectious laryngotracheitis (ILT) is an upper-respiratory disease of poultry of worldwide distribution. The disease is caused by infectious laryngotracheitis virus (ILTV) a member of family Herpesviridae, is characterized by acute respiratory signs, and is common in areas of intense poultry production. The disease is controlled by vaccination with live attenuated vaccines. Once vaccine strains have been introduced in the field the differentiation of ILTV strains is difficult because of the antigenic and genomic homogeneity of the vaccines and field viruses. In this study, polymerase chain reaction and restriction fragment polymorphism (PCR-RFLP) of genome regions was utilized to characterize ILTV isolates from commercial poultry and backyard flocks from the US. Combinations of PCR-RFLP patterns classified the infectious laryngotracheitis virus (ILTV) isolates into different groups, as well as backyard flock isolates. Some of the isolates were closely related to the CEO vaccine strains. However isolates different to the vaccine strains were also identified in commercial poultry. Sequencing analysis of multiple genome regions were used to validate genotype groups obtained by PCR-RFLP analyses. This study presents the first comparative sequencing analysis for a wide variety of ITLV isolates from the US where specific genome differences were identified for commercial poultry and backyard flock isolates from the US. In addition, the evaluation of clinical signs, viral tissue distribution in chickens, viral replication in cell culture, and plaque formation ability of several ILTV isolates categorized into different PCR-RFLP or sequencing groups showed biological differences among US ILTV isolates.