Modes of transmission and evidence for viral latency from studies of human T-cell lymphotrophic virus type I in Japanese migrant populations in Hawaii.

Human T-cell lymphotrophic virus type I (HTLV-I) seroprevalence was 20% among Hawaiian Japanese migrants (issei) and their offspring (nisei) from Okinawa compared to 35% in similarly aged men who were lifetime residents of Okinawa. A control group of migrants from a nonendemic area of Japan, Niigata, had low rates of HTLV-I antibodies, suggesting that Hawaii per se is not an endemic area for HTLV-I. Factors that were significantly associated with seropositivity in the Okinawa migrant groups were years of residence in Japan before migration (issei) and age for offspring of Okinawa migrants (nisei). Antibody titer was highest in Okinawa lifetime residents, intermediate in migrants (issei), and significantly lower in offspring of Okinawa migrants (nisei), with increasing titer observed with advancing age in the offspring of the migrant group. Based on these data, infection within the household occurring early in life appears to be a major route of HTLV-I transmission and may help to explain the curious geographic clustering of this virus in certain locales. As yet to be defined cofactors, including sexual transmission and/or environmental exposures (e.g., particularly before age 20), also may contribute to HTLV-I seropositivity. The pattern of rising seroprevalence and titer with age in the offspring of migrants who resided all of their lives in Hawaii raises the possibility that HTLV-I infection acquired early in life may become dormant and reexpressed with reactivation of latently infected T cells. The importance of this model in the process of viral leukemogenesis is supported by recent reports of adult T-cell leukemia in offspring (nisei) of Okinawa migrants.