Epidemiological Aspects and World Distribution of HTLV-1 Infection

Abstract

The human T-cell leukemia virus type 1 (HTLV-1), identified as the first human oncogenic retrovirus 30 years ago, is not an ubiquitous virus. HTLV-1 is present throughout the world, with clusters of high endemicity located often nearby areas where the virus is nearly absent. The main HTLV-1 highly endemic regions are the Southwestern part of Japan, sub-Saharan Africa and South America, the Caribbean area, and foci in Middle East and Australo-Melanesia. The origin of this puzzling geographical or rather ethnic repartition is probably linked to a founder effect in some groups with the persistence of a high viral transmission rate. Despite different socio-economic and cultural environments, the HTLV-1 prevalence increases gradually with age, especially among women in all highly endemic areas. The three modes of HTLV-1 transmission are mother to child, sexual transmission, and transmission with contaminated blood products. Twenty years ago, de Thé and Bomford estimated the total number of HTLV-1 carriers to be 10-20 millions people. At that time, large regions had not been investigated, few population-based studies were available and the assays used for HTLV-1 serology were not enough specific. Despite the fact that there is still a lot of data lacking in large areas of the world and that most of the HTLV-1 studies concern only blood donors, pregnant women, or different selected patients or high-risk groups, we shall try based on the most recent data, to revisit the world distribution and the estimates of the number of HTLV-1 infected persons. Our best estimates range from 5-10 millions HTLV-1 infected individuals. However, these results were based on only approximately 1.5 billion of individuals originating from known HTLV-1 endemic areas with reliable available epidemiological data. Correct estimates in other highly populated regions, such as China, India, the Maghreb, and East Africa, is currently not possible, thus, the current number of HTLV-1 carriers is very probably much higher.

Keywords: HTLV-1; HTLV-1 epidemiology; HTLV-1 in Africa; HTLV-1 in Asia; HTLV-1 in Europe; HTLV-1 in Oceania; HTLV-1 in the Americas; HTLV-1 world distribution.

Figure 1

Map of the geographical distribution of HTLV-1 subtypes (A–G), and the main modes of viral dissemination by movements of infected populations. Small arrows indicate the very probable interspecies transmission of STLV-1 (S) from monkeys to Humans (H) at the origin of some current HTLV-1 subtypes. These different subtypes comprise the Cosmopolitan A subtype with its different subgroups: TC (Transcontinental being the most frequent and widespread one), Awa (West African), Ana (North African), Ajp (Japanese), B or Central African being the most frequent in this large endemic area, C or Australo-Melanesian D, also from Central Africa and present especially in certain Pygmy groups and lastly E, F, G with very few strains yet reported (all in Central Africa). The main HTLV-1 molecular epidemiological studies used to draw this map are the following ones: (Gessain et al., ; Gasmi et al., ; Miura et al., ; Mahieux et al., , ; Salemi et al., ; Vandamme et al., ; Wolfe et al., ; Cassar et al., ; Gessain, 2011).

Figure 2

Geographical distribution of the main foci of HTLV-1 infection. Estimates of the number of HTLV-1 infected carriers, based on approximately 1.5 billion of individuals from known endemic areas and reliable epidemiological data obtained from studies among pregnant women and/or blood donors and/or different adult populations. In few countries, HTLV-1 endemic areas are limited to residents of certain regions such as Mashad in Iran, The Fujian Province in China, Tumaco in Colombia and Central Australia.