Trends in Human T-Cell Leukemia Virus Type-1 (HTLV-1) Prevalence and the Incidence of Adult T-Cell Leukemia/Lymphoma (ATL) in Nagasaki, Japan: A Hospital-Based and Population-Based Study.

The prevalence of HTLV-1 is mostly evaluated by the age-specific seroprevalence in blood donors, and the results have been conventionally used to estimate the age-specific incidence of ATL in Japan. However, the results may be underestimated due to an age limit (16-69 yr) for donation, a healthy donor effect, and a birth cohort effect. Data concerning the birth-year specific incidence of ATL among HTLV-1 carriers other than blood donors are scarce.

The study evaluated data of the anti-HTLV-1 antibody testing of 10,261 patients (males: 5,523, females: 4,737) who visited the Nagasaki University Hospital during 2000-2007 and data of 360 ATL cases (males: 188, females: 172) who were diagnosed in Nagasaki City (an endemic area in Japan) in a population-based Nagasaki Prefectural Cancer Registry (NPCR). To estimate birth-year specific incidence rates of ATL in population-based HTLV-1 carriers, we used the 2006 census population for Nagasaki City by applying the hospital-based seroprevalence data.

Of 10,261 patients, 1,392 (males: 653, females: 739) were HTLV-1 antibody positive. The overall HTLV-1 seroprevalence was 13.57% (95%CI: 12.90-14.23%). The seroprevalence was significantly higher in females than in males (15.60% vs. 11.82%, P<0.0001). The birth-year specific seroprevalence was 18.69% (before 1926), 17.83% (1927-1936), 15.91% (1937-1946), 13.80% (1947-1956), 9.19% (1957-1966), 4.07% (1967-1976), 2.07% (1977-1986), and 0% (after 1987) (a significantly declining trend: P <0.0001). The estimated annual number of HTLV-1 carriers by birth-year in Nagasaki city was 5257, 8093, 8151, 8083, 4434, 2180, 785, and 0, respectively. Finally, we estimated the annual incidence rate of ATLL per 100,000 HTLV-1 carriers by birth-year; 171 (before 1926), 86 (1927-1936), 41 (1937-1946), 32 (1947-1956), 11 (1957-1966), and 0 (after 1967). The crude lifetime risk of developing ATLL in HTLV-1 carriers was estimated to be 7.29% for males and 3.78% for females.

The birth-year specific HTLV-1 seroprevalnces in the present study were approximately 50% higher than those previously reported in blood donors1 (for example: 6.22% in those born before 1950). Although it is possible that our results are over-estimated2, the present study suggests that there is still a large pool of elderly HTLV-1 carriers in this endemic area. Further studies are needed to investigate the mechanism of the development of ATL among HTLV-1 carriers for preventing the development. Reference: 1) Iwanaga M et al. Int J Hematol, 2009. 2) Arisawa K et al. Int J Cancer, 2000.

No relevant conflicts of interest to declare.