The three human T-lymphotropic virus type I subtypes arose from three geographically distinct simian reservoirs.

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To investigate the origin of human T-lymphotropic virus I (HTLV-I), strains of diverse geographical origin were analysed. We sequenced the LTR and env genes of HTLV-I strains from Brazil, Central African Republic, Taiwan and Zaire, and the simian T-lymphotropic virus type I (STLV-I) strain PHSu1 from a baboon from the Sukhumi primate centre. We performed phylogenetic analyses using neighbour-joining, parsimony and maximum likelihood methods. Three separate HTLV-I clusters were identified interspersed between STLV-I clusters. The Brazilian and the Taiwanese strains were within the first well-supported cluster containing all cosmopolitan HTLV-I strains flanked by west African STLV-I strains. The HTLV-I strains from Central African Republic and Zaire fell into a central African cluster close to the chimpanzee STLV-I isolates. The third well-supported cluster included all Melanesian HTLV-I strains and had Indonesian STLV-I strains as closest neighbours. Therefore, currently known HTLV-I strains represent three HTLV-I subtypes that most probably have originated from three geographically distinct interspecies transmission events. The highly divergent PHSu1, isolated from Papio hamadryas, was closely related to PCY-991, isolated from Papio cynocephalus, both from the Sukhumi primate centre. Both clustered together with Asian wild-caught rhesus macaque STLV-I strains suggesting recent interspecies transmission of virus from rhesus macaques to colony-bred African baboons at the Sukhumi primate centre. In the rooted env trees obtained using the STLV strain PH969 as an outgroup, the Asian strains branched off before the African strains, implying an Asian origin for HTLV/STLV type I based on presently available strains.