Molecular characterization of infectious laryngotracheitis virus in naturally infected egg layer chickens in a multi-age flock in Brazil

The virus responsible for an outbreak of infectious laryngotracheitis (ILT) in a multi-age flock of egg layer chickens under quarantine in Brazil was characterized. Layer chickens from this area with circulating gallid herpesvirus 1 (GaHV 1) were evaluated using histopathology and molecular characterization techniques based on sequences of infected-cell polypeptide 4 (ICP4) and thymidine kinase (TK) genes. The infected chickens that were analyzed were PCR-positive for GaHV-1 in the trachea and negative in most trigeminal ganglia. The lack of ILT lesions in the conjunctiva and respiratory tissues, combined with detection of viral DNA in the trachea, was found to be associated with latent infection. The sequences from five farms obtained in the present study were identical, and there were no deletions within the 272- to 283-bp region of the ICP4 gene, as observed in the sequences of vaccine strains (CEO and TCO). The lack of a deletion in the ICP4 fragment analyzed in this study indicates that the chickens were infected with a field virus. The absence of the T252M mutation in a fragment of the TK gene, in addition to the low mortality rate observed, suggests that the outbreak in the state of Minas Gerais was not caused by a highly virulent strain but rather by a field virus of lower virulence. In addition, using phylogenetic reconstructions, it was found that this field strain was grouped together in a separate branch, apart from the previously characterized Brazilian strains. The introduction of vectored vaccines apparently has been effective in reducing clinical disease and lesions, and preventing new outbreaks of disease.