Integrated reticuloendotheliosis virus genomic sequences in fowlpox virus

Fowlpox is a common viral disease in domestic poultry, with a worldwide distribution. The negative impacts of the disease are reduction in egg production and retarded growth in younger birds. Vaccination with live vaccines has long been used for the control of the disease in the poultry industry and economic losses have been minimized. However, in recent years, this disease re-emerged as a new threat to poultry production, particularly in vaccinated flocks. Reticuloendotheliosis virus (REV) sequences were detected in the genomes of the fowlpox virus (FPV) field isolates causing the disease outbreaks and subsequently in archival virus isolates. It is interesting that this integration event occurred over 50 years ago. It has been found that most fowlpox field viruses contain REV provirus, while vaccine strains (except FPV S strain, an obsolete Australian vaccine strain) contain only remnants of REV long terminal repeats (LTRs). The integrated REV provirus appears to be infectious, as antibody responses to REV have been detected in infected birds. The presence of REV provirus in FPV field isolates has resulted in enhanced pathogenicity of the virus. In addition, clinical signs associated with REV, including runting disease syndrome and immunosuppression, have been demonstrated in young birds. The retention of the REV provirus may have resulted in modification of host gene (FPV genome) activities such as gene activation or inactivation, giving rise to alteration of the viral phenotype. This may have contributed to antigenic variation, leading to inadequate protection in vaccinated flocks