Vertical transmission is thought to play an important role in the maintenance of virus in areas of high endemicity. Transmission through breast milk is implicated as a major route for the maintenance of infection in high prevalence areas. Seroprevalence of HTLV-I increases with age and is twice as high in females than males. This gender difference usually emerges after 30 years and probably reflects more efficient transmission from males to females during sexually active years. HTLV- II infection is particularly common in IV drug abusers, and has been found in clusters among certain South American Indians.
Initial symptoms are bilateral weakness and stiffness of the lower extremities.
The course is slowly progressive, usually with bladder involvement, but shows considerable variations. It is likely that the spectrum of disease associations with increase with time. There is a possibility that some of the agents currently in use against HIV, especially the nucleoside analogue inhibitors, may work against HTLV-1. However, since ATL and TSP present years following infection, there appears little justification in using antiviral therapy in healthy carriers. A combination of interferon-alpha and zidovudine had been reported to be effective in treating ATL patients.