Decline in Human T‐Cell Leukemia Virus Type 1 Pregnant Carriers and Emerging Concerns About Horizontal Transmission: A Nationwide Survey in Japan

Aim To clarify the current status of human T‐cell leukemia virus type 1 (HTLV‐1) among pregnant women in Japan, with a focus on its prevalence by age and region and the contribution of horizontal transmission. Methods The Japan Association of Obstetricians and Gynecologists conducted a nationwide questionnaire survey in 2025, collecting 2024 data from 1935 delivery facilities. Facilities reported total deliveries, HTLV‐1 carrier cases, and testing details. Information was obtained on birth year, parity, residence, and prior HTLV‐1 test results for carriers. Carrier prevalence by region and age group was analyzed, and prior negative results in multiparous carriers were used to estimate horizontal transmission. Results Responses were received from 1297 facilities (response rate: 67.0%), covering 461 271 deliveries (67.2% of national births). Among these, 331 HTLV‐1 carriers were identified (0.07%), with the highest prevalence in Kyushu and Okinawa (0.25%). Carrier prevalence declined in younger cohorts (0.04% in women born ≥ 1990 vs. 0.09% before 1990; OR 0.435, p < 0.001). Among multiparous carriers, 16.3% had previously tested negative, suggesting horizontal transmission, particularly in urban regions (Kanto, Tokai, Kinki). Conclusions The prevalence of HTLV‐1 among pregnant women in Japan has declined, largely due to effective mother‐to‐child transmission prevention implemented mainly in HTLV‐1 high‐prevalence areas. However, horizontal transmission accounts for a notable percentage of cases, highlighting the need to reinforce preventive education and strategies targeting sexual transmission and unsafe medical or cosmetic practices alongside continued perinatal screening and counseling.