HTLV-1 intrafamilial transmission among Japanese immigrants in Brazil.

Human T-lymphotropic virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia/lymphoma and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The aim of this study was to investigate the intrafamilial transmission of HTLV-1 among Japanese immigrants and their descendants living in a non-endemic area of central Brazil. Six families were investigated. Thirty-seven relatives of the six index cases were tested by ELISA for the presence of anti-HTLV antibodies, and the positive cases were confirmed by Western blot. HTLV-1 isolates were genotyped by partial nucleotide sequencing (5' LTR) of the proviral DNA. All individuals, including index cases and relatives, were asymptomatic. In five families, at least one relative was infected with HTLV-1. In all, eight (22%) relatives (one mother, four wives, one brother, and two brothers-in-law) were infected. However, none of the 22 individuals under 55 years of age was infected. In each family, the HTLV-1 sequences from the relatives were identical or almost identical to that of the index case, except in one case. Pedigrees of the families, together with socio-demographic data of the HTLV-1 infected individuals, strongly suggested the occurrence of both vertical and sexual transmission, with breastfeeding as an important risk factor. Whether and why the virus transmission is less effective among younger generations deserves to be further investigated.