Fowlpox virus (FWPV) is found worldwide in poultry and wild birds. FWPV is a natural example of recombination between viruses, as reticuloendotheliosis virus (REV) fragments have been found in all poultry FWPVs and these are implicated in virulence alteration. We aimed to determine the commonality of this phenomenon and analysed FWPVs collected from 128 poultry flocks and birds over the last 10 years. Various fragments of both viruses were amplified and sequenced at the FWPV integration site, located between FWPV open reading frames 201 and 203. Seven isolates were found to contain no REV insertions, including fragments of the REV env, gag and 59 REV-long terminal repeat (LTR). We demonstrate here for the first time, the existence of poultry FWPVs without REV inserts (two from chickens, one from turkey FWPV and four from wild birds). The REV inserts were heterogeneous in size. In addition to poultry and wild bird isolates, three FWPV vaccine virus strains were examined and found to contain only remnant REV-LTR and no REV envelope gene fragments.
Fowlpox virus (FWPV) is the prototype of the genusAvipoxvirus and has worldwide distribution in the poultry industry. Infection of commercial chickens and turkeys with FWPV induces two forms of disease; the relatively mild form is characterized by the development of cutaneous lesions on unfeathered areas of the skin and the more severe diphtheritic form by lesions on mucous membranes of the respiratory and gastrointestinal tracts. In addition, FWPV infection causes reduced growth rates and egg production (Tripathy & Reed, 2003).Vaccination with various live vaccines is widely practised and was thought to be effective in preventing the disease, at least until 10 years ago; since then, the vaccination efficacy has been doubted (Fatunmbi & Reed, 1996;Tripathy et al., 1998;Singh et al., 2000). The avian retrovirus reticuloendotheliosis virus (REV) (Witter & Fadly, 2003), which has inserted various genomic fragments into FWPV (Garcia et al., 2003;Hertig et al., 1997;Kim & Tripathy, 2001;Moore et al., 2000;Singh et al., 2003), was implicated as being responsible for the FWPV virulence alteration (Garcia et al., 2003;Fatunmbi & Reed, 1996). Subsequently, a field FWPV isolate containing an REV insert was used to create a modified FWPV in which the REV inserts were eliminated (Singh et al., 2005). By comparing the lesions induced by the two versions of the same FWPV, the contribution of the REV insert to the increased virulence of the progenitor FWPV was confirmed. REV-containing FWPV isolates were also found to be responsible for immunosuppression and even dissemination of REV, if a full infective provirus is integrated into the FWPV (Singh et al., 2000).REV integration also occurred in FWPV vaccine strains, leading to their immediate elimination from use (Hertig et al., 1997). Initial observations from commercial flocks suggested that FWPV vaccine contamination with REV caused tumours in the vaccinated birds (Bendheim, 1973;Bagust & Dennett, 1977;Fadly et al., 1996). The link b...