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The geographic distribution of HTLV-1 is patchy, with a tendency to cluster in certain countries, mainly in the tropics. The highest prevalence has been observed in southern Japan, the Caribbean, equatorial Africa, parts of South America, eastern Siberia, Pacific islands such as Papua New Guinea, and certain Eskimo populations. This suggests that the virus is of ancient origin and has been maintained by vertical transmission in isolated racial groups, dating back to the days before the major waves of intercontinental migration 40,000 to 100,000 years ago when the average human life span was shorter than the incubation period of the diseases we see in HTLV-positive patients today. Spread to the Caribbean iind southeastern United States may have coincided with the much more recent slave trade from Africa.

The observed clustering of infection within particular families indicates that vertical transmission or close contact is required. Infants of seropositive mothers acquire infection more readily if breast-fed. However, the existence of additional currently undefined routes of transmission from mother to child is suggested by the observation that the prevalence of infection in the above-mentioned endemic areas is diminishing in parallel with improving living standards. The second important route of transmission is sexual intercourse, with the receptive partner being most at risk, so accounting for the somewhat higher incidence in females. Third, parenteral transmission is important.

Prevalence in blood donors varies from less than 0.1% in the United States to 5% or more in countries of high endemicity. Transmission by blood transfusion will fall away following more widespread introduction of screening of blood donors, now routine in many developed nations. Hemophiliacs appear not to be at risk, presumably because the virus is cell-associated and is effectively lost during the extraction of factor VIII from plasma. On the other hand, intravenous drug abuse is an increasing hazard, so much so that around 5% of IV drug users are already infected with HTLV; fortunately, most infections appear to be due to HTLV-2, with which no disease has yet been linked. Little is yet known of the natural history of HTLV-2, which has been found to be common in postpubertal Guaymi Indians in Panama, among whom it is considered to spread by sexual intercourse. The search continues for additional HTLV types and for possible additional disease associations.

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