HTLV in South America: Origins of a silent ancient human infection

The description of the first human retrovirus, human T-lymphotropic virus 1 (HTLV-1), was soon associated with an aggressive lymphoma and a chronic inflammatory neurodegenerative disease. Later, other associated clinical manifestations were described, affecting diverse target organs in the human body and showing the enormous burden carried by the virus and the associated diseases. The epidemiology of HTLV-1 and HTLV-2 showed that they were largely distributed around the world, although it is possible to locate geographical areas with pockets of low and very high prevalence and incidence. Aboriginal Australians and indigenous peoples of Brazil are examples of the large spread of HTLV-1 and HTLV-2, respectively. The epidemiological link of both situations is their occurrence among isolated, epidemiologically closed or semi-closed communities. The origin of the viruses in South America shows two different branches with distinct timing of entry. HTLV-1 made its probable entrance in a more recent route through the east coast of Brazil at the beginning of the slave trade from the African continent, starting in the 16th century and lasting for more than 350 years. HTLV-2 followed the ancient route of human migration from the Asian continent, crossing the Behring Strait and then splitting in South America as the population became separated by the Andes Mountains. By that time, HTLV-2c probably arose and became isolated among the indigenous populations in the Brazilian Amazon. The study of epidemiologically closed communities of indigenous populations in Brazil allowed tracing the most likely route of entry, the generation of a new molecular subtype (HTLV-2c), the elucidation of the vertical transmission of HTLV-2, the intrafamilial aggregation of cases and the escape and spread of the virus to other areas in Brazil and abroad. Despite the burden and impact of both viruses, they are maintained as silent infections among human populations because 1, health authorities in most South American countries in which national surveillance is poor have little interest in the disease, 2, the information is commonly lost as indigenous groups do not have specific policies for HTLV and other sexually transmitted infections, and 3, health access is not feasible or properly delivered.