Evolutionary strategies of human T-cell lymphotropic virus type II

Human T-cell lymphotropic virus type II (HTLV-II) primarily infects two dif ferent populations in which the virus is transmitted in very diverse ways. In endemically infected populations, the virus is propagated through sexual contact, and by mother to child transmission via breast-feeding, among int ravenous drug users (IDUs), spread is mainly due to blood-borne transmissio n via needle sharing. The phylogeny of HTLV-II strains isolated from Americ an Indian and Pygmy tribes and strains from IDUs, reveal that the virus ori ginated on the African continent as a result of a simian to human transmiss ion at least 400,000 years ago. HTLV-II was very likely introduced into the American continent during one or more migrations of HTLV-II infected Asian populations over the Bering land bridge, some 15,000-35,000 years ago. Dur ing the last few decades, HTLV-II has been transmitted from native American Indians to IDUs at least twice, followed by a rapid spread of the virus in the drug users population world-wide due to the practice of needle sharing . Molecular clock analysis showed that HTLV-II has two different evolutiona ry rates, with the molecular clock for the virus in IDUs ticking 150-350 ti mes faster than the one in endemically infected tribes: 2.7 x 10(-4) compar ed to 1.7/7.3 x 10(-7) nucleotide substitutions per site per year in the LT R region. Although many of the HTLV-II infected drug users are co-infected with HIV, the dramatic acceleration of the evolutionary rate seems to be ma inly related to the different modes of transmission in the two populations. These contrasting evolutionary rates correlate with an endemic spread of H TLV-II in infected tribes compared to an epidemic spread in IDUs. (C) 2000 Elsevier Science B.V. All rights reserved.