Entamoeba histolytica seropositivity among STI clinic attendees in Japan: implications for risk-informed screening and surveillance

Entamoeba histolytica is increasingly recognized as a sexually transmissible enteric pathogen, but its burden in sexually transmitted infection (STI) services is incompletely characterized outside historically emphasized risk groups. In Japan, interpretation of temporal surveillance trends has also become more difficult since discontinuation of widely used serologic kits in 2018. We conducted a cross-sectional study of 1,017 unique attendees at a specialized STI clinic in Japan who underwent blood testing and had stored serum available for Entamoeba histolytica serology together with corresponding clinical data between August and October 2021. Seropositivity was examined across demographic and clinic-defined behavioral subgroups, concurrent STI burden, and logistic regression models. Overall E. histolytica seropositivity was 10.7% (95% confidence interval [CI] 9.0%-12.8%), a yield comparable to Chlamydia trachomatis positivity (7.5%) in the same clinic population. Seropositivity was observed in men who have sex with men (MSM; 16.1%), female sex workers (FSWs; 9.1%), non-MSM men (8.9%), and non-FSW women (3.9%). Seropositivity increased with the number of concurrent STIs (p for trend = 0.004). In the overall multivariable model, age 40 years or older (adjusted odds ratio [aOR] 3.33, 95% CI 1.99–5.63) and N. gonorrhoeae positivity (aOR 2.99, 95% CI 1.11–7.19) were independently associated with seropositivity. In this single-clinic sample, E. histolytica exposure was common and detectable across multiple STI clinic subgroups. Because serology does not establish current shedding or direction of transmission, the findings are best interpreted as evidence supporting risk-informed assessment, counseling, and confirmatory testing pathways in metropolitan STI services rather than as proof of active spread across all sexual networks.