Approximately 20% of 52 Japanese males with invasive amebiasis confirmed by gel diffusion precipitin test (GDP) were positive by both Treponema pallidum hemagglutination test (TPHA) and nontreponemal antigen tests, and an additional 20% were positive by TPHA alone. However, none of 109 GDP-negative Japanese males without invasive amebiasis were positive on these serologic tests for syphilis. At least 2 bisexuals and 4 homosexuals were among those with invasive amebiasis. All of these biased males were positive on both TPHA and nontreponemal antigen tests. Four of the biased males had liver abscess and 2 had amebic dysentery of colitis. Bisexual or homosexual males were not found in the negative GDP group. These observations suggest that sexually transmitted amebiasis due to Entamoeba histolytica occurs among sexually biased males in Japan.
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