HTLV-1 is sexually transmitted and is found most commonly among sex workers.  Female sex workers in South America/Caribbean have an infection rate of 7-25%. Men who visit an STD clinic in the same region have infection rates of 5 - 10%.  Virus is found in semen and cervical secretions. Transmission, like HIV-1, is more common from male to female and rate depends on the number of sexual partners. Male to male transmission is also common. In Peru, a mother to child transmission rate of 10% is found where breast feeding does not occur while with breast feeding the rate is also 10% when breast feeding is less than 6 months but this rises to 32% when feeding is extended beyond 6 months. Seropositivity increases with age as is found with many others STDs.

Gotuzzo, E. HTLV-1: a new problem for Latin America ASM News, 67, 144, 2001