Amebiasis is infection with the parasitic intestinal protozoan Entamoeba histolytica (the “tissue-lysing ameba”). Most infections are probably asymptomatic, but E. histolytica can cause disease ranging from dysentery to extraintestinal infections, including liver abscesses.
Life Cycle and Transmission
E. histolytica exists in two stages: a hardy multinucleate cyst form (Fig. 209-1) and the motile trophozoite stage (Fig. 209-2). Infection (of which humans are the natural hosts) is acquired by ingestion of cysts contained in fecally contaminated food or water or, more rarely, through oral-anal sexual contact. Cysts survive stomach acidity and excyst within the small intestine to form the 20- to 50-μm trophozoite stage. Trophozoites can live within the large-bowel lumen without causing disease or can invade the intestinal mucosa, causing amebic colitis. In some cases, E. histolytica trophozoites invade through the mucosa and into the bloodstream, traveling through the portal circulation to reach the liver and causing amebic liver abscesses. Motile trophozoites may be excreted into the stool—a diagnostically important event—but are rapidly killed upon exposure to air or stomach acid and therefore are not infectious. Trophozoite cysts within the large bowel are excreted in the stool, continuing the life cycle.
Entamoeba cyst. Three of the four nuclei are clearly visible. (Courtesy of Dr. George Healy, Centers for Disease Control and Prevention.)
E. histolytica trophozoite with ingested red blood cells. Note the single nucleus with central nucleolus. (Courtesy of the Centers for Disease Control and Prevention.)
Molecular diagnostics continue to clarify what was once a confusing picture of the true incidence and prevalence of E. histolytica infection and disease. It was a staple of most textbooks that 10% of the world's population was infected with E. histolytica. We now know that most asymptomatic individuals harboring amebic trophozoites or cysts in their stools are infected with a noninvasive species: Entamoeba dispar or Entamoeba moshkovskii. E. dispar appears not to cause disease, even in the most profoundly immunosuppressed individuals; furthermore, at this time, there is little evidence to suggest that E. moshkovskii causes disease, although epidemiologic studies of this species are in their infancy. In contrast, E. histolytica infection can cause disease, although not all patients develop symptoms. It remains unclear how frequently people infected with E. histolytica do develop symptoms; in one study in a highly endemic area, only 10% of infected patients developed symptoms over a 1-year observation period. A remarkable feature of amebiasis is its more common occurrence in men than in women, although the prevalence of infection with E. histolytica does not appear to differ between the sexes. This pattern is particularly pronounced for amebic liver abscess, whose prevalence is ∼7 times higher among ...