Degenerate and specific PCR assays were developed for bovine leukaemia virus (BLV) and/or primate T cell leukaemia/lymphoma viruses (PTLV). The degenerate assays detected all major variants of the BLV/PTLV genus at a sensitivity of 10-100 copies of input DNA; the specific systems detected 1-10 copies of input target. Sensitivity was 100% in specific DNA-PCR assays done on peripheral blood from seropositive BLV-infected cattle and HTLV-I- or HTLV-II-infected humans, and 62% in RNA/DNA-PCR assays on sera from BLV seropositive cattle. The pol fragments from 21 different BLV strains, isolated from cattle in North and Central America, were cloned and sequenced, and compared to other published BLV and PTLV pol sequences. BLV and PTLV sequences differed by 42%. Sequence divergence was up to 6% among the BLV strains, and up to 36% among the PTLV strains (with PTLV-I and PTLV-II differing among themselves by 15% and 8%, respectively). Some cows were infected with several BLV strains. Among retroviruses, BLV and PTLV sequences formed a distinct clade. The data support the interpretation that BLV and PTLV evolved from a common ancestor many millennia ago, and some considerable time before the PTLV-I and PTLV-II strains diverged from each other. The dissemination of the BLV strains studied probably resulted from the export of European cattle throughout the world over the last 500 years. The relatively similar mutation rates of BLV and PTLV, after their various points of divergence, suggest that there could be a much wider genetic range of BLV than has currently been defined.


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