Human T-limphotropic virus 1 infection has a global distribution, with a high prevalence in some regions of Brazil and the world, while HTLV-2 infection is endemic mainly among indigenous people and drug users. To analyze intrafamilial transmission of HTLV-1/2 in five Kayapó indigenous peoples (Gorotire, Kararaô, Kokraimoro, Kubenkokre, and Xikrin do Bacajá), we investigated 1452 individuals who underwent serological and molecular tests. Among the 276 indigenous people with positive results, we identified intrafamily transmission in 42.7% of cases, representing 38 families. It was possible to suggest horizontal and vertical transmissions in 15.8% (6/38) and 47.4% (18/38) of the family groups, respectively. In 15.8%, it was not possible to suggest the route, which indicated that the transmission may have occurred through both vertical and horizontal routes. Through phylogenetic analyses, 35 samples positive for HTLV-2 were sequenced and classified as subtype 2c, and the two samples that tested positive for HTLV-1 were shown to belong to the cosmopolitan subtype, transcontinental subgroup (HTLV-1aA). This study confirms the intrafamilial transmission of HTLV-1/2 infection in indigenous people of the Brazilian Amazon, highlighting the importance of the sexual and mother-to-child transmission routes in maintaining the virus in these people.
