Background Amebiasis, which is caused by Entamoeba histolytica, is a re-emerging public health issue owing to sexually transmitted infection (STI) in Japan. However, epidemiological data are quite limited.
Methods To reveal the relative prevalence of sexually transmitted E. histolytica infection to other STIs, we conducted a cross-sectional study at a voluntary counselling and testing (VCT) centre in Tokyo. Seroprevalence of E. histolytica was assessed according to positivity with an enzyme-linked immunosorbent assay for E. histolytica-specific IgG in serum samples collected from anonymous VCT clients.
Results Among 2,083 samples, seropositive rate for E. histolytica was 2.64%, which was higher than that for HIV-1 (0.34%, p < 0.001) and comparable to that for syphilis (rapid plasma reagin (RPR) 2.11%, p = 0.31). Positivity for Chlamydia trachomatis in urine by transcription-mediated amplification (TMA) was 4.59%. Seropositivity for E. histolytica was high among RPR- or Treponema pallidum hemagglutination (TPHA)-positive individuals and it was not different between clients with and without other STIs. Both seropositivity of E. histolytica and RPR were high among male clients. The seropositive rate for anti-E. histolytica antibody was positively correlated with age. TMA positivity for urine C. trachomatis was high among female clients and negatively correlated with age. Regression analysis identified that male sex, older age, and TPHA-positive results are independent risk factors of E. histolytica seropositivity.
Conclusions Seroprevalence of E. histolytica was 7.9 times higher than that of HIV-1 at a VCT centre in Tokyo, with a tendency to be higher among people at risk for syphilis infection. There is a need for education and specific interventions against this parasite, as a potentially re-
65 emerging pathogen.

The design of this study was a cross-sectional study. The total 2,083 serum samples used in this study were collected at the Tokyo Metropolitan Minami Shinjuku Testing – Counselling Centre where more than 10,000 anonymous clients seek HIV-1 screening tests each year. Collected samples are transferred to the Tokyo Metropolitan Institute of Public Health for laboratory testing, then stored at 4°C. Fourth generation HIV-1 screening is performed routinely throughout the year. However, in 2 months of the year (e.g., June and December in the case of 2017), the Tokyo Metropolitan Government intensifies STI screening, and rapid plasma reagin (RPR) and Treponema pallidum hemagglutination (TPHA) tests for syphilis screening are additionally performed for all clients. In addition, urinary sampling and transcription-mediated amplification (TMA) assay testing for Chlamydia trachomatis and Neisseria gonorrhoeae are performed for clients who are willing to undergo these tests.