In the poultry industry, prophylactic measures against FWPV are achieved primarily by vaccination with live FWPV or antigenically similar pigeonpox virus strains produced in CEF cells [60] . In the past two decades, numerous outbreaks have been reported in vaccinated flocks, suggesting that vaccines used against the disease were not effective. In the United States a commercial FWPV vaccine was shown to be contaminated with REV and caused lymphoma among broiler chickens [86] . It has been shown that sequences of REV have been integrated into the DNA of FWPV vaccines as well as in field FWPV isolates [81, [87] [88] [89] [90] . The integration site is constant, while the size of the integrated fragments differs between various isolates and strains. Two different types of integrated sequences are reported; long terminal repeats (LTRs) with size of approximately 200 to 600 bp and the near-full-length REV provirus of about 800 bp [87, 90, 91] . Most vaccine strains carry only an LTR remnant while most FWPV field isolates carry the nearfull-length provirus. Singh and others [81] , however, detected REV LTRs of various lengths in the genome of two commercial FWPV vaccine strains and four field isolates, while several studies have shown that the source of REV infection was REV-contaminated FWPV [86, [92] [93] [94] and herpesvirus of turkeys vaccines [92, [94] [95] [96] [97] . Reticuloendotheliosis is a tumorigenic and immunosupressive disease. REV strains have been reported to cause diseases characterized by chronic lymphoma, non-neoplastic lesions and a runting-stunting syndrome in chickens, turkeys, and quails [98, 99] . REV are group of avian retroviruses and representatives include the defective REV-T and the non-defective REV-A, spleen necrosis virus (SNV), duck infectious anemia virus, and chick syncytial virus (CSV) [98] . The presence of REV in FWPV vaccines and the failure of currently used FWPV vaccines to evoke high level immunological protection against field challenge of FWPVs are of major concern to the poultry industry [100] , which emphasizes the need for research into alternative vaccines.