Multiple infection of chickens and turkeys with avian oncogenic viruses: prevalence and molecular analysis.

The avian herpesvirus, Marek's disease virus (MDV) and several retroviruses, reticuloendotheliosis virus (REV), avian leukosis virus (ALV) (chickens) and lymphoproliferative disease virus (turkeys) are oncogenic and immunosuppressive agents. These viruses were detected either alone, or in various combinations in blood and tumor DNAs of commercial birds using PCR. We present a 5-year retrospective study that included 207 chicken and 52 turkey flocks. Of these, 32 chicken and 18 turkey flocks were negative. Of the positive chicken and turkey flocks 76% and 75%, respectively, had a single, while the rest, 24% and 25%, had a multiple virus infection. In the chickens of the multiple virus-infected flocks, 14% and 17% of the blood and tumor DNAs carry dual MDV and REV and/or ALV sequences, that is about 30% of the PCR-positive, and about 5% of the total DNAs analysed. Multiple virus sequences were detected only in the turkey blood DNAs-11% of 84 samples. Following that quantitation we aimed to analyse the molecular status of the retrovirus sequences in order to determine whether retrovirus sequences were integrated into the herpesvirus genome. We focused on the MDV BamH1-H 132 bp tandem repeat fragment proximity using a combined PCR (cPCR) to identify chimeric PCR products. That included amplification with heterologous combinations of the MDV and retroviral LTR primers. In 13 of 35 DNAs that had both MDV and retrovirus sequences new products were produced. Of 4 MDV + REV chimeric products that were sequenced, one was homologous to the Chicken Repeat element 1 non-LTR type retrotransposon. No evidence for a retrovirus LTR integration was found in the 132 bp repeat proximity, but in two of these products we detected nucleotide stretches of 20 bp and 21 bp with a 70% and 71% homology to the REV-LTR. Also, the amplification of the chimeric products using a retrovirus primer denoted that at least short nucleotide stretches homologous to retroviral LTR primer were present in these DNAs, and that they might resemble ancient retroviral insertions, as previously demonstrated (Isfort et al., 1992).