Milk-Borne Transmission of Human T-Cell Lymphotropic Virus Type I (HTLV-I) and Its Intervention in Nagasaki

Summary Human T-cell lymphotropic virus type I (HTLV-I), a causative virus of adult T-cell leukemia (ATL), transmits through both horizontal and vertical pathways. Since ATL develops in only carriers infected early in life, vertical transmission cycle is the most important target of intervention for the purpose of the prevention of ATL. Epidemiological studies in an endemic area, Nagasaki, Japan, and animal experiments identified milk-borne maternal transmission as the major vertical pathway. The prefecture-wide intervention in Nagasaki since 1987 has revealed that the refraining from breast-feeding by carrier mothers can prevent about 85% of maternal HTLV-I transmission. Pathways for the remaining 15% await for elucidation. However, our studies argued against the possibilities of intrauterine transmission and infection via saliva. Perinatal transmission remains to be evaluated as the alternative pathway. At present, refraining from breast-feeding is the most effective measure to break the cycle of maternal HTLV-I transmission in endemic areas. It is estimated that the intervention in Nagasaki for the past 10 years has prevented 1,000 maternal transmission and 50 future ATL cases. I am reasonably confident that incidence of ATL in Nagasaki will decline to the national average level over the next few generation if the intervention program stays alive