Different population dynamics of human T cell lymphotropic virus type II in intravenous drug users compared with endemically infected tribes

The phylogeny of human T cell lymphotropic virus type II (HTLV-II) was inve stigated by using strains isolated from Amerindian and Pygmy tribes, in whi ch the virus is maintained primarily through mother-to-child transmission v ia breast-feeding, and strains from intravenous drug users (IDUs), in which spread is mainly blood-borne via needle sharing. Molecular clock analysis showed that HTLV-II has two different evolutionary rates with the molecular clock for the virus in IDUs ticking 150-350 times faster than the one in e ndemically infected tribes: 2.7 x 10(-4) compared with 1.71/7.31 x 10(-7) n ucleotide substitutions per site per year in the long terminal repeat regio n. This dramatic acceleration of the evolutionary rate seems to he related with the mode of transmission. Mathematical models showed the correlation o f these two molecular clocks with an endemic spread of HTLV-II in infected tribes compared with the epidemic spread in Ious, We also noted a sharp inc rease in the population size of the virus among IDUs during the last decade s probably caused by the worldwide increase in intravenous drug use.