Objectives: Infectious laryngotracheitis is an acute respiratory disease in chickens. Vaccinallaryngotracheitis (VLT) is another form of the disease that is caused by vaccine strains, cultivated in embryonated eggs. This vaccine strains can revert their pathogenicity through serial amplification in the field. A deletion of 12 nucleotides in the sequence of ICP4 gene is a proved marker to originate vaccine strains prepared by embryonated egg culture. Investigating the origin of the causative agent is of epidemiologic importance. Due to uncommon prevalence of ILT in laying and breeding hens, vaccination is practiced in some part of Iran, including Khorasan Razavi province. Materials & Methods: To evaluate the type of virus involved in the outbreaks, 15 suspected flocks referred to the Maad poultry professional hospital, during 2008 to 2011, were investigated by PCR amplification and direct sequence analysis of partial ICP4 gene. Results & Conclusion: Eleven out of 15 flocks studied, were found to be positive in PCR assay and confirmed by sequencing. The nucleotide comparison showed 100% similarity between all studied isolates and chicken embryo origin (CEO) vaccine strains. In phylogenetic analysis, all 11 molecular isolates also clustered along with CEO vaccine strains. This finding was in agreement with the finding reported by other researchers regarding the origin of the virus in clinical cases of ILT in some countries. This study reveals that all clinical cases were VLT, indicating that ILT outbreaks in Khorasan Razavi province are mainly due to vaccine strains which turn into the pathogenic virus by serial passages in the field. It might be concluded, when the disease is not endemic in a region, the conventional CEO ILT vaccines should not be introduced.

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