INTRODUCTION
Intestinal amebiasis is caused by the protozoan Entamoeba histolytica. There are four species of intestinal amebae with identical morphologic characteristics: E. histolytica, E. dispar, E. moshkovskii, and E. bangladeshi [1,2]. Most symptomatic disease is caused by E. histolytica; E. dispar is generally considered nonpathogenic. Reported infections with E. moshkovskii are becoming more frequent, with increasing evidence of its potential pathogenicity emerging [3]. The pathogenic potential of E. bangladeshi remains unclear [3,4].Issues related to intestinal E. histolytica infection will be reviewed here; issues related to extraintestinal E. histolytica infection are discussed separately. (See "Extraintestinal Entamoeba histolytica amebiasis".)
EPIDEMIOLOGY
Geographic distribution — Amebiasis occurs worldwide; the prevalence is disproportionately increased in resource-limited countries because of poor socioeconomic conditions and sanitation levels. Areas with high rates of amebic infection include India, Africa, Mexico, and parts of Central and South America. The overall prevalence of amebic infection may be as high as 50 percent in some areas [1].
In resource-rich countries, amebiasis is generally seen in migrants from and travelers to endemic areas. E. histolytica is not a common cause of travelers' diarrhea, and gastrointestinal infection is uncommon in travelers who have spent less than one month in endemic areas. In one prospective study of German travelers to the tropics, only 0.3 percent had pathogenic E. histolytica infection [5]. Institutionalized patients and in some areas, sexually active men who have sex with men, are also at increased risk of infection [6]. In the United States and Europe, the prevalence of E. histolytica among men who have sex with men (MSM) is relatively low; such individuals are principally colonized with nonpathogenic E. dispar [1,7]. However, in areas where E. histolytica is much more prevalent, it does occur among MSM, and invasive, extraintestinal amebiasis (eg, hepatic abscesses) is more frequent in immunosuppressed patients with HIV [1,8-10].
Transmission — The parasite exists in two forms, a cyst stage (the infective form) and a trophozoite stage (the form that causes invasive disease) (figure 1). Infection occurs following ingestion of amebic cysts; this is usually via contaminated food or water but can be associated with sexual transmission through fecal-oral contact, so infection can occur in nonendemic areas among individuals who have never traveled abroad [6,11]. Cysts can remain viable in the environment for weeks to months, and ingestion of a single cyst is sufficient to cause disease.