Stable human T-lymphotropic virus type I carrier rates for 7 years among a teenaged blood donor cohort of 1986 in Kumamoto, Japan.

The human T-lymphotropic virus type I (HTLV-I) carrier rates for blood donors in Kumamoto, Kyushu, Japan for 8 years, 1986-1993, are currently available. The data show that 16-19-year-olds in 1986 and 20-29-year-olds in 1993 represent nearly the same cohort, because the median age in both groups is 24.5 years in 1993. Therefore, comparison of the HTLV-I positive rate for the two groups gives an estimate of the change in the rate over 7 years within the cohort. In males, 265 of 22,143 donors (1.20%) were seropositive for HTLV-I among 16-19-year-olds in 1986, and 214 were seropositive among 20,076 (1.07%) donors in 20-29-year-olds in 1993. In females, 203 were seropositive among 20,677 (0.98%) donors in 16-19-year-olds in 1986, and there 154 were seropositive among 18,660 (0.83%) donors in 20-29-year-olds in 1993. Thus, the seropositive rates declined in both sexes. However, the average annual rate of immigration to Kumamoto Prefecture was 2.37%. If seropositive rates for 20-29-year-olds in 1993 are adjusted for the dilution effect due to immigration (under the assumption that all immigrants were HTLV-I negative), the adjusted carrier rate for males is 1.26% and that for females is 0.98%. The adjusted carrier rates for both sexes are almost the same as those for 16-19-year-olds in 1986. This indicates that horizontal transmission was negligible for those in the cohort who were in their early reproductive period. Using all 8 year carrier rates for 16-19-year-olds and 20-29-year-olds, chronological changes of 20-29-year-olds, in the near future was estimated. The best goodness of fit model indicated that the HTLV-I carrier rate will decline exponentially, and that the rate will decrease by 50% approximately every 10 years for both sexes. It is probable that in recent years south-west Japan has lost the conditions that are favorable for HTLV-I endemicity and the virus will be virtually non-endemic within a few generations.