A Prospective Study of Sexual Transmission of Human T Lymphotropic Virus (HTLV)–I and HTLV-II

BackgroundCross-sectional studies support sexual transmission of human T lymphotropic virus (HTLV)–I/II; however, prospective incidence data, particularly for HTLV-II, are limited MethodsA cohort of 85 HTLV-positive (30 with HTLV-I and 55 with HTLV-II) blood donors and their stable (⩾6 months) heterosexual sex partners were followed biannually over the course of a 10-year period ResultsFour of 85 initially seronegative sex partners of HTLV-I and -II carriers seroconverted, for an incidence rate (IR) of 0.6 transmissions/100 person-years (py) (95% confidence interval [CI], 0.2–1.6). This includes 2 HTLV-I transmissions/219 py (IR, 0.9 transmissions/100 py [95% CI, 0.1–3.3]) and 2 HTLV-II transmissions/411 py (IR, 0.5 transmissions/100 py [95% CI, 0.06–1.8]), with no significant difference by HTLV type. There were 2 male-to-female (IR, 1.2 transmissions/100 py [95% CI, 0.1–4.3]) and 2 female-to-male (IR, 0.4 transmissions/100 py [95% CI, 0.05–1.6) transmissions. HTLV-I or -II proviral load was 2 log10 lower in newly infected partners than in index positive partners who transmitted HTLV (P=.007) ConclusionsThe incidence of sexual transmission of HTLV-II may be similar to that of HTLV-I, and female-to-male transmission may play a more important role than previously thought. HTLV-I and -II proviral load may be lower in sexually acquired infection, because of a small infectious dose