Epidemiology, clinical features and prevention of HTLV‐I infection

Abstract

Human T-cell leukemia virus type I (HTLV-I) is the main cause of Adult T-cell leukemia/lymphoma (ATLL) and HTLV-I associated myelopathy/ Tropical spastic paraparesis (HAM/TSP). From the long-term epidemiological observation, the HTLV-I carriers are clustered in limited groups in the world, i.e., Southwestern Japanese, native Oceanians, Amerindians in Andes and Northeast British Colombia, North Iranians as well as Central Africans and their descendants in the Caribbean basin and South America. The annual risk of ATLL in endemic areas of Japan is approximately 1/1,000 over 40 years and the whole life risk is estimated at 2-5%, which are not very different in Central and South America. HAM/TSP is the second most common diseases caused by HTLV-I and its immunological feature showed high titers of anti-HTLV-I antibodies in serum and cerebrospinal fluid. The previous immunogenetic study showed that most ATLL patients have specific HLA haplotypes found only in a minority subpopulation in the HTLV-I endemic areas, while HAM/TSP patients share common HLA haplotypes appearing in the majority of Japanese. Such a different genetic background might determine the geographical epidemic pattern of ATLL and HAM/TSP in the world. The final goal of epidemiology is the establishment of preventive measures and the promotion of better human health. The following strategies for primary prevention of HTLV-I related diseases include: 1) elimination or reduction of exposure to HTLV-I; 2) promotion of the effects of protective factors for progression of HTLV-I related diseases. The familial clustering of HTLV-I carriers indicated a couple of natural transmission routes, motherto- child through breast milk and husband-to-wife through semen under natural life styles. To establish a desirable preventive measure against mother-to-child transmission of HTLV-I through breast milk, several prospective studies were conducted to evaluate the risk of HTLV-I infection among children born to HTLV-I carrier mothers in highly HTLV-I-endemic areas in South and West Kyushu, Japan. The pregnant women were informed the results of anti-HTLV-I antibody test before 30 pregnant weeks and positives were advised to stop breast feeding to prevent mother-to-child transmission of HTLV-I. If they cannot accept it, alternative advice to shorten breast feeding and stop within 3-5 months was recommended. After following up until more than 24 months of age, the seroconversion rate among bottle-feeders (2-4%) and shortterm breast-feeders (3-4%) showed much lower than those of long-term breast feeders (around 20%). The choice of being short-term breast fed for control of HTLV-I infection will provide a new gleam of hope for HTLV-I carrier mothers in the world. Recent cancer registry in Nagasaki, West Kyushu, showed actually decreasing trends in age-adjusted incidence rate of ATLL in both sexes because age distribution of HTLV-I carriers in the endemic areas is relatively aging and HTLV-I carriers in younger generation is drastically decreasing. For the future prevention of ATL, risk reduction of maternal transmission of HTLV-I by stop or short-term breastfeeding would be effective. Actually infection rate among children delivered from HTLV-I carrier mothers was drastically decreasing from more than 10% to less than 3% after starting eradication programme.