A Possible Cluster of Sexually Transmitted Entamoeba histolytica: Genetic Analysis of a Highly Virulent Strain

Background. Transmission of Entamoeba histolytica generally occurs by fecal excretion of cysts followed by oral ingestion of contaminated food or water. However, fecal-oral transmission may occur within households and long-term care institutions, and sexual transmission occurs among men who have sex with men. Epidemiologically linked clusters of E. histolytica infection are rare in industrialized countries. We report such a sexually linked cluster in Canada. Methods. An index case involving a young female with an amebic liver abscess led to an epidemiological investigation of sexual contacts. Anti-amebic serological analysis, stool specimen examinations, and abdominal ultrasounds were done for the contacts. Enzyme-linked immunosorbent assay was done for stool antigen specific to E. histolytica. Genotyping and phylogenetic analysis was performed on 1 stool isolate. Results. By tracing sexual contacts related to the index case, we uncovered a cluster of 7 cases of amebiasis (3 with liver abscesses). Oral-anal sex was common in the group; the 5 female individuals were bisexual (4) or homosexual (1). The outbreak strain was genotyped, and cluster analysis indicated that this virulent strain differed substantially from asymptomatic or diarrheal E. histolytica isolates. Conclusions. E. histolytica can be transmitted by heterosexual activity as well as male and female homosexual activity. Patients with amebiasis should be counselled about possible sexual transmission.