HTLV-1 Seroprevalence and Evidence of Seroconversion Among Men Who Have Sex with Men in Osaka, Japan

In Japan, preventing the vertical transmission of HTLV-1 has been prioritized; however, horizontal transmission through sexual contact has been increasing in urban areas. Since HTLV-1 is more frequently transmitted from men to women in sexual contact, men who have sex with men (MSM) may be at a higher risk of the horizontal transmission of HTLV-1 than heterosexual men. We have been providing HIV and sexually transmitted infection testing for MSM at clinics, and have examined the presence or absence of anti-HTLV-1 antibodies using stored sera or plasma in a testing project for MSM. Among the 4,444 stored samples, 10 were found to be positive for HTLV-1 in confirmatory testing. A retrospective review of medical records at clinics revealed that at least one antibody-positive carrier had previously tested negative. The estimated prevalence of HTLV-1 antibodies among MSM was 0.45%. These results suggest the horizontal transmission of HTLV-1 among MSM and provide the first estimate of HTLV-1 seroprevalence in this population in Japan.